go.mod: update golang.org/x dependencies

Update golang.org/x dependencies to their latest tagged versions.

Change-Id: I1dc78ceffd32c80457b284aa1bcefb6dfc2757ba
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/788222
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Gopher Robot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>

go.mod

@@ -3,9 +3,9 @@ module golang.org/x/crypto
 go 1.25.0
 
 require (
-	golang.org/x/net v0.54.0 // tagx:ignore
-	golang.org/x/sys v0.45.0
-	golang.org/x/term v0.43.0
+	golang.org/x/net v0.55.0 // tagx:ignore
+	golang.org/x/sys v0.46.0
+	golang.org/x/term v0.44.0
 )
 
-require golang.org/x/text v0.37.0 // indirect
+require golang.org/x/text v0.38.0 // indirect

go.sum

@@ -1,8 +1,8 @@
-golang.org/x/net v0.54.0 h1:2zJIZAxAHV/OHCDTCOHAYehQzLfSXuf/5SoL/Dv6w/w=
-golang.org/x/net v0.54.0/go.mod h1:Sj4oj8jK6XmHpBZU/zWHw3BV3abl4Kvi+Ut7cQcY+cQ=
-golang.org/x/sys v0.45.0 h1:dO4czNzziLiiXplLQgBCEpCvXQ3dnkn0SdaZSYdQ+FY=
-golang.org/x/sys v0.45.0/go.mod h1:4GL1E5IUh+htKOUEOaiffhrAeqysfVGipDYzABqnCmw=
-golang.org/x/term v0.43.0 h1:S4RLU2sB31O/NCl+zFN9Aru9A/Cq2aqKpTZJ6B+DwT4=
-golang.org/x/term v0.43.0/go.mod h1:lrhlHNdQJHO+1qVYiHfFKVuVioJIheAc3fBSMFYEIsk=
-golang.org/x/text v0.37.0 h1:Cqjiwd9eSg8e0QAkyCaQTNHFIIzWtidPahFWR83rTrc=
-golang.org/x/text v0.37.0/go.mod h1:a5sjxXGs9hsn/AJVwuElvCAo9v8QYLzvavO5z2PiM38=
+golang.org/x/net v0.55.0 h1:bcvxaJn3e1U6InsFWt1JUq1aSjnRxLzT2rtD2KfkDF8=
+golang.org/x/net v0.55.0/go.mod h1:L5U2KuzuOe1lY7Z+aWVIKK6qEeJXnXV9yzGA+WCHJww=
+golang.org/x/sys v0.46.0 h1:noSf2Fq6F8DBgS+LysIkx7rIExoNHJsxOAtPp4rthXw=
+golang.org/x/sys v0.46.0/go.mod h1:4GL1E5IUh+htKOUEOaiffhrAeqysfVGipDYzABqnCmw=
+golang.org/x/term v0.44.0 h1:0rLvDRCtNj0gZkyIXhCyOb2OAzEhLVqc4B+hrsBhrmc=
+golang.org/x/term v0.44.0/go.mod h1:7ze4MdzUzLXpSAoFP1H0bOI9aXDqveSvatT5vKcFh2Y=
+golang.org/x/text v0.38.0 h1:sXmwo9DwP3OK9EZ7PqAdaooSGozfl/3a6/xJcbzPRhE=
+golang.org/x/text v0.38.0/go.mod h1:YXZt3QhHUKYT53r2lLKFIVi6Ao1jdzrTR/KQ09qyxF4=

pkcs12/crypto.go

@@ -80,6 +80,10 @@ func pbDecrypterFor(algorithm pkix.AlgorithmIdentifier, password []byte) (cipher
 		return nil, 0, err
 	}
 
+	if params.Iterations < 0 || params.Iterations > maxIterations {
+		return nil, 0, NotImplementedError("iteration count is invalid or too high")
+	}
+
 	key := cipherType.deriveKey(params.Salt, password, params.Iterations)
 	iv := cipherType.deriveIV(params.Salt, password, params.Iterations)
 

pkcs12/crypto_test.go

@@ -13,6 +13,46 @@ import (
 
 var sha1WithTripleDES = asn1.ObjectIdentifier([]int{1, 2, 840, 113549, 1, 12, 1, 3})
 
+func TestPbDecrypterForIterationLimit(t *testing.T) {
+	pass, _ := bmpString("Sesame open")
+
+	tests := []struct {
+		name       string
+		iterations int
+		wantErr    bool
+	}{
+		{"at limit", maxIterations, false},
+		{"over limit", maxIterations + 1, true},
+		{"negative", -1, true},
+		{"max int", 1<<31 - 1, true},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			params, _ := asn1.Marshal(pbeParams{
+				Salt:       []byte{1, 2, 3, 4, 5, 6, 7, 8},
+				Iterations: tt.iterations,
+			})
+			alg := pkix.AlgorithmIdentifier{
+				Algorithm: sha1WithTripleDES,
+				Parameters: asn1.RawValue{
+					FullBytes: params,
+				},
+			}
+			_, _, err := pbDecrypterFor(alg, pass)
+			if tt.wantErr {
+				if _, ok := err.(NotImplementedError); !ok {
+					t.Errorf("iterations=%d: got %v, want NotImplementedError", tt.iterations, err)
+				}
+			} else {
+				if err != nil {
+					t.Errorf("iterations=%d: unexpected error: %v", tt.iterations, err)
+				}
+			}
+		})
+	}
+}
+
 func TestPbDecrypterFor(t *testing.T) {
 	params, _ := asn1.Marshal(pbeParams{
 		Salt:       []byte{1, 2, 3, 4, 5, 6, 7, 8},

pkcs12/mac.go

@@ -27,11 +27,19 @@ var (
 	oidSHA1 = asn1.ObjectIdentifier([]int{1, 3, 14, 3, 2, 26})
 )
 
+// maxIterations is a safety limit to prevent CPU exhaustion from
+// crafted PKCS#12 files with unreasonable iteration counts.
+const maxIterations = 1 << 20 // ~1 million
+
 func verifyMac(macData *macData, message, password []byte) error {
 	if !macData.Mac.Algorithm.Algorithm.Equal(oidSHA1) {
 		return NotImplementedError("unknown digest algorithm: " + macData.Mac.Algorithm.Algorithm.String())
 	}
 
+	if macData.Iterations < 0 || macData.Iterations > maxIterations {
+		return NotImplementedError("iteration count is invalid or too high")
+	}
+
 	key := pbkdf(sha1Sum, 20, 64, macData.MacSalt, password, macData.Iterations, 3, 20)
 
 	mac := hmac.New(sha1.New, key)

pkcs12/mac_test.go

@@ -5,10 +5,52 @@
 package pkcs12
 
 import (
+	"crypto/x509/pkix"
 	"encoding/asn1"
 	"testing"
 )
 
+func TestVerifyMacIterationLimit(t *testing.T) {
+	password, _ := bmpString("Sesame open")
+	message := []byte{11, 12, 13, 14, 15}
+
+	tests := []struct {
+		name       string
+		iterations int
+		wantErr    bool
+	}{
+		{"at limit", maxIterations, false},
+		{"over limit", maxIterations + 1, true},
+		{"negative", -1, true},
+		{"max int", 1<<31 - 1, true},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			td := macData{
+				Mac: digestInfo{
+					Algorithm: pkix.AlgorithmIdentifier{
+						Algorithm: oidSHA1,
+					},
+					Digest: nil, // will fail MAC check, but iteration check comes first
+				},
+				MacSalt:    []byte{1, 2, 3, 4, 5, 6, 7, 8},
+				Iterations: tt.iterations,
+			}
+			err := verifyMac(&td, message, password)
+			if tt.wantErr {
+				if _, ok := err.(NotImplementedError); !ok {
+					t.Errorf("iterations=%d: got %v, want NotImplementedError", tt.iterations, err)
+				}
+			} else {
+				if _, ok := err.(NotImplementedError); ok {
+					t.Errorf("iterations=%d: got unexpected NotImplementedError", tt.iterations)
+				}
+			}
+		})
+	}
+}
+
 func TestVerifyMac(t *testing.T) {
 	td := macData{
 		Mac: digestInfo{

ssh/agent/client.go

@@ -26,6 +26,7 @@ import (
 	"io"
 	"math/big"
 	"sync"
+	"sync/atomic"
 
 	"golang.org/x/crypto/ssh"
 )
@@ -307,17 +308,50 @@ func parseKey(in []byte) (out *Key, rest []byte, err error) {
 	}, record.Rest, nil
 }
 
+// pipelineMaxInFlight is the maximum number of outstanding requests the
+// client will pipeline to the agent before applying backpressure.
+const pipelineMaxInFlight = 32
+
 // client is a client for an ssh-agent process.
+//
+// Exactly one of pipeline / (mu, conn) is set, chosen by NewClient
+// based on whether the underlying transport implements io.Closer.
 type client struct {
-	// conn is typically a *net.UnixConn
+	// pipeline, if non-nil, dispatches requests over a pipelined
+	// connection: requests are written as soon as the wire is
+	// available and responses are routed back to per-call reply
+	// channels in FIFO order by a background reader goroutine.
+	pipeline *pipeline
+
+	// mu and conn are used in fully-serialized mode, when the
+	// transport does not implement io.Closer. Each call takes mu,
+	// writes its request, reads the matching response, and releases
+	// mu before returning. There is no background goroutine.
+	mu   sync.Mutex
 	conn io.ReadWriter
-	// mu is used to prevent concurrent access to the agent
-	mu sync.Mutex
 }
 
 // NewClient returns an Agent that talks to an ssh-agent process over
 // the given connection.
+//
+// If rw also implements io.Closer (like *net.UnixConn and ssh.Channel
+// do), the returned client pipelines concurrent requests over the
+// connection: callers can issue Sign and other operations from
+// multiple goroutines and they will be written to the agent as soon
+// as the wire is available, rather than waiting for the previous
+// responses. The ssh-agent protocol still requires responses to be
+// returned in request order, so a slow request delays subsequent
+// responses on the same connection (head-of-line blocking).
+//
+// Pipelining requires io.Closer because, on a Write error, the
+// background reader goroutine must be unblocked by closing the
+// underlying connection. When rw does not implement io.Closer
+// this is not possible, so NewClient falls back to fully
+// serializing each request: a single in-flight call at a time.
 func NewClient(rw io.ReadWriter) ExtendedAgent {
+	if rwc, ok := rw.(io.ReadWriteCloser); ok {
+		return &client{pipeline: newPipeline(rwc)}
+	}
 	return &client{conn: rw}
 }
 
@@ -340,6 +374,16 @@ func (c *client) call(req []byte) (reply interface{}, err error) {
 // bytes of the response are returned; no unmarshalling is
 // performed on the response.
 func (c *client) callRaw(req []byte) (reply []byte, err error) {
+	if c.pipeline != nil {
+		return c.pipeline.call(req)
+	}
+	return c.serialCall(req)
+}
+
+// serialCall implements the fully-serialized request/response path
+// used when the transport is not an io.Closer. It writes req under mu
+// and reads the matching response before returning.
+func (c *client) serialCall(req []byte) (reply []byte, err error) {
 	c.mu.Lock()
 	defer c.mu.Unlock()
 
@@ -577,6 +621,9 @@ func (c *client) insertKey(s interface{}, comment string, constraints []byte) er
 			Constraints: constraints,
 		})
 	case ed25519.PrivateKey:
+		if len(k) != ed25519.PrivateKeySize {
+			return fmt.Errorf("agent: bad ED25519 key size: %d", len(k))
+		}
 		req = ssh.Marshal(ed25519KeyMsg{
 			Type:        ssh.KeyAlgoED25519,
 			Pub:         []byte(k)[32:],
@@ -588,6 +635,9 @@ func (c *client) insertKey(s interface{}, comment string, constraints []byte) er
 	// general idiom is to pass ed25519.PrivateKey by value, not by pointer.
 	// We still support the pointer variant for backwards compatibility.
 	case *ed25519.PrivateKey:
+		if len(*k) != ed25519.PrivateKeySize {
+			return fmt.Errorf("agent: bad ED25519 key size: %d", len(*k))
+		}
 		req = ssh.Marshal(ed25519KeyMsg{
 			Type:        ssh.KeyAlgoED25519,
 			Pub:         []byte(*k)[32:],
@@ -712,6 +762,9 @@ func (c *client) insertCert(s interface{}, cert *ssh.Certificate, comment string
 			Constraints: constraints,
 		})
 	case ed25519.PrivateKey:
+		if len(k) != ed25519.PrivateKeySize {
+			return fmt.Errorf("agent: bad ED25519 key size: %d", len(k))
+		}
 		req = ssh.Marshal(ed25519CertMsg{
 			Type:        cert.Type(),
 			CertBytes:   cert.Marshal(),
@@ -724,6 +777,9 @@ func (c *client) insertCert(s interface{}, cert *ssh.Certificate, comment string
 	// general idiom is to pass ed25519.PrivateKey by value, not by pointer.
 	// We still support the pointer variant for backwards compatibility.
 	case *ed25519.PrivateKey:
+		if len(*k) != ed25519.PrivateKeySize {
+			return fmt.Errorf("agent: bad ED25519 key size: %d", len(*k))
+		}
 		req = ssh.Marshal(ed25519CertMsg{
 			Type:        cert.Type(),
 			CertBytes:   cert.Marshal(),
@@ -861,3 +917,170 @@ func (c *client) Extension(extensionType string, contents []byte) ([]byte, error
 
 	return buf, nil
 }
+
+// pipelineResult carries either a raw agent reply or an error back to a
+// caller waiting on the response channel.
+type pipelineResult struct {
+	reply []byte
+	err   error
+}
+
+// pipeline implements request pipelining over a single agent connection.
+//
+// Writers serialize on writeMu to both register a reply channel in the
+// pending FIFO queue and write the request bytes on the wire; the two
+// must be atomic so the queue order matches the wire order. A single
+// reader goroutine decodes responses from the connection and dispatches
+// each one to the channel at the head of the queue.
+//
+// pending is a chan-of-chan acting as a FIFO queue with a fixed
+// capacity of pipelineMaxInFlight. The outer channel provides ordering
+// (reads happen in send order) and natural backpressure (a full queue
+// blocks new writers). Each inner channel is buffered with capacity
+// one and is sent to exactly once: either by the reader goroutine
+// with the agent reply, or by shutdown with the terminal error during
+// drain. The cap-one buffer makes the producer's send non-blocking,
+// so the reader and shutdown never have to wait for the caller to be
+// scheduled on the receive.
+//
+// When the reader goroutine exits (on read error or protocol
+// violation), it closes exitCh to wake any writer blocked on the
+// pending queue, then serializes with any in-flight writer to close
+// the pending channel, and finally drains the remaining entries
+// delivering the terminal error to each waiting caller. The
+// pipeline relies on conn implementing io.Closer so a writer that
+// hits a Write error can close the connection to unblock the reader
+// goroutine; NewClient is responsible for only constructing a
+// pipeline when this guarantee holds.
+type pipeline struct {
+	conn io.ReadWriteCloser
+
+	writeMu sync.Mutex
+	// pending is the FIFO queue of reply channels with capacity
+	// pipelineMaxInFlight. See type-level documentation.
+	pending chan chan pipelineResult
+	exitCh  chan struct{}
+
+	// err carries the terminal error to callers blocked on a closed
+	// pipeline. It is stored exactly once by the reader goroutine
+	// before exitCh is closed; every read happens after observing
+	// exitCh closed, so the load synchronises through the close and
+	// is guaranteed to return the stored value (never nil).
+	err atomic.Pointer[error]
+}
+
+func newPipeline(conn io.ReadWriteCloser) *pipeline {
+	p := &pipeline{
+		conn:    conn,
+		pending: make(chan chan pipelineResult, pipelineMaxInFlight),
+		exitCh:  make(chan struct{}),
+	}
+	go p.readLoop()
+	return p
+}
+
+// readLoop decodes responses from conn and dispatches them in FIFO order
+// to reply channels in pending. On any failure it invokes shutdown.
+func (p *pipeline) readLoop() {
+	var finalErr error
+	for {
+		var sizeBuf [4]byte
+		if _, err := io.ReadFull(p.conn, sizeBuf[:]); err != nil {
+			finalErr = err
+			break
+		}
+		respSize := binary.BigEndian.Uint32(sizeBuf[:])
+		if respSize > maxAgentResponseBytes {
+			finalErr = errors.New("response too large")
+			break
+		}
+		buf := make([]byte, respSize)
+		if _, err := io.ReadFull(p.conn, buf); err != nil {
+			finalErr = err
+			break
+		}
+		// Successful writes always enqueue before sending bytes, so
+		// pending has a waiting channel for this response.
+		ch := <-p.pending
+		// The reply channel is buffered with capacity 1 and is only
+		// ever written to once, so this send cannot block.
+		ch <- pipelineResult{reply: buf}
+	}
+	p.shutdown(clientErr(finalErr))
+}
+
+// shutdown is called exactly once, from readLoop, when the reader is
+// terminating. It unblocks pending writers and fails all in-flight
+// requests with finalErr.
+func (p *pipeline) shutdown(finalErr error) {
+	// Publish the terminal error before closing exitCh so any
+	// writer that subsequently observes exitCh closed sees err.
+	p.err.Store(&finalErr)
+
+	// Wake any writer blocked waiting for a slot in the pending queue.
+	close(p.exitCh)
+
+	// Wait for any writer currently inside its critical section to
+	// complete. After this lock, no new writer can reach the send on
+	// pending: they will observe exitCh closed in the select and bail
+	// out before attempting the send.
+	p.writeMu.Lock()
+	close(p.pending)
+	p.writeMu.Unlock()
+
+	// Drain entries that were enqueued but never answered, delivering
+	// the terminal error to their waiting callers. The reply channels
+	// are buffered (cap 1) and written to exactly once, so these sends
+	// cannot block.
+	for ch := range p.pending {
+		ch <- pipelineResult{err: finalErr}
+	}
+}
+
+// call sends req to the agent and returns the matching raw response.
+func (p *pipeline) call(req []byte) ([]byte, error) {
+	replyCh := make(chan pipelineResult, 1)
+
+	p.writeMu.Lock()
+
+	// Priority check: if the reader has already finished shutdown,
+	// pending is closed and sending to it would panic. Bail out now.
+	// Once we pass this check while holding writeMu, shutdown cannot
+	// complete close(pending) until we release writeMu, so the send
+	// below is safe against concurrent closure.
+	select {
+	case <-p.exitCh:
+		p.writeMu.Unlock()
+		return nil, *p.err.Load()
+	default:
+	}
+
+	// Enqueue the reply channel before writing the request, so FIFO
+	// order on the wire matches FIFO order in the pending queue. The
+	// exitCh arm handles the case where the reader errors while we
+	// block on a full queue.
+	select {
+	case p.pending <- replyCh:
+	case <-p.exitCh:
+		p.writeMu.Unlock()
+		return nil, *p.err.Load()
+	}
+
+	msg := make([]byte, 4+len(req))
+	binary.BigEndian.PutUint32(msg, uint32(len(req)))
+	copy(msg[4:], req)
+	_, werr := p.conn.Write(msg)
+	p.writeMu.Unlock()
+
+	if werr != nil {
+		// The connection is in an undefined state. Close it so the
+		// reader unblocks promptly and triggers shutdown for every
+		// other in-flight caller. NewClient guarantees conn is a
+		// real io.Closer when the pipeline is in use.
+		p.conn.Close()
+		return nil, clientErr(werr)
+	}
+
+	res := <-replyCh
+	return res.reply, res.err
+}

ssh/agent/client_pipeline_test.go

@@ -0,0 +1,649 @@
+// Copyright 2026 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package agent
+
+import (
+	"encoding/binary"
+	"errors"
+	"fmt"
+	"io"
+	"net"
+	"strings"
+	"sync"
+	"sync/atomic"
+	"testing"
+	"time"
+)
+
+func TestPipelineParallelSigns(t *testing.T) {
+	c1, c2, err := netPipe()
+	if err != nil {
+		t.Fatalf("netPipe: %v", err)
+	}
+	defer c1.Close()
+	defer c2.Close()
+
+	keyring := NewKeyring()
+	if err := keyring.Add(AddedKey{PrivateKey: testPrivateKeys["rsa"]}); err != nil {
+		t.Fatal(err)
+	}
+	go ServeAgent(keyring, c2)
+
+	client := NewClient(c1)
+	pubKey := testPublicKeys["rsa"]
+
+	const N = 64
+	var wg sync.WaitGroup
+	errCh := make(chan error, N)
+	for i := range N {
+		wg.Go(func() {
+			data := []byte{byte(i), byte(i >> 8)}
+			sig, err := client.Sign(pubKey, data)
+			if err != nil {
+				errCh <- err
+				return
+			}
+			if err := pubKey.Verify(data, sig); err != nil {
+				errCh <- err
+			}
+		})
+	}
+	wg.Wait()
+	close(errCh)
+	for err := range errCh {
+		t.Error(err)
+	}
+}
+
+func TestPipelineBackpressure(t *testing.T) {
+	c1, c2, err := netPipe()
+	if err != nil {
+		t.Fatalf("netPipe: %v", err)
+	}
+
+	const total = pipelineMaxInFlight + 8
+
+	// Fake agent: the reader drains requests from the wire independently
+	// of response production, so the write path is never self-blocked.
+	// The responder only writes a canned success reply once unblocked
+	// via gate.
+	var reqsRead int64
+	gate := make(chan struct{}, total)
+	pending := make(chan struct{}, total+pipelineMaxInFlight)
+	readerDone := make(chan struct{})
+	responderDone := make(chan struct{})
+	go func() {
+		defer close(readerDone)
+		defer close(pending)
+		for {
+			var sizeBuf [4]byte
+			if _, err := io.ReadFull(c2, sizeBuf[:]); err != nil {
+				return
+			}
+			size := binary.BigEndian.Uint32(sizeBuf[:])
+			req := make([]byte, size)
+			if _, err := io.ReadFull(c2, req); err != nil {
+				return
+			}
+			atomic.AddInt64(&reqsRead, 1)
+			pending <- struct{}{}
+		}
+	}()
+	go func() {
+		defer close(responderDone)
+		for range pending {
+			<-gate
+			var out [5]byte
+			binary.BigEndian.PutUint32(out[:4], 1)
+			out[4] = agentSuccess
+			if _, err := c2.Write(out[:]); err != nil {
+				return
+			}
+		}
+	}()
+	defer func() {
+		c1.Close()
+		c2.Close()
+		<-readerDone
+		<-responderDone
+	}()
+
+	client := NewClient(c1)
+
+	var wg sync.WaitGroup
+	errs := make(chan error, total)
+	for range total {
+		wg.Go(func() {
+			// RemoveAll maps to a simpleCall expecting agentSuccess.
+			if err := client.RemoveAll(); err != nil {
+				errs <- err
+			}
+		})
+	}
+
+	// Wait for the client to fill the pipeline.
+	deadline := time.Now().Add(2 * time.Second)
+	for time.Now().Before(deadline) {
+		if atomic.LoadInt64(&reqsRead) >= pipelineMaxInFlight {
+			break
+		}
+		time.Sleep(5 * time.Millisecond)
+	}
+	if got := atomic.LoadInt64(&reqsRead); got != pipelineMaxInFlight {
+		t.Errorf("requests in flight before any response: got %d, want %d", got, pipelineMaxInFlight)
+	}
+
+	// Release all responses; every caller should eventually complete.
+	for range total {
+		gate <- struct{}{}
+	}
+	wg.Wait()
+	close(errs)
+	for err := range errs {
+		t.Error(err)
+	}
+
+	if got := atomic.LoadInt64(&reqsRead); got != total {
+		t.Errorf("total requests received: got %d, want %d", got, total)
+	}
+}
+
+func TestPipelineConnCloseFailsInFlight(t *testing.T) {
+	c1, c2, err := netPipe()
+	if err != nil {
+		t.Fatalf("netPipe: %v", err)
+	}
+
+	// Server that reads requests but never replies, so all calls are
+	// parked waiting for responses.
+	serverExit := make(chan struct{})
+	go func() {
+		defer close(serverExit)
+		buf := make([]byte, 4096)
+		for {
+			if _, err := c2.Read(buf); err != nil {
+				return
+			}
+		}
+	}()
+
+	client := NewClient(c1)
+
+	const N = 8
+	var wg sync.WaitGroup
+	errs := make(chan error, N)
+	for range N {
+		wg.Go(func() {
+			errs <- client.RemoveAll()
+		})
+	}
+
+	// Give callers time to enter the pipeline, then tear down the conn.
+	time.Sleep(50 * time.Millisecond)
+	c1.Close()
+	c2.Close()
+
+	// All callers must return promptly with an error.
+	doneCh := make(chan struct{})
+	go func() {
+		wg.Wait()
+		close(doneCh)
+	}()
+	select {
+	case <-doneCh:
+	case <-time.After(5 * time.Second):
+		t.Fatal("pipelined callers did not complete after conn close")
+	}
+
+	close(errs)
+	for err := range errs {
+		if err == nil {
+			t.Error("expected error after conn close, got nil")
+		}
+	}
+
+	// Subsequent calls also fail fast rather than hanging.
+	errCh := make(chan error, 1)
+	go func() { errCh <- client.RemoveAll() }()
+	select {
+	case err := <-errCh:
+		if err == nil {
+			t.Error("expected error on post-close call, got nil")
+		}
+	case <-time.After(2 * time.Second):
+		t.Fatal("post-close call hung")
+	}
+
+	<-serverExit
+}
+
+// readWriter exposes only io.Reader and io.Writer, hiding any Close
+// method on the wrapped value so NewClient cannot take the pipelined
+// path.
+type readWriter struct {
+	io.Reader
+	io.Writer
+}
+
+func TestNewClientSerialFallback(t *testing.T) {
+	c1, c2, err := netPipe()
+	if err != nil {
+		t.Fatalf("netPipe: %v", err)
+	}
+	defer c1.Close()
+	defer c2.Close()
+
+	keyring := NewKeyring()
+	if err := keyring.Add(AddedKey{PrivateKey: testPrivateKeys["rsa"]}); err != nil {
+		t.Fatal(err)
+	}
+	go ServeAgent(keyring, c2)
+
+	rw := readWriter{Reader: c1, Writer: c1}
+	ag := NewClient(rw)
+
+	ci, ok := ag.(*client)
+	if !ok {
+		t.Fatalf("NewClient: got %T, want *client", ag)
+	}
+	if ci.pipeline != nil {
+		t.Error("NewClient with non-Closer rw: pipeline must be nil")
+	}
+	if ci.conn == nil {
+		t.Error("NewClient with non-Closer rw: conn must be set")
+	}
+
+	// The serialized path correctly handles a request/response cycle.
+	keys, err := ag.List()
+	if err != nil {
+		t.Fatalf("List: %v", err)
+	}
+	if len(keys) != 1 {
+		t.Errorf("List: got %d keys, want 1", len(keys))
+	}
+}
+
+func TestSerialFallbackParallelSigns(t *testing.T) {
+	c1, c2, err := netPipe()
+	if err != nil {
+		t.Fatalf("netPipe: %v", err)
+	}
+	defer c1.Close()
+	defer c2.Close()
+
+	keyring := NewKeyring()
+	if err := keyring.Add(AddedKey{PrivateKey: testPrivateKeys["rsa"]}); err != nil {
+		t.Fatal(err)
+	}
+	go ServeAgent(keyring, c2)
+
+	rw := readWriter{Reader: c1, Writer: c1}
+	ag := NewClient(rw)
+
+	if ci, ok := ag.(*client); !ok {
+		t.Fatalf("NewClient: got %T, want *client", ag)
+	} else if ci.pipeline != nil {
+		t.Fatal("pipeline must be nil for non-Closer rw")
+	}
+
+	pubKey := testPublicKeys["rsa"]
+
+	const N = 64
+	var wg sync.WaitGroup
+	errCh := make(chan error, N)
+	for i := range N {
+		wg.Go(func() {
+			data := []byte{byte(i), byte(i >> 8)}
+			sig, err := ag.Sign(pubKey, data)
+			if err != nil {
+				errCh <- err
+				return
+			}
+			if err := pubKey.Verify(data, sig); err != nil {
+				errCh <- err
+			}
+		})
+	}
+	wg.Wait()
+	close(errCh)
+	for err := range errCh {
+		t.Error(err)
+	}
+}
+
+// writeFailConn wraps a net.Conn and forces Write to return an error when
+// the fail flag is set. Close delegates to the underlying conn so the test
+// can observe whether the pipeline closed it.
+type writeFailConn struct {
+	net.Conn
+	fail atomic.Bool
+}
+
+func (c *writeFailConn) Write(p []byte) (int, error) {
+	if c.fail.Load() {
+		return 0, errors.New("synthetic write failure")
+	}
+	return c.Conn.Write(p)
+}
+
+func TestPipelineWriteErrorClosesConn(t *testing.T) {
+	c1, c2, err := netPipe()
+	if err != nil {
+		t.Fatalf("netPipe: %v", err)
+	}
+	defer c2.Close()
+
+	// Peer that reads and discards, exiting when the conn closes.
+	serverExit := make(chan struct{})
+	go func() {
+		defer close(serverExit)
+		io.Copy(io.Discard, c2)
+	}()
+
+	wrapper := &writeFailConn{Conn: c1}
+	wrapper.fail.Store(true)
+
+	client := NewClient(wrapper)
+
+	errCh := make(chan error, 1)
+	go func() { errCh <- client.RemoveAll() }()
+
+	select {
+	case err := <-errCh:
+		if err == nil {
+			t.Fatal("expected error on forced write failure, got nil")
+		}
+		// The caller whose Write failed must receive the actual
+		// write error, not the reader's "closed connection" error
+		// from the post-shutdown drain.
+		if !strings.Contains(err.Error(), "synthetic write failure") {
+			t.Errorf("got %q, want error containing %q", err, "synthetic write failure")
+		}
+	case <-time.After(2 * time.Second):
+		t.Fatal("RemoveAll hung after write failure")
+	}
+
+	// The pipeline must have closed the underlying conn, so the peer
+	// goroutine returns from its Read.
+	select {
+	case <-serverExit:
+	case <-time.After(2 * time.Second):
+		t.Fatal("connection was not closed after write failure")
+	}
+
+	// After shutdown, subsequent calls must also fail promptly.
+	errCh2 := make(chan error, 1)
+	go func() { errCh2 <- client.RemoveAll() }()
+	select {
+	case err := <-errCh2:
+		if err == nil {
+			t.Error("expected error on post-shutdown call, got nil")
+		}
+	case <-time.After(2 * time.Second):
+		t.Fatal("post-shutdown call hung")
+	}
+}
+
+func TestPipelineResponseTooLarge(t *testing.T) {
+	c1, c2, err := netPipe()
+	if err != nil {
+		t.Fatalf("netPipe: %v", err)
+	}
+	defer c1.Close()
+	defer c2.Close()
+
+	// Server: read the request, then reply with a size header that
+	// exceeds maxAgentResponseBytes. The size check fires before the
+	// body is read, so no body needs to follow.
+	serverDone := make(chan struct{})
+	go func() {
+		defer close(serverDone)
+		var sizeBuf [4]byte
+		if _, err := io.ReadFull(c2, sizeBuf[:]); err != nil {
+			return
+		}
+		size := binary.BigEndian.Uint32(sizeBuf[:])
+		if _, err := io.CopyN(io.Discard, c2, int64(size)); err != nil {
+			return
+		}
+		var out [4]byte
+		binary.BigEndian.PutUint32(out[:], maxAgentResponseBytes+1)
+		c2.Write(out[:])
+	}()
+
+	client := NewClient(c1)
+
+	errCh := make(chan error, 1)
+	go func() { errCh <- client.RemoveAll() }()
+	select {
+	case err := <-errCh:
+		if err == nil {
+			t.Fatal("expected error, got nil")
+		}
+		if !strings.Contains(err.Error(), "response too large") {
+			t.Errorf("got %q, want error containing %q", err, "response too large")
+		}
+	case <-time.After(2 * time.Second):
+		t.Fatal("call hung")
+	}
+
+	// Subsequent calls must also fail: the pipeline is shut down.
+	errCh2 := make(chan error, 1)
+	go func() { errCh2 <- client.RemoveAll() }()
+	select {
+	case err := <-errCh2:
+		if err == nil {
+			t.Error("expected error on post-shutdown call, got nil")
+		}
+	case <-time.After(2 * time.Second):
+		t.Fatal("post-shutdown call hung")
+	}
+
+	<-serverDone
+}
+
+func TestPipelineShutdownWithBlockedWriter(t *testing.T) {
+	c1, c2, err := netPipe()
+	if err != nil {
+		t.Fatalf("netPipe: %v", err)
+	}
+	defer c1.Close()
+
+	// Server drains requests off the wire so writes never block on
+	// backpressure from the transport, but never replies. The reader
+	// goroutine therefore stays parked on Read, never dequeues from
+	// pending, and the queue fills up.
+	var reqsRead int64
+	serverExit := make(chan struct{})
+	go func() {
+		defer close(serverExit)
+		for {
+			var sizeBuf [4]byte
+			if _, err := io.ReadFull(c2, sizeBuf[:]); err != nil {
+				return
+			}
+			size := binary.BigEndian.Uint32(sizeBuf[:])
+			if _, err := io.CopyN(io.Discard, c2, int64(size)); err != nil {
+				return
+			}
+			atomic.AddInt64(&reqsRead, 1)
+		}
+	}()
+
+	client := NewClient(c1)
+
+	const total = pipelineMaxInFlight + 4
+	var wg sync.WaitGroup
+	errs := make(chan error, total)
+	for range total {
+		wg.Go(func() {
+			errs <- client.RemoveAll()
+		})
+	}
+
+	// Wait for the queue to fill: once pipelineMaxInFlight requests
+	// are on the wire, pending is at capacity and the +1 writer is
+	// blocked in the second-select arm; the rest are queued behind
+	// writeMu.
+	deadline := time.Now().Add(2 * time.Second)
+	for time.Now().Before(deadline) {
+		if atomic.LoadInt64(&reqsRead) >= pipelineMaxInFlight {
+			break
+		}
+		time.Sleep(5 * time.Millisecond)
+	}
+	if got := atomic.LoadInt64(&reqsRead); got < pipelineMaxInFlight {
+		t.Fatalf("requests on wire: got %d, want >= %d", got, pipelineMaxInFlight)
+	}
+	// Give the +1 writer a moment to actually park in the second
+	// select before we trigger shutdown.
+	time.Sleep(50 * time.Millisecond)
+
+	// Trigger shutdown: closing the server side makes the client's
+	// reader hit EOF, which calls shutdown(...).
+	c2.Close()
+
+	doneCh := make(chan struct{})
+	go func() {
+		wg.Wait()
+		close(doneCh)
+	}()
+	select {
+	case <-doneCh:
+	case <-time.After(5 * time.Second):
+		t.Fatal("blocked callers did not complete after shutdown")
+	}
+
+	close(errs)
+	for err := range errs {
+		if err == nil {
+			t.Error("expected error after shutdown, got nil")
+		}
+	}
+
+	<-serverExit
+}
+
+// multiDeviceAgent simulates an ssh-agent backed by numDevices signing
+// devices, each of which takes latency to process a request. Incoming
+// requests are round-robin dispatched to devices, and responses are
+// serialized back to the wire in request order as the protocol requires.
+//
+// The simulation does not emit a semantically meaningful reply; it just
+// writes a 1-byte SSH_AGENT_SUCCESS after the simulated latency has
+// elapsed. This is enough to exercise callRaw/simpleCall (RemoveAll).
+func multiDeviceAgent(t testing.TB, conn io.ReadWriteCloser, numDevices int, latency time.Duration) (done <-chan struct{}) {
+	t.Helper()
+
+	type job struct {
+		ready chan struct{}
+	}
+
+	devices := make([]chan job, numDevices)
+	var devicesWG sync.WaitGroup
+	for i := range numDevices {
+		devices[i] = make(chan job, 1024)
+		ch := devices[i]
+		devicesWG.Go(func() {
+			for j := range ch {
+				time.Sleep(latency)
+				close(j.ready)
+			}
+		})
+	}
+
+	// Order-preserving responder: responses are written in the same
+	// order the requests were read, regardless of which device finished
+	// first.
+	orderCh := make(chan chan struct{}, 1024)
+	responderDone := make(chan struct{})
+	go func() {
+		defer close(responderDone)
+		for ready := range orderCh {
+			<-ready
+			var out [5]byte
+			binary.BigEndian.PutUint32(out[:4], 1)
+			out[4] = agentSuccess
+			if _, err := conn.Write(out[:]); err != nil {
+				return
+			}
+		}
+	}()
+
+	finished := make(chan struct{})
+	go func() {
+		defer close(finished)
+		defer func() {
+			close(orderCh)
+			<-responderDone
+			for _, d := range devices {
+				close(d)
+			}
+			devicesWG.Wait()
+		}()
+		next := 0
+		for {
+			var sizeBuf [4]byte
+			if _, err := io.ReadFull(conn, sizeBuf[:]); err != nil {
+				return
+			}
+			size := binary.BigEndian.Uint32(sizeBuf[:])
+			if _, err := io.CopyN(io.Discard, conn, int64(size)); err != nil {
+				return
+			}
+			ready := make(chan struct{})
+			devices[next%numDevices] <- job{ready: ready}
+			next++
+			orderCh <- ready
+		}
+	}()
+
+	return finished
+}
+
+// BenchmarkPipelineMultiDevice measures throughput of the pipelined
+// agent client against a simulated multi-device agent, varying the
+// number of backend devices.
+//
+// Setup: each simulated signing device takes 2ms per request. The
+// protocol serializes responses in request order, but pipelining lets
+// the client keep multiple devices busy concurrently; throughput
+// should scale roughly linearly with numDevices up to the pipeline
+// depth (pipelineMaxInFlight).
+//
+// Run with: go test -benchmem -bench BenchmarkPipelineMultiDevice -benchtime=2s ./ssh/agent/
+func BenchmarkPipelineMultiDevice(b *testing.B) {
+	const (
+		deviceLatency = 2 * time.Millisecond
+		parallelism   = 32
+	)
+
+	for _, numDevices := range []int{1, 2, 4, 8, 16} {
+		b.Run(fmt.Sprintf("Devices=%d", numDevices), func(b *testing.B) {
+			c1, c2, err := netPipe()
+			if err != nil {
+				b.Fatalf("netPipe: %v", err)
+			}
+			serverDone := multiDeviceAgent(b, c2, numDevices, deviceLatency)
+
+			client := NewClient(c1)
+
+			b.ResetTimer()
+			b.SetParallelism(parallelism)
+			b.RunParallel(func(pb *testing.PB) {
+				for pb.Next() {
+					if err := client.RemoveAll(); err != nil {
+						b.Fatal(err)
+					}
+				}
+			})
+			b.StopTimer()
+
+			c1.Close()
+			c2.Close()
+			<-serverDone
+		})
+	}
+}

ssh/agent/client_test.go

@@ -6,6 +6,7 @@ package agent
 
 import (
 	"bytes"
+	"crypto/ed25519"
 	"crypto/rand"
 	"encoding/binary"
 	"errors"
@@ -654,3 +655,36 @@ func TestAddConstraintExtensionsWireFormat(t *testing.T) {
 		}
 	}
 }
+
+func TestAddRejectsShortEd25519Key(t *testing.T) {
+	short := ed25519.PrivateKey(make([]byte, 16))
+	// insertCert reaches its ed25519 branch only when AddedKey.Certificate
+	// is non-nil. The length check returns before cert.Type/Marshal run, so
+	// an empty certificate is sufficient to drive the path.
+	emptyCert := &ssh.Certificate{}
+
+	cases := []struct {
+		name string
+		add  AddedKey
+	}{
+		{"insertKey value", AddedKey{PrivateKey: short}},
+		{"insertKey pointer", AddedKey{PrivateKey: &short}},
+		{"insertCert value", AddedKey{PrivateKey: short, Certificate: emptyCert}},
+		{"insertCert pointer", AddedKey{PrivateKey: &short, Certificate: emptyCert}},
+	}
+
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			client, cleanup := startKeyringAgent(t)
+			defer cleanup()
+
+			err := client.Add(tc.add)
+			if err == nil {
+				t.Fatal("Add accepted ed25519 key shorter than 64 bytes")
+			}
+			if !strings.Contains(err.Error(), "bad ED25519 key size") {
+				t.Errorf("got error %q, want substring %q", err.Error(), "bad ED25519 key size")
+			}
+		})
+	}
+}

ssh/agent/server.go

@@ -240,13 +240,35 @@ func setConstraints(key *AddedKey, constraintBytes []byte) error {
 	return nil
 }
 
+// checkRSAKeyParams enforces the same bounds as parseRSA in the ssh
+// package, and additionally caps the prime factors. Without this,
+// the rsa.PrivateKey built from an Add request would call Precompute()
+// on arbitrary inputs; the CRT coefficient recomputation is cubic in
+// |p| and can consume excessive CPU on oversized keys.
+func checkRSAKeyParams(N, E, P, Q *big.Int) error {
+	if N.BitLen() > 8192 {
+		return errors.New("agent: RSA modulus too large")
+	}
+	if P.BitLen() > 4096 || Q.BitLen() > 4096 {
+		return errors.New("agent: RSA prime too large")
+	}
+	if E.BitLen() > 24 {
+		return errors.New("agent: RSA public exponent too large")
+	}
+	e := E.Int64()
+	if e < 3 || e&1 == 0 {
+		return errors.New("agent: incorrect RSA public exponent")
+	}
+	return nil
+}
+
 func parseRSAKey(req []byte) (*AddedKey, error) {
 	var k rsaKeyMsg
 	if err := ssh.Unmarshal(req, &k); err != nil {
 		return nil, err
 	}
-	if k.E.BitLen() > 30 {
-		return nil, errors.New("agent: RSA public exponent too large")
+	if err := checkRSAKeyParams(k.N, k.E, k.P, k.Q); err != nil {
+		return nil, err
 	}
 	priv := &rsa.PrivateKey{
 		PublicKey: rsa.PublicKey{
@@ -399,8 +421,8 @@ func parseRSACert(req []byte) (*AddedKey, error) {
 		return nil, fmt.Errorf("agent: Unmarshal failed to parse public key: %v", err)
 	}
 
-	if rsaPub.E.BitLen() > 30 {
-		return nil, errors.New("agent: RSA public exponent too large")
+	if err := checkRSAKeyParams(rsaPub.N, rsaPub.E, k.P, k.Q); err != nil {
+		return nil, err
 	}
 
 	priv := rsa.PrivateKey{

ssh/channel.go

@@ -634,7 +634,10 @@ func (ch *channel) SendRequest(name string, wantReply bool, payload []byte) (boo
 	drain:
 		for {
 			select {
-			case <-ch.msg:
+			case _, ok := <-ch.msg:
+				if !ok {
+					break drain
+				}
 			default:
 				break drain
 			}

ssh/client.go

@@ -88,6 +88,32 @@ func NewClientConn(c net.Conn, addr string, config *ClientConfig) (Conn, <-chan
 	return conn, conn.mux.incomingChannels, conn.mux.incomingRequests, nil
 }
 
+// NewControlClientConn establishes an SSH connection over an OpenSSH
+// ControlMaster socket c in proxy mode.
+//
+// Note that this package only implements the client side of the multiplexing
+// protocol. The provided net.Conn must be a local, secure connection (such as a
+// Unix domain socket) connected to an already-running OpenSSH process acting as
+// the ControlMaster.
+//
+// WARNING: Because proxy mode bypasses the standard cryptographic handshake
+// passing a standard network connection (e.g., TCP) will result in plaintext
+// data leakage.
+//
+// The Request and NewChannel channels must be serviced or the connection
+// will hang.
+func NewControlClientConn(c net.Conn) (Conn, <-chan NewChannel, <-chan *Request, error) {
+	conn := &connection{
+		sshConn: sshConn{conn: c},
+	}
+	var err error
+	if conn.transport, err = handshakeControlProxy(c); err != nil {
+		return nil, nil, nil, fmt.Errorf("ssh: control proxy handshake failed: %w", err)
+	}
+	conn.mux = newMux(conn.transport)
+	return conn, conn.mux.incomingChannels, conn.mux.incomingRequests, nil
+}
+
 // clientHandshake performs the client side key exchange. See RFC 4253 Section
 // 7.
 func (c *connection) clientHandshake(dialAddress string, config *ClientConfig) error {
@@ -197,6 +223,59 @@ type HostKeyCallback func(hostname string, remote net.Addr, key PublicKey) error
 // the server. A BannerCallback receives the message sent by the remote server.
 type BannerCallback func(message string) error
 
+// ClientAuthContext contains information about the current state of the
+// authentication process, passed to [ClientAuthCallback].
+type ClientAuthContext struct {
+	// Metadata contains the connection metadata.
+	Metadata ConnMetadata
+
+	// Algorithms contains the negotiated algorithms.
+	Algorithms NegotiatedAlgorithms
+
+	// AllowedMethods lists the authentication methods currently accepted
+	// by the server. These are the protocol-level names defined in RFC 4252
+	// such as "publickey", "password".
+	AllowedMethods []string
+
+	// PartialSuccessMethods lists the authentication methods that have already
+	// succeeded, indicating a multi-step authentication flow. This list
+	// represents the exact sequence of partial successes and may contain
+	// duplicates if the same method succeeded multiple times.
+	PartialSuccessMethods []string
+
+	// TriedMethods lists the methods that have already been attempted and
+	// failed during this session. This list represents the exact sequence of
+	// failures and may contain duplicates. This allows the callback to also
+	// track the number of failed attempts for a specific method.
+	TriedMethods []string
+}
+
+// ClientAuthCallback is a hook invoked before each authentication attempt. It
+// allows the client to dynamically select an authentication method based on the
+// current context, server capabilities, or previous failures.
+//
+// The callback is invoked after the initial "none" authentication method, once
+// the server's supported authentication methods are known.
+//
+// Return values:
+//   - (AuthMethod, nil): The client will attempt this specific method next.
+//     The returned method does NOT need to be present in [ClientConfig.Auth].
+//     This allows for dynamic authentication strategies (e.g., prompting
+//     for a password only if public key auth fails). Callers should inspect
+//     [ClientAuthContext.TriedMethods] to avoid repeatedly returning the
+//     same failing method.
+//   - (nil, nil): The client selects from [ClientConfig.Auth] the first
+//     instance of a method that has not been tried yet, or aborts if none
+//     are left. If authentication is not successful, the callback is invoked
+//     again before the following attempt.
+//   - (nil, error): The authentication process is aborted immediately,
+//     causing the ongoing SSH handshake to fail with the provided error.
+//
+// To bound resource use, the client caps the total number of authentication
+// attempts (failures and partial successes combined) at 64. If the cap is
+// exceeded the handshake aborts with an error.
+type ClientAuthCallback func(ctx *ClientAuthContext) (AuthMethod, error)
+
 // A ClientConfig structure is used to configure a Client. It must not be
 // modified after having been passed to an SSH function.
 type ClientConfig struct {
@@ -210,6 +289,9 @@ type ClientConfig struct {
 	// Auth contains possible authentication methods to use with the
 	// server. Only the first instance of a particular RFC 4252 method will
 	// be used during authentication.
+	//
+	// If AuthCallback is set, these AuthMethod are only used if the
+	// callback returns nil.
 	Auth []AuthMethod
 
 	// HostKeyCallback is called during the cryptographic
@@ -240,6 +322,9 @@ type ClientConfig struct {
 	//
 	// A Timeout of zero means no timeout.
 	Timeout time.Duration
+
+	// AuthCallback, if non-nil, is invoked before each authentication attempt.
+	AuthCallback ClientAuthCallback
 }
 
 // InsecureIgnoreHostKey returns a function that can be used for

ssh/client_auth.go

@@ -21,6 +21,12 @@ const (
 	authSuccess
 )
 
+// maxAuthClientTried bounds the total number of authentication attempts
+// (failures and partial successes combined) the client makes before
+// aborting the loop, to prevent unbounded growth when an AuthCallback
+// keeps supplying methods.
+const maxAuthClientTried = 64
+
 // clientAuthenticate authenticates with the remote server. See RFC 4252.
 func (c *connection) clientAuthenticate(config *ClientConfig) error {
 	// initiate user auth session
@@ -67,32 +73,62 @@ func (c *connection) clientAuthenticate(config *ClientConfig) error {
 	// then any untried methods suggested by the server.
 	var tried []string
 	var lastMethods []string
+	var partialSuccess []string
 
 	sessionID := c.transport.getSessionID()
 	for auth := AuthMethod(new(noneAuth)); auth != nil; {
 		ok, methods, err := auth.auth(sessionID, config.User, c.transport, config.Rand, extensions)
 		if err != nil {
 			// On disconnect, return error immediately
-			if _, ok := err.(*disconnectMsg); ok {
+			if _, isDisconnect := err.(*disconnectMsg); isDisconnect {
 				return err
 			}
-			// We return the error later if there is no other method left to
-			// try.
+			// We return the error later if there is no other method
+			// left to try.
 			ok = authFailure
 		}
-		if ok == authSuccess {
-			// success
+
+		switch ok {
+		case authSuccess:
 			return nil
-		} else if ok == authFailure {
-			if m := auth.method(); !slices.Contains(tried, m) {
-				tried = append(tried, m)
-			}
+		case authPartialSuccess:
+			partialSuccess = append(partialSuccess, auth.method())
+		case authFailure:
+			tried = append(tried, auth.method())
 		}
+		if len(partialSuccess)+len(tried) > maxAuthClientTried {
+			return fmt.Errorf("ssh: too many authentication attempts (%d), aborting",
+				len(partialSuccess)+len(tried))
+		}
+
 		if methods == nil {
 			methods = lastMethods
 		}
 		lastMethods = methods
 
+		// If AuthCallback is set it takes precedence: it picks the next
+		// AuthMethod dynamically. The returned method need not be in
+		// config.Auth. If the callback returns (nil, nil) we fall back to
+		// selecting the next untried method from config.Auth below; on
+		// (nil, error) the handshake aborts.
+		if config.AuthCallback != nil {
+			ctx := &ClientAuthContext{
+				Metadata:              c,
+				Algorithms:            c.Algorithms(),
+				AllowedMethods:        slices.Clone(methods),
+				PartialSuccessMethods: slices.Clone(partialSuccess),
+				TriedMethods:          slices.Clone(tried),
+			}
+			altAuth, cbErr := config.AuthCallback(ctx)
+			if cbErr != nil {
+				return cbErr
+			}
+			if altAuth != nil {
+				auth = altAuth
+				continue
+			}
+		}
+
 		auth = nil
 
 	findNext:
@@ -377,11 +413,11 @@ func (cb publicKeyCallback) auth(session []byte, user string, c packetConn, rand
 			return authFailure, nil, err
 		}
 
-		// If authentication succeeds or the list of available methods does not
-		// contain the "publickey" method, do not attempt to authenticate with any
-		// other keys.  According to RFC 4252 Section 7, the latter can occur when
-		// additional authentication methods are required.
-		if success == authSuccess || !slices.Contains(methods, cb.method()) {
+		// If authentication succeeds or partially succeeds, return immediately
+		// so the caller can select the next auth method. According to RFC 4252
+		// Section 7, if the server no longer lists "publickey" among its
+		// allowed methods, do not attempt to authenticate with any other keys.
+		if success == authSuccess || success == authPartialSuccess || !slices.Contains(methods, cb.method()) {
 			return success, methods, err
 		}
 	}

ssh/client_auth_test.go

@@ -169,6 +169,689 @@ func TestClientAuthThirdKey(t *testing.T) {
 	}
 }
 
+// partialSuccessPublicKeyAndKbdInteractiveServer returns a server config
+// where the rsa key gets partial success and the server offers both publickey
+// and keyboard-interactive as next methods. The VerifiedPublicKeyCallback
+// tracks state so that the same key only triggers partial success once,
+// avoiding an impossible auth loop.
+func partialSuccessPublicKeyAndKbdInteractiveServer() *ServerConfig {
+	rsaDone := false
+	serverConf := &ServerConfig{
+		PublicKeyCallback: func(conn ConnMetadata, key PublicKey) (*Permissions, error) {
+			if bytes.Equal(key.Marshal(), testPublicKeys["rsa"].Marshal()) {
+				return nil, nil
+			}
+			return nil, errors.New("invalid credentials")
+		},
+		VerifiedPublicKeyCallback: func(conn ConnMetadata, key PublicKey, permissions *Permissions, signatureAlgorithm string) (*Permissions, error) {
+			if !rsaDone && bytes.Equal(key.Marshal(), testPublicKeys["rsa"].Marshal()) {
+				rsaDone = true
+				return nil, &PartialSuccessError{
+					Next: ServerAuthCallbacks{
+						PublicKeyCallback: func(conn ConnMetadata, key PublicKey) (*Permissions, error) {
+							if bytes.Equal(key.Marshal(), testPublicKeys["rsa"].Marshal()) {
+								return nil, nil
+							}
+							return nil, errors.New("invalid credentials")
+						},
+						KeyboardInteractiveCallback: func(conn ConnMetadata, client KeyboardInteractiveChallenge) (*Permissions, error) {
+							answers, err := client("", "", []string{"token: "}, []bool{true})
+							if err != nil {
+								return nil, err
+							}
+							if len(answers) == 1 && answers[0] == "correct-token" {
+								return nil, nil
+							}
+							return nil, errors.New("invalid token")
+						},
+					},
+				}
+			}
+			return nil, errors.New("invalid credentials")
+		},
+	}
+	serverConf.AddHostKey(testSigners["rsa"])
+	return serverConf
+}
+
+// partialSuccessMultiKeyServer returns a server config that requires two
+// public keys in sequence: the first key (rsa) triggers partial success
+// requiring a second key (ecdsa) to complete authentication.
+func partialSuccessMultiKeyServer() *ServerConfig {
+	firstKeyDone := false
+	serverConf := &ServerConfig{
+		PublicKeyCallback: func(conn ConnMetadata, key PublicKey) (*Permissions, error) {
+			if bytes.Equal(key.Marshal(), testPublicKeys["rsa"].Marshal()) {
+				return nil, nil
+			}
+			if bytes.Equal(key.Marshal(), testPublicKeys["ecdsa"].Marshal()) {
+				return nil, nil
+			}
+			return nil, errors.New("invalid credentials")
+		},
+		VerifiedPublicKeyCallback: func(conn ConnMetadata, key PublicKey, permissions *Permissions, signatureAlgorithm string) (*Permissions, error) {
+			if !firstKeyDone && bytes.Equal(key.Marshal(), testPublicKeys["rsa"].Marshal()) {
+				firstKeyDone = true
+				return nil, &PartialSuccessError{
+					Next: ServerAuthCallbacks{
+						PublicKeyCallback: func(conn ConnMetadata, key PublicKey) (*Permissions, error) {
+							if bytes.Equal(key.Marshal(), testPublicKeys["ecdsa"].Marshal()) {
+								return nil, nil
+							}
+							return nil, errors.New("invalid credentials")
+						},
+					},
+				}
+			}
+			if firstKeyDone && bytes.Equal(key.Marshal(), testPublicKeys["ecdsa"].Marshal()) {
+				return nil, nil
+			}
+			return nil, errors.New("invalid credentials")
+		},
+	}
+	serverConf.AddHostKey(testSigners["rsa"])
+	return serverConf
+}
+
+func TestStaticMultiKeyPartialSuccess(t *testing.T) {
+	c1, c2, err := netPipe()
+	if err != nil {
+		t.Fatalf("netPipe: %v", err)
+	}
+	defer c1.Close()
+	defer c2.Close()
+
+	clientConf := &ClientConfig{
+		User: "user",
+		Auth: []AuthMethod{
+			PublicKeys(testSigners["rsa"], testSigners["ecdsa"]),
+		},
+		HostKeyCallback: InsecureIgnoreHostKey(),
+	}
+
+	go NewServerConn(c1, partialSuccessMultiKeyServer())
+
+	_, _, _, err = NewClientConn(c2, "", clientConf)
+	if err != nil {
+		t.Fatalf("client login error: %s", err)
+	}
+}
+
+func TestStaticPartialSuccess(t *testing.T) {
+	c1, c2, err := netPipe()
+	if err != nil {
+		t.Fatalf("netPipe: %v", err)
+	}
+	defer c1.Close()
+	defer c2.Close()
+
+	clientConf := &ClientConfig{
+		User: "user",
+		Auth: []AuthMethod{
+			PublicKeys(testSigners["rsa"]),
+			KeyboardInteractive(func(name, instruction string, questions []string, echos []bool) ([]string, error) {
+				return []string{"correct-token"}, nil
+			}),
+		},
+		HostKeyCallback: InsecureIgnoreHostKey(),
+	}
+
+	go NewServerConn(c1, partialSuccessPublicKeyAndKbdInteractiveServer())
+
+	_, _, _, err = NewClientConn(c2, "", clientConf)
+	if err != nil {
+		t.Fatalf("client login error: %s", err)
+	}
+}
+
+// partialSuccessKeyAThenKeyBServer returns a server config where keyA (rsa)
+// gets partial success and the server offers BOTH publickey and
+// keyboard-interactive. keyB (ecdsa) then completes auth successfully.
+func partialSuccessKeyAThenKeyBServer() *ServerConfig {
+	keyADone := false
+	serverConf := &ServerConfig{
+		PublicKeyCallback: func(conn ConnMetadata, key PublicKey) (*Permissions, error) {
+			if bytes.Equal(key.Marshal(), testPublicKeys["rsa"].Marshal()) {
+				return nil, nil
+			}
+			if bytes.Equal(key.Marshal(), testPublicKeys["ecdsa"].Marshal()) {
+				return nil, nil
+			}
+			return nil, errors.New("invalid credentials")
+		},
+		VerifiedPublicKeyCallback: func(conn ConnMetadata, key PublicKey, permissions *Permissions, signatureAlgorithm string) (*Permissions, error) {
+			if !keyADone && bytes.Equal(key.Marshal(), testPublicKeys["rsa"].Marshal()) {
+				keyADone = true
+				return nil, &PartialSuccessError{
+					Next: ServerAuthCallbacks{
+						PublicKeyCallback: func(conn ConnMetadata, key PublicKey) (*Permissions, error) {
+							if bytes.Equal(key.Marshal(), testPublicKeys["ecdsa"].Marshal()) {
+								return nil, nil
+							}
+							return nil, errors.New("invalid credentials")
+						},
+						KeyboardInteractiveCallback: func(conn ConnMetadata, client KeyboardInteractiveChallenge) (*Permissions, error) {
+							answers, err := client("", "", []string{"token: "}, []bool{true})
+							if err != nil {
+								return nil, err
+							}
+							if len(answers) == 1 && answers[0] == "correct-token" {
+								return nil, nil
+							}
+							return nil, errors.New("invalid token")
+						},
+					},
+				}
+			}
+			if keyADone && bytes.Equal(key.Marshal(), testPublicKeys["ecdsa"].Marshal()) {
+				return nil, nil
+			}
+			return nil, errors.New("invalid credentials")
+		},
+	}
+	serverConf.AddHostKey(testSigners["rsa"])
+	return serverConf
+}
+
+func TestStaticPartialSuccessKeyAThenKeyB(t *testing.T) {
+	c1, c2, err := netPipe()
+	if err != nil {
+		t.Fatalf("netPipe: %v", err)
+	}
+	defer c1.Close()
+	defer c2.Close()
+
+	clientConf := &ClientConfig{
+		User: "user",
+		Auth: []AuthMethod{
+			PublicKeys(testSigners["rsa"], testSigners["ecdsa"]),
+		},
+		HostKeyCallback: InsecureIgnoreHostKey(),
+	}
+
+	go NewServerConn(c1, partialSuccessKeyAThenKeyBServer())
+
+	_, _, _, err = NewClientConn(c2, "", clientConf)
+	if err != nil {
+		t.Fatalf("client login error: %s", err)
+	}
+}
+
+// partialSuccessMultiKeyThenKbdInteractiveServer returns a server config
+// that strictly requires keyA (rsa) AND keyB (ecdsa) as public keys before
+// allowing keyboard-interactive. After keyA, ONLY publickey is offered
+// (for keyB). After keyB, ONLY keyboard-interactive is offered.
+func partialSuccessMultiKeyThenKbdInteractiveServer() *ServerConfig {
+	serverConf := &ServerConfig{
+		PublicKeyCallback: func(conn ConnMetadata, key PublicKey) (*Permissions, error) {
+			if bytes.Equal(key.Marshal(), testPublicKeys["rsa"].Marshal()) {
+				return nil, &PartialSuccessError{
+					Next: ServerAuthCallbacks{
+						PublicKeyCallback: func(conn ConnMetadata, key PublicKey) (*Permissions, error) {
+							if bytes.Equal(key.Marshal(), testPublicKeys["ecdsa"].Marshal()) {
+								return nil, &PartialSuccessError{
+									Next: ServerAuthCallbacks{
+										KeyboardInteractiveCallback: func(conn ConnMetadata, client KeyboardInteractiveChallenge) (*Permissions, error) {
+											answers, err := client("", "", []string{"token: "}, []bool{true})
+											if err != nil {
+												return nil, err
+											}
+											if len(answers) == 1 && answers[0] == "correct-token" {
+												return nil, nil
+											}
+											return nil, errors.New("invalid token")
+										},
+									},
+								}
+							}
+							return nil, errors.New("invalid credentials")
+						},
+					},
+				}
+			}
+			return nil, errors.New("invalid credentials")
+		},
+	}
+	serverConf.AddHostKey(testSigners["rsa"])
+	return serverConf
+}
+
+func TestStaticMultiKeyPartialThenKbdInteractive(t *testing.T) {
+	c1, c2, err := netPipe()
+	if err != nil {
+		t.Fatalf("netPipe: %v", err)
+	}
+	defer c1.Close()
+	defer c2.Close()
+
+	clientConf := &ClientConfig{
+		User: "user",
+		Auth: []AuthMethod{
+			PublicKeys(testSigners["rsa"], testSigners["ecdsa"]),
+			KeyboardInteractive(func(name, instruction string, questions []string, echos []bool) ([]string, error) {
+				return []string{"correct-token"}, nil
+			}),
+		},
+		HostKeyCallback: InsecureIgnoreHostKey(),
+	}
+
+	go NewServerConn(c1, partialSuccessMultiKeyThenKbdInteractiveServer())
+
+	_, _, _, err = NewClientConn(c2, "", clientConf)
+	if err != nil {
+		t.Fatalf("client login error: %s", err)
+	}
+}
+
+func TestAuthMethodDynamic(t *testing.T) {
+	config := &ClientConfig{
+		User:            "testuser",
+		Auth:            nil,
+		HostKeyCallback: InsecureIgnoreHostKey(),
+		AuthCallback: func(ctx *ClientAuthContext) (AuthMethod, error) {
+			if len(ctx.Metadata.SessionID()) == 0 {
+				t.Error("empty sessionID")
+			}
+			if ctx.Algorithms.HostKey == "" {
+				t.Error("empty Host Key")
+			}
+			if slices.Contains(ctx.AllowedMethods, "password") {
+				return Password(clientPassword), nil
+			}
+			return nil, errors.New("auth callback error")
+		},
+	}
+
+	if err := tryAuth(t, config); err != nil {
+		t.Fatalf("unable to dial remote side: %s", err)
+	}
+}
+
+func TestAuthCallbackError(t *testing.T) {
+	config := &ClientConfig{
+		User: "testuser",
+		Auth: []AuthMethod{
+			Password(clientPassword),
+		},
+		HostKeyCallback: InsecureIgnoreHostKey(),
+		AuthCallback: func(ctx *ClientAuthContext) (AuthMethod, error) {
+			return nil, errors.New("auth callback error")
+		},
+	}
+
+	if err := tryAuth(t, config); err == nil {
+		t.Fatal("authentication unexpectedly succeeded after AuthCallback returned an error")
+	}
+}
+
+func TestAuthCallbackPartialSuccess(t *testing.T) {
+	c1, c2, err := netPipe()
+	if err != nil {
+		t.Fatalf("netPipe: %v", err)
+	}
+	defer c1.Close()
+	defer c2.Close()
+
+	clientConf := &ClientConfig{
+		User:            "user",
+		HostKeyCallback: InsecureIgnoreHostKey(),
+		AuthCallback: func(ctx *ClientAuthContext) (AuthMethod, error) {
+			if slices.Contains(ctx.AllowedMethods, "publickey") && !slices.Contains(ctx.PartialSuccessMethods, "publickey") {
+				return PublicKeys(testSigners["rsa"]), nil
+			}
+			if slices.Contains(ctx.PartialSuccessMethods, "publickey") && slices.Contains(ctx.AllowedMethods, "keyboard-interactive") {
+				return KeyboardInteractive(func(name, instruction string, questions []string, echos []bool) ([]string, error) {
+					return []string{"correct-token"}, nil
+				}), nil
+			}
+			return nil, fmt.Errorf("unexpected auth state: allowed=%v partial=%v", ctx.AllowedMethods, ctx.PartialSuccessMethods)
+		},
+	}
+
+	go NewServerConn(c1, partialSuccessPublicKeyAndKbdInteractiveServer())
+
+	_, _, _, err = NewClientConn(c2, "", clientConf)
+	if err != nil {
+		t.Fatalf("client login error: %s", err)
+	}
+}
+
+func TestAuthCallbackMultiKeyPartialSuccess(t *testing.T) {
+	c1, c2, err := netPipe()
+	if err != nil {
+		t.Fatalf("netPipe: %v", err)
+	}
+	defer c1.Close()
+	defer c2.Close()
+
+	clientConf := &ClientConfig{
+		User:            "user",
+		HostKeyCallback: InsecureIgnoreHostKey(),
+		AuthCallback: func(ctx *ClientAuthContext) (AuthMethod, error) {
+			if slices.Contains(ctx.AllowedMethods, "publickey") && !slices.Contains(ctx.PartialSuccessMethods, "publickey") {
+				return PublicKeys(testSigners["rsa"]), nil
+			}
+			if slices.Contains(ctx.PartialSuccessMethods, "publickey") && slices.Contains(ctx.AllowedMethods, "publickey") {
+				return PublicKeys(testSigners["ecdsa"]), nil
+			}
+			return nil, fmt.Errorf("unexpected auth state: allowed=%v partial=%v", ctx.AllowedMethods, ctx.PartialSuccessMethods)
+		},
+	}
+
+	go NewServerConn(c1, partialSuccessMultiKeyServer())
+
+	_, _, _, err = NewClientConn(c2, "", clientConf)
+	if err != nil {
+		t.Fatalf("client login error: %s", err)
+	}
+}
+
+func TestAuthCallbackMultiKeyPartialThenKbdInteractive(t *testing.T) {
+	c1, c2, err := netPipe()
+	if err != nil {
+		t.Fatalf("netPipe: %v", err)
+	}
+	defer c1.Close()
+	defer c2.Close()
+
+	clientConf := &ClientConfig{
+		User:            "user",
+		HostKeyCallback: InsecureIgnoreHostKey(),
+		AuthCallback: func(ctx *ClientAuthContext) (AuthMethod, error) {
+			partialCount := len(ctx.PartialSuccessMethods)
+			if partialCount == 0 && slices.Contains(ctx.AllowedMethods, "publickey") {
+				return PublicKeys(testSigners["rsa"]), nil
+			}
+			if partialCount == 1 && slices.Contains(ctx.AllowedMethods, "publickey") {
+				return PublicKeys(testSigners["ecdsa"]), nil
+			}
+			if partialCount >= 2 && slices.Contains(ctx.AllowedMethods, "keyboard-interactive") {
+				return KeyboardInteractive(func(name, instruction string, questions []string, echos []bool) ([]string, error) {
+					return []string{"correct-token"}, nil
+				}), nil
+			}
+			return nil, fmt.Errorf("unexpected auth state: allowed=%v partial=%v tried=%v",
+				ctx.AllowedMethods, ctx.PartialSuccessMethods, ctx.TriedMethods)
+		},
+	}
+
+	go NewServerConn(c1, partialSuccessMultiKeyThenKbdInteractiveServer())
+
+	_, _, _, err = NewClientConn(c2, "", clientConf)
+	if err != nil {
+		t.Fatalf("client login error: %s", err)
+	}
+}
+
+func TestAuthCallbackPartialSuccessKeyAThenKeyB(t *testing.T) {
+	c1, c2, err := netPipe()
+	if err != nil {
+		t.Fatalf("netPipe: %v", err)
+	}
+	defer c1.Close()
+	defer c2.Close()
+
+	clientConf := &ClientConfig{
+		User:            "user",
+		HostKeyCallback: InsecureIgnoreHostKey(),
+		AuthCallback: func(ctx *ClientAuthContext) (AuthMethod, error) {
+			if len(ctx.PartialSuccessMethods) == 0 && slices.Contains(ctx.AllowedMethods, "publickey") {
+				return PublicKeys(testSigners["rsa"]), nil
+			}
+			if len(ctx.PartialSuccessMethods) == 1 && slices.Contains(ctx.AllowedMethods, "publickey") {
+				return PublicKeys(testSigners["ecdsa"]), nil
+			}
+			return nil, fmt.Errorf("unexpected auth state: allowed=%v partial=%v",
+				ctx.AllowedMethods, ctx.PartialSuccessMethods)
+		},
+	}
+
+	go NewServerConn(c1, partialSuccessKeyAThenKeyBServer())
+
+	_, _, _, err = NewClientConn(c2, "", clientConf)
+	if err != nil {
+		t.Fatalf("client login error: %s", err)
+	}
+}
+
+func TestAuthCallbackPartialSuccessWithStaticAuth(t *testing.T) {
+	c1, c2, err := netPipe()
+	if err != nil {
+		t.Fatalf("netPipe: %v", err)
+	}
+	defer c1.Close()
+	defer c2.Close()
+
+	clientConf := &ClientConfig{
+		User: "user",
+		Auth: []AuthMethod{
+			PublicKeys(testSigners["rsa"]),
+		},
+		HostKeyCallback: InsecureIgnoreHostKey(),
+		AuthCallback: func(ctx *ClientAuthContext) (AuthMethod, error) {
+			if slices.Contains(ctx.PartialSuccessMethods, "publickey") && slices.Contains(ctx.AllowedMethods, "keyboard-interactive") {
+				return KeyboardInteractive(func(name, instruction string, questions []string, echos []bool) ([]string, error) {
+					return []string{"correct-token"}, nil
+				}), nil
+			}
+			return nil, nil
+		},
+	}
+
+	go NewServerConn(c1, partialSuccessPublicKeyAndKbdInteractiveServer())
+
+	_, _, _, err = NewClientConn(c2, "", clientConf)
+	if err != nil {
+		t.Fatalf("client login error: %s", err)
+	}
+}
+
+func partialSuccessPubKeyForbiddenAfterPartialServer(maxTries int) *ServerConfig {
+	serverConf := &ServerConfig{
+		PublicKeyCallback: func(conn ConnMetadata, key PublicKey) (*Permissions, error) {
+			if bytes.Equal(key.Marshal(), testPublicKeys["rsa"].Marshal()) {
+				return nil, nil
+			}
+			return nil, errors.New("invalid credentials")
+		},
+		VerifiedPublicKeyCallback: func(conn ConnMetadata, key PublicKey, permissions *Permissions, signatureAlgorithm string) (*Permissions, error) {
+			if bytes.Equal(key.Marshal(), testPublicKeys["rsa"].Marshal()) {
+				return nil, &PartialSuccessError{
+					Next: ServerAuthCallbacks{
+						PublicKeyCallback: func(conn ConnMetadata, key PublicKey) (*Permissions, error) {
+							return nil, errors.New("legacy public key authentication is forbidden")
+						},
+						KeyboardInteractiveCallback: func(conn ConnMetadata, client KeyboardInteractiveChallenge) (*Permissions, error) {
+							answers, err := client("", "", []string{"token: "}, []bool{true})
+							if err != nil {
+								return nil, err
+							}
+							if len(answers) == 1 && answers[0] == "correct-token" {
+								return nil, nil
+							}
+							return nil, errors.New("invalid token")
+						},
+					},
+				}
+			}
+			return nil, errors.New("invalid credentials")
+		},
+		MaxAuthTries: maxTries,
+	}
+	serverConf.AddHostKey(testSigners["rsa"])
+	return serverConf
+}
+
+func TestAuthCallbackPartialSuccessWithStaticMultiKey(t *testing.T) {
+	c1, c2, err := netPipe()
+	if err != nil {
+		t.Fatalf("netPipe: %v", err)
+	}
+	defer c1.Close()
+	defer c2.Close()
+
+	clientConf := &ClientConfig{
+		User: "user",
+		Auth: []AuthMethod{
+			PublicKeys(
+				testSigners["rsa"],
+				testSigners["ecdsa"],
+				testSigners["ed25519"],
+			),
+		},
+		HostKeyCallback: InsecureIgnoreHostKey(),
+		AuthCallback: func(ctx *ClientAuthContext) (AuthMethod, error) {
+			if slices.Contains(ctx.PartialSuccessMethods, "publickey") && slices.Contains(ctx.AllowedMethods, "keyboard-interactive") {
+				return KeyboardInteractive(func(name, instruction string, questions []string, echos []bool) ([]string, error) {
+					return []string{"correct-token"}, nil
+				}), nil
+			}
+			return nil, nil
+		},
+	}
+
+	go NewServerConn(c1, partialSuccessPubKeyForbiddenAfterPartialServer(2))
+
+	_, _, _, err = NewClientConn(c2, "", clientConf)
+	if err != nil {
+		t.Fatalf("client login error: %s", err)
+	}
+}
+
+func TestAuthCallbackPartialSuccessShortCircuit(t *testing.T) {
+	c1, c2, err := netPipe()
+	if err != nil {
+		t.Fatalf("netPipe: %v", err)
+	}
+	defer c1.Close()
+	defer c2.Close()
+
+	serverConf := partialSuccessPubKeyForbiddenAfterPartialServer(2)
+
+	pubkeyAttempts := 0
+	clientConf := &ClientConfig{
+		User:            "user",
+		HostKeyCallback: InsecureIgnoreHostKey(),
+		AuthCallback: func(ctx *ClientAuthContext) (AuthMethod, error) {
+			if slices.Contains(ctx.AllowedMethods, "publickey") && !slices.Contains(ctx.PartialSuccessMethods, "publickey") {
+				pubkeyAttempts++
+				return PublicKeys(testSigners["rsa"], testSigners["ecdsa"], testSigners["ed25519"]), nil
+			}
+			if slices.Contains(ctx.PartialSuccessMethods, "publickey") && slices.Contains(ctx.AllowedMethods, "keyboard-interactive") {
+				return KeyboardInteractive(func(name, instruction string, questions []string, echos []bool) ([]string, error) {
+					return []string{"correct-token"}, nil
+				}), nil
+			}
+			return nil, fmt.Errorf("unexpected auth state: allowed=%v partial=%v tried=%v",
+				ctx.AllowedMethods, ctx.PartialSuccessMethods, ctx.TriedMethods)
+		},
+	}
+
+	go NewServerConn(c1, serverConf)
+
+	_, _, _, err = NewClientConn(c2, "", clientConf)
+	if err != nil {
+		t.Fatalf("client login error: %s", err)
+	}
+	if pubkeyAttempts != 1 {
+		t.Errorf("expected publickey to be attempted once before short-circuit, got %d", pubkeyAttempts)
+	}
+}
+
+func TestAuthCallbackTriedLoopBound(t *testing.T) {
+	c1, c2, err := netPipe()
+	if err != nil {
+		t.Fatalf("netPipe: %v", err)
+	}
+	defer c1.Close()
+	defer c2.Close()
+
+	serverConf := &ServerConfig{
+		MaxAuthTries: -1,
+		PasswordCallback: func(conn ConnMetadata, pass []byte) (*Permissions, error) {
+			return nil, errors.New("password rejected")
+		},
+	}
+	serverConf.AddHostKey(testSigners["rsa"])
+
+	invocations := 0
+	clientConf := &ClientConfig{
+		User:            "user",
+		HostKeyCallback: InsecureIgnoreHostKey(),
+		AuthCallback: func(ctx *ClientAuthContext) (AuthMethod, error) {
+			invocations++
+			return Password("wrong"), nil
+		},
+	}
+
+	go NewServerConn(c1, serverConf)
+
+	_, _, _, err = NewClientConn(c2, "", clientConf)
+	if err == nil {
+		t.Fatal("expected the client to abort after exceeding the attempts cap")
+	}
+	if !strings.Contains(err.Error(), "too many authentication attempts") {
+		t.Errorf("unexpected error: %v", err)
+	}
+	if invocations != maxAuthClientTried {
+		t.Errorf("AuthCallback invoked %d times; expected %d",
+			invocations, maxAuthClientTried)
+	}
+}
+
+func alwaysPartialPubKeyServer() *ServerConfig {
+	var alwaysPartial func(ConnMetadata, PublicKey) (*Permissions, error)
+	alwaysPartial = func(conn ConnMetadata, key PublicKey) (*Permissions, error) {
+		if !bytes.Equal(key.Marshal(), testPublicKeys["rsa"].Marshal()) {
+			return nil, errors.New("invalid credentials")
+		}
+		return nil, &PartialSuccessError{
+			Next: ServerAuthCallbacks{
+				PublicKeyCallback: alwaysPartial,
+			},
+		}
+	}
+	serverConf := &ServerConfig{
+		MaxAuthTries:      -1,
+		PublicKeyCallback: alwaysPartial,
+	}
+	serverConf.AddHostKey(testSigners["rsa"])
+	return serverConf
+}
+
+func TestAuthCallbackPartialSuccessLoopBound(t *testing.T) {
+	c1, c2, err := netPipe()
+	if err != nil {
+		t.Fatalf("netPipe: %v", err)
+	}
+	defer c1.Close()
+	defer c2.Close()
+
+	invocations := 0
+	clientConf := &ClientConfig{
+		User:            "user",
+		HostKeyCallback: InsecureIgnoreHostKey(),
+		AuthCallback: func(ctx *ClientAuthContext) (AuthMethod, error) {
+			invocations++
+			return PublicKeys(testSigners["rsa"]), nil
+		},
+	}
+
+	go NewServerConn(c1, alwaysPartialPubKeyServer())
+
+	_, _, _, err = NewClientConn(c2, "", clientConf)
+	if err == nil {
+		t.Fatal("expected the client to abort after exceeding the attempts cap")
+	}
+	if !strings.Contains(err.Error(), "too many authentication attempts") {
+		t.Errorf("unexpected error: %v", err)
+	}
+	if invocations != maxAuthClientTried {
+		t.Errorf("AuthCallback invoked %d times; expected %d",
+			invocations, maxAuthClientTried)
+	}
+}
+
 func TestAuthMethodPassword(t *testing.T) {
 	config := &ClientConfig{
 		User: "testuser",
@@ -971,6 +1654,74 @@ func TestAuthMethodGSSAPIWithMIC(t *testing.T) {
 	}
 }
 
+type maliciousGSSAPIWithMICAuthMethod struct{}
+
+func (maliciousGSSAPIWithMICAuthMethod) method() string {
+	// Reuse an advertised auth method name so the client attempts this
+	// AuthMethod even when gssapi-with-mic is not advertised.
+	return "password"
+}
+
+func (maliciousGSSAPIWithMICAuthMethod) auth(session []byte, user string, c packetConn, rand io.Reader, _ map[string][]byte) (authResult, []string, error) {
+	req := &userAuthRequestMsg{
+		User:    user,
+		Service: serviceSSH,
+		Method:  "gssapi-with-mic",
+	}
+	req.Payload = appendU32(req.Payload, 1)
+	req.Payload = appendString(req.Payload, string(krb5OID))
+	if err := c.writePacket(Marshal(req)); err != nil {
+		return authFailure, nil, err
+	}
+
+	packet, err := c.readPacket()
+	if err != nil {
+		return authFailure, nil, err
+	}
+	switch packet[0] {
+	case msgUserAuthFailure:
+		var msg userAuthFailureMsg
+		if err := Unmarshal(packet, &msg); err != nil {
+			return authFailure, nil, err
+		}
+		if msg.PartialSuccess {
+			return authPartialSuccess, msg.Methods, nil
+		}
+		return authFailure, msg.Methods, nil
+	default:
+		return authFailure, nil, unexpectedMessageError(msgUserAuthFailure, packet[0])
+	}
+}
+
+func TestGSSAPIWithMICPartialConfigRejectedAtRuntime(t *testing.T) {
+	config := &ClientConfig{
+		User: "testuser",
+		Auth: []AuthMethod{
+			maliciousGSSAPIWithMICAuthMethod{},
+		},
+		HostKeyCallback: InsecureIgnoreHostKey(),
+	}
+
+	clientErr, serverErrs := tryAuthBothSides(t, config, &GSSAPIWithMICConfig{
+		Server: &FakeServer{},
+	})
+
+	if clientErr == nil || !strings.Contains(clientErr.Error(), "ssh: unable to authenticate") {
+		t.Fatalf("client got %v, want authentication failure", clientErr)
+	}
+
+	found := false
+	for _, err := range serverErrs {
+		if err != nil && strings.Contains(err.Error(), "ssh: gssapi-with-mic auth not configured") {
+			found = true
+			break
+		}
+	}
+	if !found {
+		t.Fatalf("server auth errors = %v, want gssapi-with-mic auth not configured", serverErrs)
+	}
+}
+
 func TestCompatibleAlgoAndSignatures(t *testing.T) {
 	type testcase struct {
 		algo       string

ssh/connection.go

@@ -91,9 +91,17 @@ func DiscardRequests(in <-chan *Request) {
 	}
 }
 
+// A connTransport represents the transport for a connection.
+type connTransport interface {
+	packetConn
+	getAlgorithms() NegotiatedAlgorithms
+	getSessionID() []byte
+	waitSession() error
+}
+
 // A connection represents an incoming connection.
 type connection struct {
-	transport *handshakeTransport
+	transport connTransport
 	sshConn
 
 	// The connection protocol.

ssh/control.go

@@ -0,0 +1,155 @@
+// Copyright 2026 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package ssh
+
+import (
+	"encoding/binary"
+	"errors"
+	"fmt"
+	"io"
+
+	"golang.org/x/crypto/cryptobyte"
+)
+
+const (
+	muxProtocolVersion = 4
+
+	muxMsgHello = 0x00000001
+	muxCProxy   = 0x1000000f
+	muxSProxy   = 0x8000000f
+)
+
+const controlProxyRequestID = 0
+
+// handshakeControlProxy attempts to establish a transport connection with an
+// OpenSSH ControlMaster socket in proxy mode. For details see:
+// https://github.com/openssh/openssh-portable/blob/master/PROTOCOL.mux
+func handshakeControlProxy(rw io.ReadWriteCloser) (connTransport, error) {
+	if err := controlProxyWritePacket(rw, func(b *cryptobyte.Builder) {
+		b.AddUint32(muxMsgHello)
+		b.AddUint32(muxProtocolVersion)
+	}); err != nil {
+		return nil, fmt.Errorf("mux hello write failed: %w", err)
+	}
+	if err := controlProxyWritePacket(rw, func(b *cryptobyte.Builder) {
+		b.AddUint32(muxCProxy)
+		b.AddUint32(controlProxyRequestID)
+	}); err != nil {
+		return nil, fmt.Errorf("mux client proxy write failed: %w", err)
+	}
+
+	messageType, body, err := controlProxyReadMessage(rw)
+	if err != nil {
+		return nil, fmt.Errorf("mux hello read failed: %w", err)
+	}
+	if messageType != muxMsgHello {
+		return nil, fmt.Errorf("expected hello response, got %v", messageType)
+	}
+	var v uint32
+	if !body.ReadUint32(&v) {
+		return nil, errors.New("EOF reading mux protocol version")
+	}
+	if v != muxProtocolVersion {
+		return nil, fmt.Errorf("mux server has unsupported version %v", v)
+	}
+	messageType, body, err = controlProxyReadMessage(rw)
+	if err != nil {
+		return nil, fmt.Errorf("mux server proxy read failed: %w", err)
+	}
+	if messageType != muxSProxy {
+		return nil, fmt.Errorf("expected server proxy response, got %v", messageType)
+	}
+	var reqID uint32
+	if !body.ReadUint32(&reqID) {
+		return nil, errors.New("EOF reading request id")
+	}
+	if reqID != controlProxyRequestID {
+		return nil, fmt.Errorf("expected request id %v, got %v", controlProxyRequestID, reqID)
+	}
+	return &controlProxyTransport{rw}, nil
+}
+
+// controlProxyTransport implements the connTransport interface for
+// ControlMaster connections. Each controlMessage has zero length padding and
+// no MAC.
+type controlProxyTransport struct {
+	rw io.ReadWriteCloser
+}
+
+func (p *controlProxyTransport) Close() error {
+	return p.rw.Close()
+}
+
+func (p *controlProxyTransport) writePacket(controlMessage []byte) error {
+	return controlProxyWritePacket(p.rw, func(b *cryptobyte.Builder) {
+		b.AddUint8(0) // Padding length.
+		b.AddBytes(controlMessage)
+	})
+}
+
+func (p *controlProxyTransport) readPacket() ([]byte, error) {
+	buf, err := controlProxyReadPacket(p.rw)
+	if err != nil {
+		return nil, fmt.Errorf("ssh: error reading control message: %w", err)
+	}
+	// Discard the padding length.
+	if len(buf) < 1 {
+		return nil, errors.New("ssh: EOF reading padding length")
+	}
+	if buf[0] != 0 {
+		return nil, errors.New("ssh: unexpected non-zero padding in control message")
+	}
+	return buf[1:], nil
+}
+
+func (p *controlProxyTransport) getAlgorithms() NegotiatedAlgorithms {
+	return NegotiatedAlgorithms{}
+}
+
+func (p *controlProxyTransport) getSessionID() []byte {
+	return nil
+}
+
+func (p *controlProxyTransport) waitSession() error {
+	return nil
+}
+
+func controlProxyWritePacket(w io.Writer, f cryptobyte.BuilderContinuation) error {
+	var buf []byte
+	b := cryptobyte.NewBuilder(buf)
+	b.AddUint32LengthPrefixed(f)
+	out, err := b.Bytes()
+	if err != nil {
+		return err
+	}
+	_, err = w.Write(out)
+	return err
+}
+
+func controlProxyReadPacket(r io.Reader) (cryptobyte.String, error) {
+	var l uint32
+	if err := binary.Read(r, binary.BigEndian, &l); err != nil {
+		return nil, err
+	}
+	if l > maxPacket {
+		return nil, fmt.Errorf("message length %v exceeds maximum %v", l, maxPacket)
+	}
+	buf := make([]byte, l)
+	if _, err := io.ReadFull(r, buf); err != nil {
+		return nil, err
+	}
+	return buf, nil
+}
+
+func controlProxyReadMessage(r io.Reader) (messageType uint32, body cryptobyte.String, err error) {
+	body, err = controlProxyReadPacket(r)
+	if err != nil {
+		return 0, nil, fmt.Errorf("error reading message body: %w", err)
+	}
+	if !body.ReadUint32(&messageType) {
+		return 0, nil, errors.New("EOF reading message type")
+	}
+	return messageType, body, nil
+}

ssh/control_test.go

@@ -0,0 +1,275 @@
+// Copyright 2026 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package ssh
+
+import (
+	"bytes"
+	"io"
+	"strings"
+	"testing"
+)
+
+func TestControlClientHandshake(t *testing.T) {
+	reqs := [][]byte{
+		// Hello request.
+		{0x00, 0x00, 0x00, 0x08, 0x00, 0x00, 0x00, 0x01, 0x00, 0x00, 0x00, 0x04},
+		// Client proxy request.
+		{0x00, 0x00, 0x00, 0x08, 0x10, 0x00, 0x00, 0x0f, 0x00, 0x00, 0x00, 0x00},
+	}
+	respsNormal := [][]byte{
+		// Hello response.
+		{0x00, 0x00, 0x00, 0x08, 0x00, 0x00, 0x00, 0x01, 0x00, 0x00, 0x00, 0x04},
+		// Server proxy response.
+		{0x00, 0x00, 0x00, 0x08, 0x80, 0x00, 0x00, 0x0f, 0x00, 0x00, 0x00, 0x00},
+	}
+	for _, tt := range []struct {
+		name        string
+		resps       [][]byte
+		expectedErr string
+	}{
+		{
+			name:  "normal handshake",
+			resps: respsNormal,
+		},
+		{
+			name: "length greater than max",
+			resps: [][]byte{
+				{0xff, 0xff, 0xff, 0xff},
+				respsNormal[1],
+			},
+			expectedErr: "message length 4294967295 exceeds maximum",
+		},
+		{
+			name: "missing hello response",
+			resps: [][]byte{
+				{},
+			},
+			expectedErr: "use of closed network connection",
+		},
+		{
+			name: "hello response too short",
+			resps: [][]byte{
+				{0x00, 0x00, 0x00, 0x06, 0x00, 0x00, 0x00, 0x01, 0x00, 0x00},
+				respsNormal[1],
+			},
+			expectedErr: "EOF",
+		},
+		{
+			name: "bad hello response type",
+			resps: [][]byte{
+				{0x00, 0x00, 0x00, 0x08, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x04},
+				respsNormal[1],
+			},
+			expectedErr: "expected hello response, got 0",
+		},
+		{
+			name: "bad protocol version",
+			resps: [][]byte{
+				{0x00, 0x00, 0x00, 0x08, 0x00, 0x00, 0x00, 0x01, 0x00, 0x00, 0x00, 0x00},
+				respsNormal[1],
+			},
+			expectedErr: "mux server has unsupported version 0",
+		},
+		{
+			name: "missing server proxy response",
+			resps: [][]byte{
+				respsNormal[0],
+			},
+			expectedErr: "use of closed network connection",
+		},
+		{
+			name: "server proxy response too short",
+			resps: [][]byte{
+				respsNormal[0],
+				{0x00, 0x00, 0x00, 0x06, 0x80, 0x00, 0x00, 0x0f, 0x00, 0x00},
+			},
+			expectedErr: "EOF",
+		},
+		{
+			name: "bad server proxy response type",
+			resps: [][]byte{
+				respsNormal[0],
+				{0x00, 0x00, 0x00, 0x08, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00},
+			},
+			expectedErr: "expected server proxy response, got 0",
+		},
+		{
+			name: "bad request id",
+			resps: [][]byte{
+				respsNormal[0],
+				{0x00, 0x00, 0x00, 0x08, 0x80, 0x00, 0x00, 0x0f, 0x00, 0x00, 0x00, 0x01},
+			},
+			expectedErr: "expected request id 0, got 1",
+		},
+	} {
+		t.Run(tt.name, func(t *testing.T) {
+			done := make(chan error, 1)
+
+			ok := func() bool {
+				c1, c2, err := netPipe()
+				if err != nil {
+					t.Fatalf("netPipe: %v", err)
+				}
+				defer c1.Close()
+				defer c2.Close()
+
+				go func() {
+					defer close(done)
+					_, _, _, err := NewControlClientConn(c2)
+					c2.Write([]byte{0}) // Dummy message to unblock the final read.
+					done <- err
+				}()
+
+				i := 0
+				for ; i < len(reqs) && i < len(tt.resps); i++ {
+					expected := reqs[i]
+					buf := make([]byte, len(expected))
+					if _, err := io.ReadFull(c1, buf); err != nil {
+						t.Errorf("error reading message %d: %v", i+1, err)
+						return false
+					}
+					if !bytes.Equal(buf, expected) {
+						t.Errorf(
+							"unexpected message %d: got %v, want %v",
+							i+1, buf, expected,
+						)
+						return false
+					}
+					_, err = c1.Write(tt.resps[i])
+					if err != nil {
+						t.Errorf("error writing message %d: %v", i+1, err)
+						return false
+					}
+				}
+				// Wait for the next message so that the final response can be read.
+				buf := make([]byte, 1)
+				c1.Read(buf)
+				return true
+			}()
+			if !ok {
+				return
+			}
+
+			err := <-done
+			if tt.expectedErr != "" {
+				if err == nil || !strings.Contains(err.Error(), tt.expectedErr) {
+					t.Fatalf("got err %q; want err containing %q", err, tt.expectedErr)
+				}
+				return
+			}
+			if err != nil {
+				t.Fatalf("got err %q; want no err", err)
+			}
+		})
+	}
+}
+
+func TestControlClientTransport(t *testing.T) {
+	type response struct {
+		status  bool
+		payload []byte
+		err     error
+	}
+
+	for _, tt := range []struct {
+		name        string
+		resp        []byte
+		respStatus  bool
+		respPayload []byte
+		expectedErr string
+	}{
+		{
+			name:       "successful request",
+			resp:       []byte{0x00, 0x00, 0x00, 0x02, 0x00, 0x51},
+			respStatus: true,
+		},
+		{
+			name:       "failed request",
+			resp:       []byte{0x00, 0x00, 0x00, 0x02, 0x00, 0x52},
+			respStatus: false,
+		},
+		{
+			name:        "short response",
+			resp:        []byte{0x00, 0x00, 0x00, 0x00},
+			expectedErr: "EOF",
+		},
+		{
+			name:        "response with payload",
+			resp:        []byte{0x00, 0x00, 0x00, 0x05, 0x00, 0x51, 0x01, 0x02, 0x03},
+			respStatus:  true,
+			respPayload: []byte{1, 2, 3},
+		},
+	} {
+		t.Run(tt.name, func(t *testing.T) {
+			c1, c2, err := netPipe()
+			if err != nil {
+				t.Fatalf("netPipe: %v", err)
+			}
+			defer c1.Close()
+			defer c2.Close()
+
+			// Handshake responses.
+			c1.Write([]byte{
+				0x00, 0x00, 0x00, 0x08, 0x00, 0x00, 0x00, 0x01, 0x00, 0x00, 0x00, 0x04,
+				0x00, 0x00, 0x00, 0x08, 0x80, 0x00, 0x00, 0x0f, 0x00, 0x00, 0x00, 0x00,
+			})
+
+			conn, chans, reqs, err := NewControlClientConn(c2)
+			if err != nil {
+				t.Fatal(err)
+			}
+			client := NewClient(conn, chans, reqs)
+
+			done := make(chan response, 1)
+			go func() {
+				defer close(done)
+				status, payload, err := client.SendRequest("hello", true, nil)
+				if err != nil {
+					done <- response{err: err}
+					return
+				}
+				done <- response{
+					status:  status,
+					payload: payload,
+				}
+			}()
+
+			// Discard handshake.
+			io.CopyN(io.Discard, c1, 24)
+
+			expectedReq := []byte{
+				0x00, 0x00, 0x00, 0x0c, 0x00, 0x50,
+				0x00, 0x00, 0x00, 0x05, 'h', 'e', 'l', 'l', 'o',
+				0x01,
+			}
+			buf := make([]byte, len(expectedReq))
+			if _, err := io.ReadFull(c1, buf); err != nil {
+				t.Fatalf("reading request: %v", err)
+			}
+			if !bytes.Equal(buf, expectedReq) {
+				t.Fatalf("got request %v; want %v", buf, expectedReq)
+			}
+
+			c1.Write(tt.resp)
+
+			resp := <-done
+			if tt.expectedErr != "" {
+				if resp.err == nil || !strings.Contains(resp.err.Error(), tt.expectedErr) {
+					t.Fatalf("got err %q; want err containing %q", resp.err, tt.expectedErr)
+				}
+				return
+			}
+			if resp.err != nil {
+				t.Fatalf("got err %q; want no err", resp.err)
+			}
+			if resp.status != tt.respStatus {
+				t.Fatalf("got status %v; want %v", resp.status, tt.respStatus)
+			}
+			if !bytes.Equal(resp.payload, tt.respPayload) {
+				t.Errorf("got payload %v; want %v", resp.payload, tt.respPayload)
+			}
+		})
+	}
+}

ssh/kex.go

@@ -16,6 +16,7 @@ import (
 	"io"
 	"math/big"
 	"slices"
+	"sync"
 
 	"golang.org/x/crypto/curve25519"
 )
@@ -718,15 +719,9 @@ func (gex *dhGEXSHA) Server(c packetConn, randSource io.Reader, magics *handshak
 			kexDHGexRequest.MaxBits, kexDHGexRequest.PreferredBits)
 	}
 
-	var p *big.Int
-	// We hardcode sending Oakley Group 14 (2048 bits), Oakley Group 15 (3072
-	// bits) or Oakley Group 16 (4096 bits), based on the requested max size.
-	if kexDHGexRequest.MaxBits < 3072 {
-		p, _ = new(big.Int).SetString(oakleyGroup14, 16)
-	} else if kexDHGexRequest.MaxBits < 4096 {
-		p, _ = new(big.Int).SetString(oakleyGroup15, 16)
-	} else {
-		p, _ = new(big.Int).SetString(oakleyGroup16, 16)
+	p, err := chooseDH(kexDHGexRequest)
+	if err != nil {
+		return nil, err
 	}
 
 	g := big.NewInt(2)
@@ -805,3 +800,65 @@ func (gex *dhGEXSHA) Server(c packetConn, randSource io.Reader, magics *handshak
 		Hash:      gex.hashFunc,
 	}, err
 }
+
+type dhKEXGroup struct {
+	size int
+	p    *big.Int
+}
+
+// supportedDHKEXGroups returns the DH groups the server is willing to offer
+// for diffie-hellman-group-exchange-* key exchanges. The list is built lazily
+// on first use to keep the hex-to-big.Int parse out of package initialization.
+var supportedDHKEXGroups = sync.OnceValue(func() []dhKEXGroup {
+	specs := []struct {
+		size int
+		hex  string
+	}{
+		{2048, oakleyGroup14},
+		{3072, oakleyGroup15},
+		{4096, oakleyGroup16},
+	}
+	out := make([]dhKEXGroup, 0, len(specs))
+	for _, s := range specs {
+		p, _ := new(big.Int).SetString(s.hex, 16)
+		out = append(out, dhKEXGroup{size: s.size, p: p})
+	}
+	return out
+})
+
+// chooseDH picks a DH group for the given client request, mirroring the
+// algorithm used by OpenSSH's choose_dh in dh.c: prefer the smallest known
+// group larger than  or equal to the client's PreferredBits, and otherwise pick
+// the largest group within the accepted [MinBits, MaxBits] range.
+func chooseDH(req kexDHGexRequestMsg) (*big.Int, error) {
+	var best *big.Int
+	bestSize := 0
+	wantBits := int(req.PreferredBits)
+
+	for _, group := range supportedDHKEXGroups() {
+		if uint32(group.size) < req.MinBits || uint32(group.size) > req.MaxBits {
+			continue
+		}
+
+		if bestSize == 0 {
+			best = group.p
+			bestSize = group.size
+			continue
+		}
+
+		closerFromAbove := group.size >= wantBits && group.size < bestSize
+		closerFromBelow := group.size > bestSize && bestSize < wantBits
+
+		if closerFromAbove || closerFromBelow {
+			best = group.p
+			bestSize = group.size
+		}
+	}
+
+	if bestSize == 0 {
+		return nil, fmt.Errorf("ssh: no suitable DH group found for request min: %d, preferred: %d, max: %d",
+			req.MinBits, req.PreferredBits, req.MaxBits)
+	}
+
+	return best, nil
+}

ssh/kex_test.go

@@ -9,6 +9,7 @@ package ssh
 import (
 	"crypto/rand"
 	"fmt"
+	"math/big"
 	"reflect"
 	"sync"
 	"testing"
@@ -65,6 +66,88 @@ func TestKexes(t *testing.T) {
 	}
 }
 
+func TestChooseDH(t *testing.T) {
+	oakley := map[int]string{
+		2048: oakleyGroup14,
+		3072: oakleyGroup15,
+		4096: oakleyGroup16,
+	}
+	expected := func(size int) *big.Int {
+		hex, ok := oakley[size]
+		if !ok {
+			t.Fatalf("test setup: no Oakley group for size %d", size)
+		}
+		p, _ := new(big.Int).SetString(hex, 16)
+		return p
+	}
+
+	tests := []struct {
+		name    string
+		request kexDHGexRequestMsg
+		want    int // expected bit size; 0 means error expected
+		wantErr bool
+	}{
+		{
+			name:    "Standard 2048 request",
+			request: kexDHGexRequestMsg{MinBits: 1024, PreferredBits: 2048, MaxBits: 8192},
+			want:    2048,
+		},
+		{
+			name:    "Standard 3072 request",
+			request: kexDHGexRequestMsg{MinBits: 1024, PreferredBits: 3072, MaxBits: 8192},
+			want:    3072,
+		},
+		{
+			name:    "Standard 4096 request",
+			request: kexDHGexRequestMsg{MinBits: 1024, PreferredBits: 4096, MaxBits: 8192},
+			want:    4096,
+		},
+		{
+			name:    "Preferred 2500 -> Expect 3072 (round up)",
+			request: kexDHGexRequestMsg{MinBits: 1024, PreferredBits: 2500, MaxBits: 8192},
+			want:    3072,
+		},
+		{
+			name:    "Preferred 3500 -> Expect 4096 (round up)",
+			request: kexDHGexRequestMsg{MinBits: 1024, PreferredBits: 3500, MaxBits: 8192},
+			want:    4096,
+		},
+		{
+			name:    "Preferred too high (8000) -> Expect 4096 (cap at max available)",
+			request: kexDHGexRequestMsg{MinBits: 1024, PreferredBits: 8000, MaxBits: 8192},
+			want:    4096,
+		},
+		{
+			name:    "No group in range",
+			request: kexDHGexRequestMsg{MinBits: 2500, PreferredBits: 2500, MaxBits: 2900},
+			wantErr: true,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got, err := chooseDH(tt.request)
+
+			if (err != nil) != tt.wantErr {
+				t.Errorf("chooseDH() error = %v, wantErr %t", err, tt.wantErr)
+				return
+			}
+			if tt.wantErr {
+				return
+			}
+			if got == nil {
+				t.Fatalf("chooseDH() returned nil big.Int but expected success")
+			}
+			if got.BitLen() != tt.want {
+				t.Errorf("chooseDH() got size = %d, want %d", got.BitLen(), tt.want)
+			}
+			if want := expected(tt.want); got.Cmp(want) != 0 {
+				t.Errorf("chooseDH() returned the wrong group for size %d", tt.want)
+			}
+		})
+	}
+}
+
 func BenchmarkKexes(b *testing.B) {
 	type kexResultErr struct {
 		result *kexResult

ssh/keys.go

@@ -76,7 +76,7 @@ func parsePubKey(in []byte, algo string) (pubKey PublicKey, rest []byte, err err
 	case InsecureKeyAlgoDSA:
 		return parseDSA(in)
 	case KeyAlgoECDSA256, KeyAlgoECDSA384, KeyAlgoECDSA521:
-		return parseECDSA(in)
+		return parseECDSA(in, algo)
 	case KeyAlgoSKECDSA256:
 		return parseSKECDSA(in)
 	case KeyAlgoED25519:
@@ -806,7 +806,7 @@ func supportedEllipticCurve(curve elliptic.Curve) bool {
 }
 
 // parseECDSA parses an ECDSA key according to RFC 5656, section 3.1.
-func parseECDSA(in []byte) (out PublicKey, rest []byte, err error) {
+func parseECDSA(in []byte, expectedType string) (out PublicKey, rest []byte, err error) {
 	var w struct {
 		Curve    string
 		KeyBytes []byte
@@ -817,6 +817,12 @@ func parseECDSA(in []byte) (out PublicKey, rest []byte, err error) {
 		return nil, nil, err
 	}
 
+	actualType := "ecdsa-sha2-" + w.Curve
+	if expectedType != actualType {
+		return nil, nil, fmt.Errorf("ssh: algorithm type mismatch: expected %q, found curve %q (type %q)",
+			expectedType, w.Curve, actualType)
+	}
+
 	key := new(ecdsa.PublicKey)
 
 	switch w.Curve {
@@ -1466,6 +1472,17 @@ func passphraseProtectedOpenSSHKey(passphrase []byte) openSSHDecryptFunc {
 			return nil, err
 		}
 
+		// OpenSSH does not impose an upper bound on the bcrypt round count
+		// stored in the key file, but bcrypt_pbkdf cost is linear in rounds:
+		// the default is 16, ssh-keygen lets users pick anything up to
+		// INT_MAX. Cap at 2048 (128x the default, a few seconds of CPU) so
+		// that an oversized value in the file cannot tie up the caller for
+		// months.
+		const maxRounds = 1 << 11
+		if opts.Rounds > maxRounds {
+			return nil, fmt.Errorf("ssh: bcrypt KDF rounds %d exceed maximum %d", opts.Rounds, maxRounds)
+		}
+
 		k, err := bcrypt_pbkdf.Key(passphrase, []byte(opts.Salt), int(opts.Rounds), 32+16)
 		if err != nil {
 			return nil, err
@@ -1635,10 +1652,28 @@ func parseOpenSSHPrivateKey(key []byte, decrypt openSSHDecryptFunc) (crypto.Priv
 			return nil, err
 		}
 
+		// Mirror the validation done in parseRSA for public keys: cap the
+		// modulus at the same limit enforced by crypto/tls, reject oversized
+		// or invalid exponents, and additionally bound the prime factors to
+		// avoid the expensive CRT coefficient recomputation in pk.Precompute.
+		if key.N.BitLen() > 8192 {
+			return nil, errors.New("ssh: rsa modulus too large")
+		}
+		if key.P.BitLen() > 4096 || key.Q.BitLen() > 4096 {
+			return nil, errors.New("ssh: rsa prime too large")
+		}
+		if key.E.BitLen() > 24 {
+			return nil, errors.New("ssh: exponent too large")
+		}
+		e := key.E.Int64()
+		if e < 3 || e&1 == 0 {
+			return nil, errors.New("ssh: incorrect exponent")
+		}
+
 		pk := &rsa.PrivateKey{
 			PublicKey: rsa.PublicKey{
 				N: key.N,
-				E: int(key.E.Int64()),
+				E: int(e),
 			},
 			D:      key.D,
 			Primes: []*big.Int{key.P, key.Q},

ssh/keys_test.go

@@ -232,6 +232,200 @@ func TestParseRSAModulusTooLarge(t *testing.T) {
 	}
 }
 
+func TestParsePrivateKeyRSAModulusTooLarge(t *testing.T) {
+	rsa16384 := []byte(`-----BEGIN OPENSSH PRIVATE KEY-----
+b3BlbnNzaC1rZXktdjEAAAAABG5vbmUAAAAEbm9uZQAAAAAAAAABAAAIFwAAAAdzc2gtcn
+NhAAAAAwEAAQAACAEAy58lBbrFRB4r18MqVA625r4nzxjTkMlN5LxoG7ARC9L7px5lPs4o
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+QHAxNnM=
+-----END OPENSSH PRIVATE KEY-----
+`)
+	_, err := ParseRawPrivateKey(rsa16384)
+	if err == nil {
+		t.Fatal("ParseRawPrivateKey accepted a 16384-bit modulus; expected it to be rejected")
+	}
+
+	expectedError := "rsa modulus too large"
+	if !strings.Contains(err.Error(), expectedError) {
+		t.Errorf("unexpected error message: got %q, want substring %q", err.Error(), expectedError)
+	}
+}
+
 func TestParseECPrivateKey(t *testing.T) {
 	key := testPrivateKeys["ecdsa"]
 
@@ -312,6 +506,38 @@ func TestParseEncryptedPrivateKeysWithIncorrectPassphrase(t *testing.T) {
 	}
 }
 
+func TestParseEncryptedPrivateKeyExcessiveBcryptRounds(t *testing.T) {
+	// Craft a minimal openssh-key-v1 blob whose KdfOpts declares a bcrypt
+	// round count above the accepted maximum. The check must reject the file
+	// before bcrypt_pbkdf is invoked, so the rest of the blob (public key,
+	// encrypted body) can be empty.
+	kdfOpts := Marshal(struct {
+		Salt   []byte
+		Rounds uint32
+	}{
+		Salt:   []byte("salt-not-used"),
+		Rounds: (1 << 11) + 1,
+	})
+	header := Marshal(openSSHEncryptedPrivateKey{
+		CipherName: "aes256-ctr",
+		KdfName:    "bcrypt",
+		KdfOpts:    string(kdfOpts),
+		NumKeys:    1,
+	})
+	pemBytes := pem.EncodeToMemory(&pem.Block{
+		Type:  "OPENSSH PRIVATE KEY",
+		Bytes: append([]byte(privateKeyAuthMagic), header...),
+	})
+
+	_, err := ParseRawPrivateKeyWithPassphrase(pemBytes, []byte("password"))
+	if err == nil {
+		t.Fatal("expected error for excessive bcrypt rounds, got nil")
+	}
+	if !strings.Contains(err.Error(), "bcrypt KDF rounds") {
+		t.Errorf("got error %q, want substring %q", err.Error(), "bcrypt KDF rounds")
+	}
+}
+
 func TestParseDSA(t *testing.T) {
 	// We actually exercise the ParsePrivateKey codepath here, as opposed to
 	// using the ParseRawPrivateKey+NewSignerFromKey path that testdata_test.go
@@ -1235,3 +1461,39 @@ cLYUOHfQDw==
 		t.Fatal("parsing an SSH certificate using another certificate as signature key succeeded; expected failure")
 	}
 }
+
+func TestParseECDSAAlgorithmMismatch(t *testing.T) {
+	cases := []struct {
+		keyName    string // key fixture in testPublicKeys
+		nativeAlgo string // algorithm actually carried in the key blob
+		askedAlgo  string // algorithm passed to parsePubKey
+	}{
+		{"ecdsap256", "ecdsa-sha2-nistp256", "ecdsa-sha2-nistp384"},
+		{"ecdsap256", "ecdsa-sha2-nistp256", "ecdsa-sha2-nistp521"},
+		{"ecdsap384", "ecdsa-sha2-nistp384", "ecdsa-sha2-nistp256"},
+		{"ecdsap384", "ecdsa-sha2-nistp384", "ecdsa-sha2-nistp521"},
+		{"ecdsap521", "ecdsa-sha2-nistp521", "ecdsa-sha2-nistp256"},
+		{"ecdsap521", "ecdsa-sha2-nistp521", "ecdsa-sha2-nistp384"},
+	}
+
+	for _, tc := range cases {
+		t.Run(tc.nativeAlgo+"_as_"+tc.askedAlgo, func(t *testing.T) {
+			pubKey := testPublicKeys[tc.keyName]
+			algo, in, ok := parseString(pubKey.Marshal())
+			if !ok {
+				t.Fatal("unable to parse public key wire format")
+			}
+			if string(algo) != tc.nativeAlgo {
+				t.Fatalf("test setup failed: expected %q, got %q", tc.nativeAlgo, algo)
+			}
+
+			_, _, err := parsePubKey(in, tc.askedAlgo)
+			if err == nil {
+				t.Fatal("expected error due to algorithm mismatch, but got nil")
+			}
+			if !strings.Contains(err.Error(), "algorithm type mismatch") {
+				t.Fatalf("unexpected error message: %v", err)
+			}
+		})
+	}
+}

ssh/knownhosts/knownhosts.go

@@ -178,7 +178,7 @@ func nextWord(line []byte) (string, []byte) {
 		return string(line), nil
 	}
 
-	return string(line[:i]), bytes.TrimSpace(line[i:])
+	return string(line[:i]), trimSpace(line[i:])
 }
 
 func parseLine(line []byte) (marker, host string, key ssh.PublicKey, err error) {
@@ -188,12 +188,17 @@ func parseLine(line []byte) (marker, host string, key ssh.PublicKey, err error)
 	}
 
 	host, line = nextWord(line)
+	// If the extracted 'host' starts with '@', it means we either encountered
+	// a second marker (e.g., "@cert-authority @revoked") or an unknown marker
+	// (e.g., "@unknown"). Both are invalid.
+	if len(host) > 0 && host[0] == '@' {
+		return "", "", nil, fmt.Errorf("knownhosts: unexpected marker: %q", host)
+	}
 	if len(line) == 0 {
 		return "", "", nil, errors.New("knownhosts: missing host pattern")
 	}
 
-	// ignore the keytype as it's in the key blob anyway.
-	_, line = nextWord(line)
+	wantType, line := nextWord(line)
 	if len(line) == 0 {
 		return "", "", nil, errors.New("knownhosts: missing key type pattern")
 	}
@@ -209,6 +214,10 @@ func parseLine(line []byte) (marker, host string, key ssh.PublicKey, err error)
 		return "", "", nil, err
 	}
 
+	if key.Type() != wantType {
+		return "", "", nil, fmt.Errorf("knownhosts: key type mismatch: found %q, want %q", key.Type(), wantType)
+	}
+
 	return marker, host, key, nil
 }
 
@@ -387,7 +396,7 @@ func (db *hostKeyDB) Read(r io.Reader, filename string) error {
 	for scanner.Scan() {
 		lineNum++
 		line := scanner.Bytes()
-		line = bytes.TrimSpace(line)
+		line = trimSpace(line)
 		if len(line) == 0 || line[0] == '#' {
 			continue
 		}
@@ -535,3 +544,10 @@ func newHashedHost(encoded string) (*hashedHost, error) {
 func (h *hashedHost) match(a addr) bool {
 	return bytes.Equal(hashHost(Normalize(a.String()), h.salt), h.hash)
 }
+
+// trimSpace removes leading and trailing ASCII whitespace (space and tab). It
+// is used instead of bytes.TrimSpace to match OpenSSH behavior, which strictly
+// parses only ASCII space (0x20) and tab (0x09) as whitespace.
+func trimSpace(in []byte) []byte {
+	return bytes.Trim(in, " \t")
+}

ssh/knownhosts/knownhosts_test.go

@@ -8,9 +8,11 @@ import (
 	"bytes"
 	"crypto/ed25519"
 	"crypto/rand"
+	"encoding/base64"
 	"fmt"
 	"net"
 	"reflect"
+	"strings"
 	"testing"
 
 	"golang.org/x/crypto/ssh"
@@ -407,3 +409,94 @@ func TestIssue36126(t *testing.T) {
 		t.Errorf("should have passed the check, got %v", err)
 	}
 }
+
+func TestUnicodeSpace(t *testing.T) {
+	line := fmt.Sprintf("server.org %s  %s", edKey.Type(), base64.StdEncoding.EncodeToString(edKey.Marshal()))
+
+	db := newHostKeyDB()
+	err := db.Read(bytes.NewBufferString(line), "testdb")
+
+	if err == nil {
+		t.Fatal("Read succeeded on line with Unicode space, expected error due to strict ASCII parsing")
+	}
+}
+
+func TestUnicodeSpaceTrimming(t *testing.T) {
+	const unicodeSpace = " "
+	line := fmt.Sprintf("%sserver.org %s", unicodeSpace, edKeyStr)
+
+	db := newHostKeyDB()
+	err := db.Read(bytes.NewBufferString(line), "testdb")
+
+	if err != nil {
+		t.Fatalf("Read failed: %v", err)
+	}
+
+	lines := db.lines
+	if len(lines) != 1 {
+		t.Fatalf("Expected 1 line, got %d", len(lines))
+	}
+
+	cleanAddr := addr{host: "server.org", port: "22"}
+	dirtyAddr := addr{host: unicodeSpace + "server.org", port: "22"}
+
+	if lines[0].match(cleanAddr) {
+		t.Errorf("Matched clean host 'server.org', implying Unicode space was trimmed")
+	}
+	if !lines[0].match(dirtyAddr) {
+		t.Errorf("Did not match dirty host, implying parsing issue")
+	}
+}
+
+func TestKeyTypeMismatch(t *testing.T) {
+	line := fmt.Sprintf("server.org ssh-rsa %s", base64.StdEncoding.EncodeToString(edKey.Marshal()))
+
+	db := newHostKeyDB()
+	err := db.Read(bytes.NewBufferString(line), "testdb")
+	if err == nil {
+		t.Fatal("Read succeeded on line with key type mismatch, expected error")
+	}
+
+	expectedErr := `knownhosts: key type mismatch: found "ssh-ed25519", want "ssh-rsa"`
+	if !strings.Contains(err.Error(), expectedErr) {
+		t.Fatalf("got error %q, want to contain %q", err.Error(), expectedErr)
+	}
+}
+
+func TestMultipleMarkers(t *testing.T) {
+	lineDouble := fmt.Sprintf("@cert-authority @revoked server.org %s %s", edKey.Type(), base64.StdEncoding.EncodeToString(edKey.Marshal()))
+	db := newHostKeyDB()
+	err := db.Read(bytes.NewBufferString(lineDouble), "testdb")
+	if err == nil {
+		t.Fatal("Read succeeded on line with multiple markers, expected error")
+	}
+	expectedError := "unexpected marker"
+	if !strings.Contains(err.Error(), expectedError) {
+		t.Errorf("got error %q, want it to contain %q", err.Error(), expectedError)
+	}
+
+	lineUnknown := fmt.Sprintf("@unknown-marker server.org %s %s", edKey.Type(), base64.StdEncoding.EncodeToString(edKey.Marshal()))
+	err = db.Read(bytes.NewBufferString(lineUnknown), "testdb")
+	if err == nil {
+		t.Fatal("Read succeeded on line with unknown marker, expected error")
+	}
+	if !strings.Contains(err.Error(), expectedError) {
+		t.Errorf("got error %q, want it to contain %q", err.Error(), expectedError)
+	}
+}
+
+func TestUnknownMarker(t *testing.T) {
+	line := fmt.Sprintf("@unknown-marker server.org %s %s", edKey.Type(), base64.StdEncoding.EncodeToString(edKey.Marshal()))
+
+	db := newHostKeyDB()
+	err := db.Read(bytes.NewBufferString(line), "testdb")
+
+	if err == nil {
+		t.Fatal("Read succeeded on line with unknown marker, expected error")
+	}
+
+	expectedError := "unexpected marker"
+	if !strings.Contains(err.Error(), expectedError) {
+		t.Errorf("got error %q, want it to contain %q", err.Error(), expectedError)
+	}
+}

ssh/mux.go

@@ -155,7 +155,10 @@ func (m *mux) SendRequest(name string, wantReply bool, payload []byte) (bool, []
 	drain:
 		for {
 			select {
-			case <-m.globalResponses:
+			case _, ok := <-m.globalResponses:
+				if !ok {
+					break drain
+				}
 			default:
 				break drain
 			}

ssh/mux_test.go

@@ -10,6 +10,7 @@ import (
 	"io"
 	"sync"
 	"testing"
+	"time"
 )
 
 func muxPair() (*mux, *mux) {
@@ -1366,3 +1367,79 @@ func TestChannelResponseAcceptedWhilePending(t *testing.T) {
 	writer.Close()
 	<-serverDone
 }
+
+func TestChannelSendRequestAfterCloseNoSpinloop(t *testing.T) {
+	writer, reader, mux := channelPair(t)
+	defer writer.Close()
+	defer mux.Close()
+
+	// Close the channel, this closes reader.msg.
+	if err := reader.Close(); err != nil {
+		t.Fatalf("Close: %v", err)
+	}
+	// Wait for reader.msg to actually be closed by the mux loop.
+	for {
+		select {
+		case _, ok := <-reader.msg:
+			if !ok {
+				goto closed
+			}
+		case <-time.After(2 * time.Second):
+			t.Fatal("reader.msg was not closed")
+		}
+	}
+closed:
+
+	done := make(chan error, 1)
+	go func() {
+		_, err := reader.SendRequest("ping", true, nil)
+		done <- err
+	}()
+
+	select {
+	case err := <-done:
+		if !errors.Is(err, io.EOF) {
+			t.Fatalf("SendRequest after close: got %v, want io.EOF", err)
+		}
+	case <-time.After(2 * time.Second):
+		t.Fatal("SendRequest spinloop on closed ch.msg")
+	}
+}
+
+func TestMuxSendRequestAfterCloseNoSpinloop(t *testing.T) {
+	a, b := muxPair()
+	defer a.Close()
+
+	// Closing the underlying conn causes mux.loop to exit and close
+	// b.globalResponses.
+	if err := b.Close(); err != nil {
+		t.Fatalf("Close: %v", err)
+	}
+	// Wait for globalResponses to actually be closed by loop().
+	for {
+		select {
+		case _, ok := <-b.globalResponses:
+			if !ok {
+				goto closed
+			}
+		case <-time.After(2 * time.Second):
+			t.Fatal("globalResponses was not closed")
+		}
+	}
+closed:
+
+	done := make(chan error, 1)
+	go func() {
+		_, _, err := b.SendRequest("ping", true, nil)
+		done <- err
+	}()
+
+	select {
+	case err := <-done:
+		if err == nil {
+			t.Fatal("SendRequest after close: got nil error")
+		}
+	case <-time.After(2 * time.Second):
+		t.Fatal("SendRequest spinloop on closed globalResponses")
+	}
+}

ssh/server.go

@@ -54,6 +54,9 @@ type Permissions struct {
 	ExtraData map[any]any
 }
 
+// GSSAPIWithMICConfig includes the server callbacks for gssapi-with-mic
+// authentication. If either field is nil, gssapi-with-mic is considered not
+// configured.
 type GSSAPIWithMICConfig struct {
 	// AllowLogin, must be set, is called when gssapi-with-mic
 	// authentication is selected (RFC 4462 section 3). The srcName is from the
@@ -68,6 +71,10 @@ type GSSAPIWithMICConfig struct {
 	Server GSSAPIServer
 }
 
+func gssapiWithMICConfigured(config *GSSAPIWithMICConfig) bool {
+	return config != nil && config.AllowLogin != nil && config.Server != nil
+}
+
 // SendAuthBanner implements [ServerPreAuthConn].
 func (s *connection) SendAuthBanner(msg string) error {
 	return s.transport.writePacket(Marshal(&userAuthBannerMsg{
@@ -382,8 +389,7 @@ func (s *connection) serverHandshake(config *ServerConfig) (*Permissions, error)
 	}
 
 	if !config.NoClientAuth && config.PasswordCallback == nil && config.PublicKeyCallback == nil &&
-		config.KeyboardInteractiveCallback == nil && (config.GSSAPIWithMICConfig == nil ||
-		config.GSSAPIWithMICConfig.AllowLogin == nil || config.GSSAPIWithMICConfig.Server == nil) {
+		config.KeyboardInteractiveCallback == nil && !gssapiWithMICConfigured(config.GSSAPIWithMICConfig) {
 		return nil, errors.New("ssh: no authentication methods configured but NoClientAuth is also false")
 	}
 
@@ -607,6 +613,15 @@ func (b *BannerError) Error() string {
 	return b.Err.Error()
 }
 
+// maxAuthServerAttempts caps the total number of SSH_MSG_USERAUTH_REQUEST
+// messages the server will process on a single connection, regardless of
+// outcome (failure, partial success, public key query, or none). It is a
+// backstop against clients that drive the authentication loop indefinitely
+// without ever incurring a real failure — for example by repeatedly
+// triggering PartialSuccessError or by spamming public key offer queries —
+// neither of which increment the MaxAuthTries failure counter.
+const maxAuthServerAttempts = 128
+
 func (s *connection) serverAuthenticate(config *ServerConfig) (*Permissions, error) {
 	if config.PreAuthConnCallback != nil {
 		config.PreAuthConnCallback(s)
@@ -617,6 +632,7 @@ func (s *connection) serverAuthenticate(config *ServerConfig) (*Permissions, err
 	var perms *Permissions
 
 	authFailures := 0
+	authAttempts := 0
 	noneAuthCount := 0
 	var authErrs []error
 	var calledBannerCallback bool
@@ -645,6 +661,19 @@ userAuthLoop:
 			return nil, &ServerAuthError{Errors: authErrs}
 		}
 
+		if authAttempts >= maxAuthServerAttempts {
+			discMsg := &disconnectMsg{
+				Reason:  2,
+				Message: "too many authentication attempts",
+			}
+			if err := s.transport.writePacket(Marshal(discMsg)); err != nil {
+				return nil, err
+			}
+			authErrs = append(authErrs, discMsg)
+			return nil, &ServerAuthError{Errors: authErrs}
+		}
+		authAttempts++
+
 		var userAuthReq userAuthRequestMsg
 		if packet, err := s.transport.readPacket(); err != nil {
 			if err == io.EOF {
@@ -846,7 +875,7 @@ userAuthLoop:
 				}
 			}
 		case "gssapi-with-mic":
-			if authConfig.GSSAPIWithMICConfig == nil {
+			if !gssapiWithMICConfigured(authConfig.GSSAPIWithMICConfig) {
 				authErr = errors.New("ssh: gssapi-with-mic auth not configured")
 				break
 			}
@@ -979,8 +1008,7 @@ userAuthLoop:
 		if authConfig.KeyboardInteractiveCallback != nil {
 			failureMsg.Methods = append(failureMsg.Methods, "keyboard-interactive")
 		}
-		if authConfig.GSSAPIWithMICConfig != nil && authConfig.GSSAPIWithMICConfig.Server != nil &&
-			authConfig.GSSAPIWithMICConfig.AllowLogin != nil {
+		if gssapiWithMICConfigured(authConfig.GSSAPIWithMICConfig) {
 			failureMsg.Methods = append(failureMsg.Methods, "gssapi-with-mic")
 		}
 

ssh/server_test.go

@@ -16,6 +16,7 @@ import (
 	"math/big"
 	"net"
 	"reflect"
+	"slices"
 	"strings"
 	"sync/atomic"
 	"testing"
@@ -165,6 +166,52 @@ func TestMaxAuthTriesNoneMethod(t *testing.T) {
 	}
 }
 
+func TestMaxAuthServerAttempts(t *testing.T) {
+	c1, c2, err := netPipe()
+	if err != nil {
+		t.Fatalf("netPipe: %v", err)
+	}
+	defer c1.Close()
+	defer c2.Close()
+
+	invocations := 0
+	clientConf := &ClientConfig{
+		User:            "user",
+		HostKeyCallback: InsecureIgnoreHostKey(),
+		AuthCallback: func(ctx *ClientAuthContext) (AuthMethod, error) {
+			invocations++
+			return PublicKeys(testSigners["rsa"]), nil
+		},
+	}
+
+	go NewServerConn(c1, alwaysPartialPubKeyServer())
+
+	_, _, _, err = NewClientConn(c2, "", clientConf)
+	if err == nil {
+		t.Fatal("expected the server to disconnect after exceeding the attempts cap")
+	}
+	// The error must be the server's disconnect, not the client-side
+	// "too many authentication attempts (N), aborting" bound. The
+	// disconnect format is set by disconnectMsg.Error().
+	if !strings.Contains(err.Error(), "ssh: disconnect") ||
+		!strings.Contains(err.Error(), "too many authentication attempts") {
+		t.Fatalf("expected server disconnect, got: %v", err)
+	}
+	// Server-side request sequence per AuthCallback iteration:
+	//   1 "none"       (initial, before any AuthCallback)
+	//   2 pubkey query (AuthCallback #k, server returns OK)
+	//   3 pubkey signed (AuthCallback #k, server returns partial)
+	// So after k complete callbacks the server has processed 1+2k requests.
+	// The (k+1)-th callback's query is processed as request number 2(k+1);
+	// the subsequent signed request is rejected by the cap check
+	// (authAttempts >= maxAuthServerAttempts), so 2(k+1) = maxAuthServerAttempts
+	// and invocations = maxAuthServerAttempts / 2.
+	expected := maxAuthServerAttempts / 2
+	if invocations != expected {
+		t.Errorf("AuthCallback invoked %d times; want %d", invocations, expected)
+	}
+}
+
 func TestMaxAuthTriesFirstNoneAuthErrorIgnored(t *testing.T) {
 	username := "testuser"
 	serverConfig := &ServerConfig{
@@ -531,12 +578,18 @@ func TestVerifiedPublicCallbackPartialSuccess(t *testing.T) {
 	serverConf.AddHostKey(testSigners["rsa"])
 
 	clientConf := ClientConfig{
-		User: "user",
-		Auth: []AuthMethod{
-			PublicKeys(testSigners["rsa"]),
-			Password(clientPassword),
-		},
+		User:            "user",
+		Auth:            nil,
 		HostKeyCallback: InsecureIgnoreHostKey(),
+		AuthCallback: func(ctx *ClientAuthContext) (AuthMethod, error) {
+			if slices.Contains(ctx.AllowedMethods, "publickey") && !slices.Contains(ctx.PartialSuccessMethods, "publickey") {
+				return PublicKeys(testSigners["rsa"]), nil
+			}
+			if slices.Contains(ctx.AllowedMethods, "password") && slices.Contains(ctx.PartialSuccessMethods, "publickey") {
+				return Password(clientPassword), nil
+			}
+			return nil, nil
+		},
 	}
 
 	go NewServerConn(c1, serverConf)

ssh/session.go

@@ -423,6 +423,9 @@ func (s *Session) wait(reqs <-chan *Request) error {
 	for msg := range reqs {
 		switch msg.Type {
 		case "exit-status":
+			if len(msg.Payload) < 4 {
+				return errors.New("ssh: malformed exit-status request")
+			}
 			wm.status = int(binary.BigEndian.Uint32(msg.Payload))
 		case "exit-signal":
 			var sigval struct {

ssh/session_test.go

@@ -334,6 +334,25 @@ func TestExitWithoutStatusOrSignal(t *testing.T) {
 	}
 }
 
+func TestExitStatusShortPayload(t *testing.T) {
+	for _, payload := range [][]byte{nil, {0}, {0, 0}, {0, 0, 0}} {
+		conn := dial(exitStatusShortPayloadHandler(payload), t)
+		session, err := conn.NewSession()
+		if err != nil {
+			t.Fatalf("unable to request new session: %v", err)
+		}
+		if err := session.Shell(); err != nil {
+			t.Fatalf("unable to execute command: %v", err)
+		}
+		err = session.Wait()
+		if err == nil {
+			t.Fatalf("expected error for payload length %d", len(payload))
+		}
+		session.Close()
+		conn.Close()
+	}
+}
+
 // windowTestBytes is the number of bytes that we'll send to the SSH server.
 const windowTestBytes = 16000 * 200
 
@@ -482,6 +501,17 @@ func exitWithoutSignalOrStatus(ch Channel, in <-chan *Request, t *testing.T) {
 	readLine(shell, t)
 }
 
+func exitStatusShortPayloadHandler(payload []byte) serverType {
+	return func(ch Channel, in <-chan *Request, t *testing.T) {
+		defer ch.Close()
+		shell := newServerShell(ch, in, "> ")
+		readLine(shell, t)
+		if _, err := ch.SendRequest("exit-status", false, payload); err != nil {
+			t.Errorf("unable to send status: %v", err)
+		}
+	}
+}
+
 func shellHandler(ch Channel, in <-chan *Request, t *testing.T) {
 	defer ch.Close()
 	// this string is returned to stdout

ssh/test/sshcli_test.go

@@ -7,11 +7,13 @@ package test
 import (
 	"bytes"
 	"fmt"
+	"net"
 	"os"
 	"os/exec"
 	"path/filepath"
 	"runtime"
 	"testing"
+	"time"
 
 	"golang.org/x/crypto/internal/testenv"
 	"golang.org/x/crypto/ssh"
@@ -88,7 +90,8 @@ func TestSSHCLIAuth(t *testing.T) {
 	}
 
 	// test public key authentication.
-	cmd := testenv.Command(t, sshCLI, "-vvv", "-i", keyPrivPath, "-o", "StrictHostKeyChecking=no",
+	cmd := testenv.Command(t, sshCLI, "-F", "none", "-vvv", "-i", keyPrivPath,
+		"-o", "StrictHostKeyChecking=no", "-o", "IdentityAgent=none",
 		"-p", port, "testpubkey@127.0.0.1", "true")
 	out, err := cmd.CombinedOutput()
 	if err != nil {
@@ -97,7 +100,8 @@ func TestSSHCLIAuth(t *testing.T) {
 	// Test SSH user certificate authentication.
 	// The username must match one of the principals included in the certificate.
 	// The certificate "rsa-user-testcertificate" has "testcertificate" as principal.
-	cmd = testenv.Command(t, sshCLI, "-vvv", "-i", keyPrivPath, "-o", "StrictHostKeyChecking=no",
+	cmd = testenv.Command(t, sshCLI, "-F", "none", "-vvv", "-i", keyPrivPath,
+		"-o", "StrictHostKeyChecking=no", "-o", "IdentityAgent=none",
 		"-p", port, "testcertificate@127.0.0.1", "true")
 	out, err = cmd.CombinedOutput()
 	if err != nil {
@@ -119,7 +123,7 @@ func TestSSHCLIKeyExchanges(t *testing.T) {
 	keyExchanges := append(ssh.SupportedAlgorithms().KeyExchanges, ssh.InsecureAlgorithms().KeyExchanges...)
 	for _, kex := range keyExchanges {
 		t.Run(kex, func(t *testing.T) {
-			cmd := testenv.Command(t, sshCLI, "-Q", "kex")
+			cmd := testenv.Command(t, sshCLI, "-F", "none", "-Q", "kex")
 			out, err := cmd.CombinedOutput()
 			if err != nil {
 				t.Fatalf("%s failed to check if the KEX is supported, error: %v, command output %q", kex, err, string(out))
@@ -152,7 +156,8 @@ func TestSSHCLIKeyExchanges(t *testing.T) {
 				t.Fatalf("unable to get server port: %v", err)
 			}
 
-			cmd = testenv.Command(t, sshCLI, "-vvv", "-i", keyPrivPath, "-o", "StrictHostKeyChecking=no",
+			cmd = testenv.Command(t, sshCLI, "-F", "none", "-vvv", "-i", keyPrivPath,
+				"-o", "StrictHostKeyChecking=no", "-o", "IdentityAgent=none",
 				"-o", fmt.Sprintf("KexAlgorithms=%s", kex), "-p", port, "testpubkey@127.0.0.1", "true")
 			out, err = cmd.CombinedOutput()
 			if err != nil {
@@ -161,3 +166,87 @@ func TestSSHCLIKeyExchanges(t *testing.T) {
 		})
 	}
 }
+
+func TestSSHCLIControlClientConn(t *testing.T) {
+	if runtime.GOOS == "windows" {
+		t.Skipf("always fails on Windows, see #64403")
+	}
+	sshCLI := sshClient(t)
+	keyFiles := map[string][]byte{
+		"rsa":     testdata.PEMBytes["rsa"],
+		"rsa.pub": ssh.MarshalAuthorizedKey(testPublicKeys["rsa"]),
+	}
+	keyPrivPath := setupSSHCLIKeys(t, keyFiles, "rsa")
+
+	config := &ssh.ServerConfig{
+		PublicKeyCallback: func(conn ssh.ConnMetadata, key ssh.PublicKey) (*ssh.Permissions, error) {
+			if conn.User() == "testcontrolproxy" && bytes.Equal(key.Marshal(), testPublicKeys["rsa"].Marshal()) {
+				return nil, nil
+			}
+			return nil, fmt.Errorf("pubkey for %q not acceptable", conn.User())
+		},
+	}
+	config.AddHostKey(testSigners["rsa"])
+
+	server, err := newTestServer(config)
+	if err != nil {
+		t.Fatalf("unable to start test server: %v", err)
+	}
+	defer server.Close()
+
+	port, err := server.port()
+	if err != nil {
+		t.Fatalf("unable to get server port: %v", err)
+	}
+
+	dir, err := os.MkdirTemp("", "controlSocket")
+	if err != nil {
+		t.Fatalf("unable to create temp dir for control socket: %v", err)
+	}
+	defer os.RemoveAll(dir)
+	csPath := filepath.Join(dir, "c")
+	cmd := testenv.Command(t, sshCLI, "-vvv", "-i", keyPrivPath, "-o", "StrictHostKeyChecking=no",
+		"-p", port, "-o", "ControlPath="+csPath, "-o", "ControlMaster=yes", "-N", "testcontrolproxy@127.0.0.1")
+	var output bytes.Buffer
+	cmd.Stdout = &output
+	cmd.Stderr = &output
+	if err := cmd.Start(); err != nil {
+		t.Fatalf("control socket master start failed, error: %v", err)
+	}
+	defer func() {
+		cmd.Process.Kill()
+		cmd.Wait()
+		if t.Failed() {
+			t.Logf("OpenSSH output:\n\n%s", cmd.Stdout)
+		}
+	}()
+	for i := range 10 {
+		if _, err := os.Stat(csPath); err == nil {
+			break
+		} else if !os.IsNotExist(err) {
+			t.Fatalf("unable to stat control socket: %v", err)
+		}
+		time.Sleep((1 << i) * 5 * time.Millisecond)
+	}
+
+	conn, err := net.Dial("unix", csPath)
+	if err != nil {
+		t.Fatalf("unable to open control socket: %v", err)
+	}
+	defer conn.Close()
+	cc, chans, reqs, err := ssh.NewControlClientConn(conn)
+	if err != nil {
+		t.Fatalf("unable to create client: %v", err)
+	}
+	client := ssh.NewClient(cc, chans, reqs)
+	defer client.Close()
+	session, err := client.NewSession()
+	if err != nil {
+		t.Fatalf("unable to create session: %v", err)
+	}
+	defer session.Close()
+	out, err := session.CombinedOutput("true")
+	if err != nil {
+		t.Fatalf("command execution failed, error: %v, command output %q", err, string(out))
+	}
+}