go.mod: update golang.org/x dependencies

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

Change-Id: Ia739869d49c750c7fa578b9dbd7bb998d8c87087
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/781683
Reviewed-by: Roland Shoemaker <roland@golang.org>
Reviewed-by: Nicholas Husin <nsh@golang.org>
Auto-Submit: Gopher Robot <gobot@golang.org>
Reviewed-by: Nicholas Husin <husin@google.com>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Damien Neil <dneil@google.com>

blake2b/blake2b_test.go

@@ -14,6 +14,8 @@ import (
 	"testing"
 )
 
+var _ hash.XOF = (*xof)(nil)
+
 func fromHex(s string) []byte {
 	b, err := hex.DecodeString(s)
 	if err != nil {

blake2b/go125.go

@@ -1,11 +0,0 @@
-// Copyright 2025 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.
-
-//go:build go1.25
-
-package blake2b
-
-import "hash"
-
-var _ hash.XOF = (*xof)(nil)

chacha20poly1305/chacha20poly1305_amd64.go

@@ -20,7 +20,7 @@ func chacha20Poly1305Open(dst []byte, key []uint32, src, ad []byte) bool
 func chacha20Poly1305Seal(dst []byte, key []uint32, src, ad []byte)
 
 var (
-	useAVX2 = cpu.X86.HasAVX2 && cpu.X86.HasBMI2
+	useAVX2 = cpu.X86.HasSSSE3 && cpu.X86.HasAVX2 && cpu.X86.HasBMI2
 )
 
 // setupState writes a ChaCha20 input matrix to state. See
@@ -47,7 +47,7 @@ func setupState(state *[16]uint32, key *[32]byte, nonce []byte) {
 }
 
 func (c *chacha20poly1305) seal(dst, nonce, plaintext, additionalData []byte) []byte {
-	if !cpu.X86.HasSSSE3 {
+	if !useAVX2 {
 		return c.sealGeneric(dst, nonce, plaintext, additionalData)
 	}
 
@@ -66,7 +66,7 @@ func (c *chacha20poly1305) seal(dst, nonce, plaintext, additionalData []byte) []
 }
 
 func (c *chacha20poly1305) open(dst, nonce, ciphertext, additionalData []byte) ([]byte, error) {
-	if !cpu.X86.HasSSSE3 {
+	if !useAVX2 {
 		return c.openGeneric(dst, nonce, ciphertext, additionalData)
 	}
 

go.mod

@@ -3,8 +3,8 @@ module golang.org/x/crypto
 go 1.25.0
 
 require (
-	golang.org/x/net v0.53.0 // tagx:ignore
-	golang.org/x/sys v0.44.0
+	golang.org/x/net v0.54.0 // tagx:ignore
+	golang.org/x/sys v0.45.0
 	golang.org/x/term v0.43.0
 )
 

go.sum

@@ -1,7 +1,7 @@
-golang.org/x/net v0.53.0 h1:d+qAbo5L0orcWAr0a9JweQpjXF19LMXJE8Ey7hwOdUA=
-golang.org/x/net v0.53.0/go.mod h1:JvMuJH7rrdiCfbeHoo3fCQU24Lf5JJwT9W3sJFulfgs=
-golang.org/x/sys v0.44.0 h1:ildZl3J4uzeKP07r2F++Op7E9B29JRUy+a27EibtBTQ=
-golang.org/x/sys v0.44.0/go.mod h1:4GL1E5IUh+htKOUEOaiffhrAeqysfVGipDYzABqnCmw=
+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=

ssh/agent/client.go

@@ -663,6 +663,13 @@ func (c *client) Add(key AddedKey) error {
 		constraints = append(constraints, agentConstrainConfirm)
 	}
 
+	for _, ext := range key.ConstraintExtensions {
+		constraints = append(constraints, ssh.Marshal(constrainExtensionAgentMsg{
+			ExtensionName:    ext.ExtensionName,
+			ExtensionDetails: ext.ExtensionDetails,
+		})...)
+	}
+
 	cert := key.Certificate
 	if cert == nil {
 		return c.insertKey(key.PrivateKey, key.Comment, constraints)

ssh/agent/client_test.go

@@ -594,3 +594,63 @@ func TestAgentExtensions(t *testing.T) {
 		t.Fatal("should have gotten agent extension failure")
 	}
 }
+
+// capturingAgent records the AddedKey observed on the server side of the
+// agent protocol. It forwards to a keyring with constraint fields stripped,
+// so the keyring's own rejection of unsupported constraints does not
+// interfere with what this test is measuring: the client-side wire
+// serialization of ConstraintExtensions.
+type capturingAgent struct {
+	Agent
+	lastAdd AddedKey
+}
+
+func newCapturingAgent() *capturingAgent {
+	return &capturingAgent{Agent: NewKeyring()}
+}
+
+func (a *capturingAgent) Add(key AddedKey) error {
+	a.lastAdd = key
+	stripped := key
+	stripped.ConstraintExtensions = nil
+	stripped.ConfirmBeforeUse = false
+	return a.Agent.Add(stripped)
+}
+
+// TestAddConstraintExtensionsWireFormat verifies that client.Add serializes
+// ConstraintExtensions into the SSH_AGENTC_ADD_IDENTITY payload and the
+// server deserializes them back into the AddedKey delivered to the backend.
+// Regressions in the client marshal loop (missing, swapped fields, wrong
+// framing) would be invisible to a keyring-based rejection test, which
+// signals only "extensions were present", not "the right ones arrived".
+func TestAddConstraintExtensionsWireFormat(t *testing.T) {
+	capturing := newCapturingAgent()
+	client, cleanup := startAgent(t, capturing)
+	defer cleanup()
+
+	constraints := []ConstraintExtension{
+		{ExtensionName: "ext-one@example.com", ExtensionDetails: []byte("details-one")},
+		{ExtensionName: "ext-two@example.com", ExtensionDetails: []byte("\x00\x01\x02\xff")},
+	}
+
+	if err := client.Add(AddedKey{
+		PrivateKey:           testPrivateKeys["rsa"],
+		Comment:              "wire-format-test",
+		ConstraintExtensions: constraints,
+	}); err != nil {
+		t.Fatalf("client.Add: %v", err)
+	}
+
+	got := capturing.lastAdd.ConstraintExtensions
+	if len(got) != len(constraints) {
+		t.Fatalf("server received %d extensions, want %d", len(got), len(constraints))
+	}
+	for i, want := range constraints {
+		if got[i].ExtensionName != want.ExtensionName {
+			t.Errorf("extension[%d] name: got %q, want %q", i, got[i].ExtensionName, want.ExtensionName)
+		}
+		if !bytes.Equal(got[i].ExtensionDetails, want.ExtensionDetails) {
+			t.Errorf("extension[%d] details: got %x, want %x", i, got[i].ExtensionDetails, want.ExtensionDetails)
+		}
+	}
+}

ssh/agent/keyring.go

@@ -32,8 +32,10 @@ type keyring struct {
 
 var errLocked = errors.New("agent: locked")
 
-// NewKeyring returns an Agent that holds keys in memory.  It is safe
-// for concurrent use by multiple goroutines.
+// NewKeyring returns an Agent that holds keys in memory.  It is safe for
+// concurrent use by multiple goroutines.
+//
+// The returned Agent only supports the "lifetime" constraint.
 func NewKeyring() Agent {
 	return &keyring{}
 }
@@ -143,15 +145,26 @@ func (r *keyring) List() ([]*Key, error) {
 	return ids, nil
 }
 
-// Insert adds a private key to the keyring. If a certificate
-// is given, that certificate is added as public key. Note that
-// any constraints given are ignored.
+// Add adds a private key to the keyring. If a certificate is given, that
+// certificate is added as public key.
+//
+// Add returns an error if key contains ConstraintExtensions or
+// ConfirmBeforeUse.
 func (r *keyring) Add(key AddedKey) error {
 	r.mu.Lock()
 	defer r.mu.Unlock()
 	if r.locked {
 		return errLocked
 	}
+
+	if key.ConfirmBeforeUse {
+		return errors.New("agent: confirm before use constraint is not supported")
+	}
+
+	if len(key.ConstraintExtensions) > 0 {
+		return errors.New("agent: constraint extensions are present but not supported")
+	}
+
 	signer, err := ssh.NewSignerFromKey(key.PrivateKey)
 
 	if err != nil {

ssh/agent/keyring_test.go

@@ -120,3 +120,42 @@ func TestAddDuplicateKey(t *testing.T) {
 		t.Fatal("key with the updated comment not found")
 	}
 }
+
+func TestAddKeyWithConstraints(t *testing.T) {
+	// Verifies the keyring refuses keys carrying constraint extensions it
+	// cannot enforce.
+	agent, cleanup := startKeyringAgent(t)
+	defer cleanup()
+
+	constraints := []ConstraintExtension{
+		{
+			ExtensionName:    "extension1",
+			ExtensionDetails: []byte("details1"),
+		},
+	}
+
+	key := testPrivateKeys["rsa"]
+
+	err := agent.Add(AddedKey{
+		PrivateKey:           key,
+		ConstraintExtensions: constraints,
+	})
+	if err == nil {
+		t.Fatal("adding a key with unsupported constraints succeeded")
+	}
+}
+
+func TestAddKeyWithConfirmBeforeUse(t *testing.T) {
+	agent, cleanup := startKeyringAgent(t)
+	defer cleanup()
+
+	key := testPrivateKeys["rsa"]
+
+	err := agent.Add(AddedKey{
+		PrivateKey:       key,
+		ConfirmBeforeUse: true,
+	})
+	if err == nil {
+		t.Fatal("adding a key with confirm before use constraint succeeded")
+	}
+}

ssh/agent/server.go

@@ -270,6 +270,9 @@ func parseEd25519Key(req []byte) (*AddedKey, error) {
 	if err := ssh.Unmarshal(req, &k); err != nil {
 		return nil, err
 	}
+	if len(k.Priv) != ed25519.PrivateKeySize {
+		return nil, fmt.Errorf("agent: bad ED25519 key size: %d", len(k.Priv))
+	}
 	priv := ed25519.PrivateKey(k.Priv)
 
 	addedKey := &AddedKey{PrivateKey: &priv, Comment: k.Comments}
@@ -336,6 +339,9 @@ func parseEd25519Cert(req []byte) (*AddedKey, error) {
 	if err != nil {
 		return nil, err
 	}
+	if len(k.Priv) != ed25519.PrivateKeySize {
+		return nil, fmt.Errorf("agent: bad ED25519 key size: %d", len(k.Priv))
+	}
 	priv := ed25519.PrivateKey(k.Priv)
 	cert, ok := pubKey.(*ssh.Certificate)
 	if !ok {

ssh/agent/server_test.go

@@ -271,3 +271,35 @@ func TestParseConstraints(t *testing.T) {
 		t.Errorf("unexpected error: %v", err)
 	}
 }
+
+func TestParseEd25519KeyShortPanic(t *testing.T) {
+	msg := ssh.Marshal(ed25519KeyMsg{
+		Type: ssh.KeyAlgoED25519,
+		Pub:  []byte{1, 2, 3},
+		Priv: []byte{1, 2, 3, 4, 5},
+	})
+
+	a, b, err := netPipe()
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer a.Close()
+	defer b.Close()
+
+	done := make(chan error, 1)
+	go func() { done <- ServeAgent(NewKeyring(), a) }()
+
+	c := NewClient(b)
+	_, err = c.(*client).call(msg)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	keys, err := c.List()
+	if err != nil {
+		t.Fatalf("agent died: %v", err)
+	}
+	if len(keys) != 0 {
+		t.Error("short ed25519 key was accepted into keyring")
+	}
+}

ssh/certs.go

@@ -348,6 +348,9 @@ func (c *CertChecker) CheckHostKey(addr string, remote net.Addr, key PublicKey)
 	if cert.CertType != HostCert {
 		return fmt.Errorf("ssh: certificate presented as a host key has type %d", cert.CertType)
 	}
+	if c.IsHostAuthority == nil {
+		return errors.New("ssh: cannot verify certificate, IsHostAuthority not set")
+	}
 	if !c.IsHostAuthority(cert.SignatureKey, addr) {
 		return fmt.Errorf("ssh: no authorities for hostname: %v", addr)
 	}
@@ -375,6 +378,9 @@ func (c *CertChecker) Authenticate(conn ConnMetadata, pubKey PublicKey) (*Permis
 	if cert.CertType != UserCert {
 		return nil, fmt.Errorf("ssh: cert has type %d", cert.CertType)
 	}
+	if c.IsUserAuthority == nil {
+		return nil, errors.New("ssh: cannot verify certificate, IsUserAuthority not set")
+	}
 	if !c.IsUserAuthority(cert.SignatureKey) {
 		return nil, fmt.Errorf("ssh: certificate signed by unrecognized authority")
 	}
@@ -438,7 +444,17 @@ func (c *CertChecker) CheckCert(principal string, cert *Certificate) error {
 	if before := int64(cert.ValidBefore); cert.ValidBefore != uint64(CertTimeInfinity) && (unixNow >= before || before < 0) {
 		return fmt.Errorf("ssh: cert has expired")
 	}
-	if err := cert.SignatureKey.Verify(cert.bytesForSigning(), cert.Signature); err != nil {
+	// Match OpenSSH: the SK user-presence flag is never enforced on a
+	// certificate's CA signature. OpenSSH calls sshkey_verify with
+	// detailsp==NULL in sshkey.c:cert_parse, so the UP/UV flags are
+	// not even extracted. The UP bit on a CA signature reflects the
+	// CA operator's presence at signing time, which has no bearing on
+	// whether the user being authenticated is present now; enforcing
+	// it here would only break interop with certificates issued by
+	// non-interactive SK CAs. skKeyWithoutUP is a no-op for non-SK
+	// keys (the common case).
+	caKey := skKeyWithoutUP(cert.SignatureKey)
+	if err := caKey.Verify(cert.bytesForSigning(), cert.Signature); err != nil {
 		return fmt.Errorf("ssh: certificate signature does not verify")
 	}
 

ssh/certs_test.go

@@ -9,8 +9,10 @@ import (
 	"crypto/ecdsa"
 	"crypto/elliptic"
 	"crypto/rand"
+	"crypto/sha256"
 	"fmt"
 	"io"
+	"math/big"
 	"net"
 	"reflect"
 	"testing"
@@ -128,6 +130,103 @@ func TestValidateCert(t *testing.T) {
 	}
 }
 
+// signSKCert builds an SK-ECDSA signature over cert.bytesForSigning()
+// using caKey as the simulated hardware token. The SK signing flags
+// byte (UP bit and others) is caller-controlled so tests can exercise
+// the user-presence enforcement paths. caApp is the SK application
+// string (typically "ssh:").
+func signSKCert(t *testing.T, cert *Certificate, caKey *ecdsa.PrivateKey, caApp string, flags byte) {
+	t.Helper()
+	cert.SignatureKey = &skECDSAPublicKey{
+		application: caApp,
+		PublicKey:   caKey.PublicKey,
+	}
+	cert.Nonce = make([]byte, 32)
+	if _, err := rand.Read(cert.Nonce); err != nil {
+		t.Fatal(err)
+	}
+
+	h := sha256.New()
+	h.Write([]byte(caApp))
+	appDigest := h.Sum(nil)
+	h.Reset()
+	h.Write(cert.bytesForSigning())
+	dataDigest := h.Sum(nil)
+
+	var counter uint32 = 1
+	blob := struct {
+		ApplicationDigest []byte `ssh:"rest"`
+		Flags             byte
+		Counter           uint32
+		MessageDigest     []byte `ssh:"rest"`
+	}{appDigest, flags, counter, dataDigest}
+	h.Reset()
+	h.Write(Marshal(blob))
+	digest := h.Sum(nil)
+
+	r, s, err := ecdsa.Sign(rand.Reader, caKey, digest)
+	if err != nil {
+		t.Fatal(err)
+	}
+	cert.Signature = &Signature{
+		Format: KeyAlgoSKECDSA256,
+		Blob:   Marshal(struct{ R, S *big.Int }{r, s}),
+		Rest: Marshal(struct {
+			Flags   byte
+			Counter uint32
+		}{flags, counter}),
+	}
+}
+
+// TestCheckCertSKAuthorityNoUPRequired pins the OpenSSH-parity
+// behavior of CertChecker.CheckCert for SK CA signatures: a
+// certificate signed by an SK CA that produced a UP=0 signature is
+// accepted, because the UP bit on a CA signature has no bearing on
+// the current user authentication (see sshkey.c:cert_parse, which
+// passes detailsp==NULL to sshkey_verify). Without this, certs issued
+// by non-interactive SK CAs would fail to verify.
+func TestCheckCertSKAuthorityNoUPRequired(t *testing.T) {
+	caKey, err := ecdsa.GenerateKey(elliptic.P256(), rand.Reader)
+	if err != nil {
+		t.Fatal(err)
+	}
+	userKey, err := ecdsa.GenerateKey(elliptic.P256(), rand.Reader)
+	if err != nil {
+		t.Fatal(err)
+	}
+	userPub, err := NewPublicKey(&userKey.PublicKey)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	mkCert := func(flags byte) *Certificate {
+		c := &Certificate{
+			CertType:        UserCert,
+			Key:             userPub,
+			ValidBefore:     CertTimeInfinity,
+			ValidPrincipals: []string{"user"},
+		}
+		signSKCert(t, c, caKey, "ssh:", flags)
+		return c
+	}
+
+	checker := CertChecker{
+		IsUserAuthority: func(k PublicKey) bool {
+			caSK := &skECDSAPublicKey{application: "ssh:", PublicKey: caKey.PublicKey}
+			return bytes.Equal(k.Marshal(), caSK.Marshal())
+		},
+	}
+
+	// Both UP=0 and UP=1 CA signatures verify: UP is not enforced
+	// here, matching OpenSSH.
+	if err := checker.CheckCert("user", mkCert(0)); err != nil {
+		t.Errorf("UP=0 CA signature must verify (OpenSSH parity): %v", err)
+	}
+	if err := checker.CheckCert("user", mkCert(flagUserPresence)); err != nil {
+		t.Errorf("UP=1 CA signature must verify: %v", err)
+	}
+}
+
 func TestValidateCertTime(t *testing.T) {
 	cert := Certificate{
 		ValidPrincipals: []string{"user"},

ssh/channel.go

@@ -11,6 +11,7 @@ import (
 	"io"
 	"log"
 	"sync"
+	"sync/atomic"
 )
 
 const (
@@ -131,11 +132,17 @@ func (r RejectionReason) String() string {
 	return fmt.Sprintf("unknown reason %d", int(r))
 }
 
-func min(a uint32, b int) uint32 {
-	if a < uint32(b) {
-		return a
+// minPayloadSize returns min(limit, length) clamped to a uint32. It is used
+// to compute the size of the next channel data packet from the remaining
+// payload. The comparison is done in int64 because length is an int — on
+// 64-bit systems len(data) can exceed 2^32, and a direct uint32(length)
+// cast would silently truncate to 0 at every multiple of 2^32, causing
+// WriteExtended's loop to spin without making progress.
+func minPayloadSize(limit uint32, length int) uint32 {
+	if int64(length) > int64(limit) {
+		return limit
 	}
-	return uint32(b)
+	return uint32(length)
 }
 
 type channelDirection uint8
@@ -177,6 +184,12 @@ type channel struct {
 	// with WantReply=true outstanding.  This lock is held by a
 	// goroutine that has such an outgoing request pending.
 	sentRequestMu sync.Mutex
+	// sentRequestPending is set to true while a SendRequest call with
+	// WantReply=true is in flight. handlePacket uses it as a gate: responses
+	// arriving while no request is pending are dropped to prevent a
+	// misbehaving peer from stalling the mux read loop by filling ch.msg
+	// with unsolicited channelRequestSuccess/Failure messages.
+	sentRequestPending atomic.Bool
 
 	incomingRequests chan *Request
 
@@ -251,7 +264,7 @@ func (ch *channel) WriteExtended(data []byte, extendedCode uint32) (n int, err e
 	ch.writeMu.Unlock()
 
 	for len(data) > 0 {
-		space := min(ch.maxRemotePayload, len(data))
+		space := minPayloadSize(ch.maxRemotePayload, len(data))
 		if space, err = ch.remoteWin.reserve(space); err != nil {
 			return n, err
 		}
@@ -460,6 +473,18 @@ func (ch *channel) handlePacket(packet []byte) error {
 		}
 
 		ch.incomingRequests <- &req
+	case *channelRequestSuccessMsg, *channelRequestFailureMsg:
+		// Drop responses that arrive when no SendRequest is waiting, to
+		// prevent a malicious peer from filling ch.msg and stalling the
+		// mux read loop. The non-blocking send additionally protects the
+		// loop if a well-behaved caller is slow to read.
+		if !ch.sentRequestPending.Load() {
+			return nil
+		}
+		select {
+		case ch.msg <- msg:
+		default:
+		}
 	default:
 		ch.msg <- msg
 	}
@@ -530,7 +555,17 @@ func (ch *channel) Reject(reason RejectionReason, message string) error {
 		Language: "en",
 	}
 	ch.decided = true
-	return ch.sendMessage(reject)
+	err := ch.sendMessage(reject)
+
+	// Remove the channel from the mux to prevent memory leaks.
+	// Do not call ch.close() here: no goroutine holds a reference to a
+	// rejected channel's internal channels (msg, incomingRequests), so
+	// removing it from chanList is sufficient for GC. Calling close()
+	// would race with the mux loop goroutine (handlePacket or dropAll),
+	// causing a panic from closing an already-closed channel.
+	ch.mux.chanList.remove(ch.localId)
+
+	return err
 }
 
 func (ch *channel) Read(data []byte) (int, error) {
@@ -586,6 +621,24 @@ func (ch *channel) SendRequest(name string, wantReply bool, payload []byte) (boo
 	if wantReply {
 		ch.sentRequestMu.Lock()
 		defer ch.sentRequestMu.Unlock()
+
+		// Open the gate so that responses arriving while this request is in
+		// flight are allowed to reach ch.msg. Responses arriving while no
+		// request is pending are dropped by handlePacket.
+		ch.sentRequestPending.Store(true)
+		defer ch.sentRequestPending.Store(false)
+
+		// Drain any spurious responses that may have been buffered. This
+		// prevents a previously buffered unexpected response from being
+		// consumed instead of the actual response for this request.
+	drain:
+		for {
+			select {
+			case <-ch.msg:
+			default:
+				break drain
+			}
+		}
 	}
 
 	msg := channelRequestMsg{

ssh/channel_test.go

@@ -0,0 +1,140 @@
+// Copyright 2025 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 (
+	"math/bits"
+	"testing"
+	"time"
+	"unsafe"
+)
+
+func TestMinPayloadSize(t *testing.T) {
+	// 4 GiB (2^32). Declared as a var (not a const) so that int(bigPayload)
+	// is a runtime conversion: a constant conversion would fail to compile
+	// on 32-bit platforms with "constant 4294967296 overflows int". On
+	// 32-bit the value truncates to 0 at runtime, but the is64Bit cases
+	// that reference it are skipped by the runtime check below.
+	var bigPayload int64 = 1 << 32
+
+	tests := []struct {
+		name       string
+		maxPayload uint32
+		dataLen    int
+		want       uint32
+		is64Bit    bool // Flag to run only on 64-bit architectures
+	}{
+		{
+			name:       "Normal Case - Data fits in payload",
+			maxPayload: 32768,
+			dataLen:    1000,
+			want:       1000,
+		},
+		{
+			name:       "Normal Case - Data larger than payload",
+			maxPayload: 32768,
+			dataLen:    50000,
+			want:       32768,
+		},
+		{
+			name:       "Boundary Case - Data zero",
+			maxPayload: 32768,
+			dataLen:    0,
+			want:       0,
+		},
+		{
+			name:       "Overflow Case - Data is exactly 4GB (1<<32)",
+			maxPayload: 32768,
+			dataLen:    int(bigPayload),
+			want:       32768,
+			is64Bit:    true,
+		},
+		{
+			name:       "Overflow Case - Data is 4GB + small amount",
+			maxPayload: 32768,
+			dataLen:    int(bigPayload + 100),
+			want:       32768,
+			is64Bit:    true,
+		},
+	}
+
+	is64Bit := bits.UintSize == 64
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if tt.is64Bit && !is64Bit {
+				t.Skip("Skipping test requiring 64-bit int")
+			}
+			got := minPayloadSize(tt.maxPayload, tt.dataLen)
+			if got != tt.want {
+				t.Errorf("minPayloadSize(%d, %d) = %d; want %d", tt.maxPayload, tt.dataLen, got, tt.want)
+			}
+		})
+	}
+}
+
+// TestWriteExtendedNoInfiniteLoopOnLargeWrite is an end-to-end regression
+// test for the integer-overflow bug in WriteExtended. Before the fix, a
+// write whose len(data) was a multiple of 2^32 caused minPayloadSize to
+// return 0; WriteExtended then spun forever, reserving 0 bytes per
+// iteration and never advancing the data slice.
+//
+// We exercise the real WriteExtended path with a slice whose declared
+// length is exactly 2^32. Allocating 4 GiB is unnecessary: each iteration
+// only reads up to maxRemotePayload bytes from the head of the slice, and
+// the loop blocks in remoteWin.reserve() once the channel window is
+// exhausted — before the slice base advances past the underlying buffer.
+//
+// With the fix, the loop blocks in reserve(); we detect that via
+// waitWriterBlocked(), then close the window to let WriteExtended return.
+// With the bug, the loop never blocks and the test times out.
+//
+//go:nocheckptr
+func TestWriteExtendedNoInfiniteLoopOnLargeWrite(t *testing.T) {
+	if bits.UintSize < 64 {
+		t.Skip("test requires 64-bit int to construct a slice with len >= 2^32")
+	}
+
+	reader, writer, mux := channelPair(t)
+	defer reader.Close()
+	defer writer.Close()
+	defer mux.Close()
+
+	// Sized to hold the full pre-update remote window so that no iteration
+	// reads past the backing buffer before reserve() blocks.
+	backing := make([]byte, channelWindowSize)
+	var bigLen int64 = 1 << 32
+	bigSlice := unsafe.Slice(&backing[0], int(bigLen))
+
+	done := make(chan int, 1)
+	go func() {
+		n, _ := writer.Write(bigSlice)
+		done <- n
+	}()
+
+	blocked := make(chan struct{})
+	go func() {
+		writer.remoteWin.waitWriterBlocked()
+		close(blocked)
+	}()
+
+	select {
+	case <-blocked:
+		// Good — the loop made progress and is now blocked in reserve().
+		// Close the window to let WriteExtended return.
+		writer.remoteWin.close()
+	case <-time.After(2 * time.Second):
+		t.Fatal("WriteExtended did not block in reserve within 2s — minPayloadSize likely returned 0 (integer overflow regression)")
+	}
+
+	select {
+	case n := <-done:
+		if n == 0 {
+			t.Fatalf("WriteExtended returned n=0; expected progress")
+		}
+	case <-time.After(2 * time.Second):
+		t.Fatal("WriteExtended did not return after closing the window")
+	}
+}

ssh/cipher.go

@@ -407,7 +407,7 @@ func (c *gcmCipher) readCipherPacket(seqNum uint32, r io.Reader) ([]byte, error)
 		return nil, fmt.Errorf("ssh: illegal padding %d", padding)
 	}
 
-	if int(padding+1) >= len(plain) {
+	if int(padding)+1 >= len(plain) {
 		return nil, fmt.Errorf("ssh: padding %d too large", padding)
 	}
 	plain = plain[1 : length-uint32(padding)]

ssh/keys.go

@@ -469,6 +469,12 @@ func parseRSA(in []byte) (out PublicKey, rest []byte, err error) {
 		return nil, nil, err
 	}
 
+	// 8192 bits is also the maximum RSA key size accepted by crypto/tls for
+	// signature verification:
+	// https://github.com/golang/go/blob/69801b25/src/crypto/tls/handshake_client.go#L1096
+	if w.N.BitLen() > 8192 {
+		return nil, nil, errors.New("ssh: rsa modulus too large")
+	}
 	if w.E.BitLen() > 24 {
 		return nil, nil, errors.New("ssh: exponent too large")
 	}
@@ -574,6 +580,24 @@ func checkDSAParams(param *dsa.Parameters) error {
 		return fmt.Errorf("ssh: unsupported DSA key size %d", l)
 	}
 
+	// FIPS 186-2 specifies that Q must be exactly 160 bits. We must enforce
+	// this to prevent DoS attacks where an attacker sends a huge Q which makes
+	// verification slow.
+	if l := param.Q.BitLen(); l != 160 {
+		return fmt.Errorf("ssh: unsupported DSA sub-prime size %d", l)
+	}
+
+	// The generator G is an element of the group, so it must be strictly less
+	// than the modulus P.
+	if param.G.Cmp(param.P) >= 0 {
+		return errors.New("ssh: DSA generator larger than modulus")
+	}
+
+	// G must be positive.
+	if param.G.Sign() <= 0 {
+		return errors.New("ssh: DSA generator must be positive")
+	}
+
 	return nil
 }
 
@@ -596,6 +620,14 @@ func parseDSA(in []byte) (out PublicKey, rest []byte, err error) {
 		return nil, nil, err
 	}
 
+	// The public value Y must be a non-zero element of the group, i.e.
+	// strictly between 0 and P. crypto/dsa.Verify does not range-check Y,
+	// so we reject out-of-range values here to prevent a maliciously
+	// oversized Y from slowing verification.
+	if w.Y.Sign() <= 0 || w.Y.Cmp(w.P) >= 0 {
+		return nil, nil, errors.New("ssh: DSA public value Y out of range")
+	}
+
 	key := &dsaPublicKey{
 		Parameters: param,
 		Y:          w.Y,
@@ -869,11 +901,25 @@ type skFields struct {
 	Counter uint32
 }
 
+// flagUserPresence is the "user present" bit (UP) in the SK signature
+// flags, matching the FIDO CTAP2 authenticatorData UP flag. See
+// openssh/PROTOCOL.u2f.
+const flagUserPresence = 0x01
+
+// errSKMissingUserPresence is returned by SK key Verify methods when
+// the signature does not assert user presence and the key was not
+// marked as no-touch-required.
+var errSKMissingUserPresence = errors.New("ssh: signature missing required user presence flag")
+
 type skECDSAPublicKey struct {
 	// application is a URL-like string, typically "ssh:" for SSH.
 	// see openssh/PROTOCOL.u2f for details.
 	application string
 	ecdsa.PublicKey
+	// noTouchRequired, when true, disables the default user-presence
+	// check in Verify. It is set by skKeyWithoutUP on a clone of the
+	// key, never on an instance shared across authentication attempts.
+	noTouchRequired bool
 }
 
 func (k *skECDSAPublicKey) Type() string {
@@ -959,6 +1005,10 @@ func (k *skECDSAPublicKey) Verify(data []byte, sig *Signature) error {
 		return err
 	}
 
+	if skf.Flags&flagUserPresence == 0 && !k.noTouchRequired {
+		return errSKMissingUserPresence
+	}
+
 	blob := struct {
 		ApplicationDigest []byte `ssh:"rest"`
 		Flags             byte
@@ -992,6 +1042,10 @@ type skEd25519PublicKey struct {
 	// see openssh/PROTOCOL.u2f for details.
 	application string
 	ed25519.PublicKey
+	// noTouchRequired, when true, disables the default user-presence
+	// check in Verify. It is set by skKeyWithoutUP on a clone of the
+	// key, never on an instance shared across authentication attempts.
+	noTouchRequired bool
 }
 
 func (k *skEd25519PublicKey) Type() string {
@@ -1066,6 +1120,10 @@ func (k *skEd25519PublicKey) Verify(data []byte, sig *Signature) error {
 		return err
 	}
 
+	if skf.Flags&flagUserPresence == 0 && !k.noTouchRequired {
+		return errSKMissingUserPresence
+	}
+
 	blob := struct {
 		ApplicationDigest []byte `ssh:"rest"`
 		Flags             byte

ssh/keys_test.go

@@ -12,6 +12,7 @@ import (
 	"crypto/elliptic"
 	"crypto/rand"
 	"crypto/rsa"
+	"crypto/sha256"
 	"crypto/x509"
 	"encoding/base64"
 	"encoding/hex"
@@ -19,6 +20,7 @@ import (
 	"errors"
 	"fmt"
 	"io"
+	"math/big"
 	"reflect"
 	"strings"
 	"testing"
@@ -216,6 +218,20 @@ func TestParseRSAPrivateKey(t *testing.T) {
 	}
 }
 
+func TestParseRSAModulusTooLarge(t *testing.T) {
+	rsa16384 := "ssh-rsa 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"
+	_, _, _, _, err := ParseAuthorizedKey([]byte(rsa16384))
+
+	if err == nil {
+		t.Fatal("ParseAuthorizedKey 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"]
 
@@ -272,18 +288,18 @@ func TestParseEncryptedPrivateKeysWithPassphrase(t *testing.T) {
 }
 
 func TestParseEncryptedPrivateKeysWithUnsupportedCiphers(t *testing.T) {
-    for _, tt := range testdata.UnsupportedCipherData {
-        t.Run(tt.Name, func(t *testing.T){
-            _, err := ParsePrivateKeyWithPassphrase(tt.PEMBytes, []byte(tt.EncryptionKey))
-            if err == nil {
-                t.Fatalf("expected 'unknown cipher' error for %q, got nil", tt.Name)
-                // If this cipher is now supported, remove it from testdata.UnsupportedCipherData
-            }
-            if !strings.Contains(err.Error(), "unknown cipher") {
-                t.Errorf("wanted 'unknown cipher' error, got %v", err.Error())
-            }
-        })
-    }
+	for _, tt := range testdata.UnsupportedCipherData {
+		t.Run(tt.Name, func(t *testing.T) {
+			_, err := ParsePrivateKeyWithPassphrase(tt.PEMBytes, []byte(tt.EncryptionKey))
+			if err == nil {
+				t.Fatalf("expected 'unknown cipher' error for %q, got nil", tt.Name)
+				// If this cipher is now supported, remove it from testdata.UnsupportedCipherData
+			}
+			if !strings.Contains(err.Error(), "unknown cipher") {
+				t.Errorf("wanted 'unknown cipher' error, got %v", err.Error())
+			}
+		})
+	}
 }
 
 func TestParseEncryptedPrivateKeysWithIncorrectPassphrase(t *testing.T) {
@@ -356,6 +372,72 @@ func TestMarshalParsePublicKey(t *testing.T) {
 	}
 }
 
+func TestParseDSAHugeQ(t *testing.T) {
+	P := new(big.Int).Lsh(big.NewInt(1), 1023)
+	Q := new(big.Int).Lsh(big.NewInt(1), 20000) // very large
+	// G and Y: Dummy values, just needs to be < P to pass that specific check
+	G := big.NewInt(2)
+	Y := big.NewInt(5)
+
+	rawKey := struct {
+		P, Q, G, Y *big.Int
+	}{
+		P: P,
+		Q: Q,
+		G: G,
+		Y: Y,
+	}
+
+	inputBytes := Marshal(&rawKey)
+
+	_, _, err := parseDSA(inputBytes)
+	if err == nil {
+		t.Fatal("parseDSA accepted a DSA key with large Q")
+	}
+
+	expectedError := "ssh: unsupported DSA sub-prime size"
+	if !strings.Contains(err.Error(), expectedError) {
+		t.Errorf("unexpected error message: got %q, want substring %q", err.Error(), expectedError)
+	}
+}
+
+func TestParseDSAYOutOfRange(t *testing.T) {
+	// Valid 1024/160 parameters (values don't need to be a real DSA group,
+	// they only need to pass the checkDSAParams bit-length checks and the
+	// G < P / G > 0 checks).
+	P := new(big.Int).Lsh(big.NewInt(1), 1023)
+	P.SetBit(P, 0, 1) // make P odd so it can pass as a prime candidate shape
+	Q := new(big.Int).Lsh(big.NewInt(1), 159)
+	Q.SetBit(Q, 0, 1)
+	G := big.NewInt(2)
+
+	for _, tc := range []struct {
+		name string
+		Y    *big.Int
+	}{
+		{"Y_zero", big.NewInt(0)},
+		{"Y_negative", big.NewInt(-1)},
+		{"Y_equals_P", new(big.Int).Set(P)},
+		{"Y_greater_than_P", new(big.Int).Add(P, big.NewInt(1))},
+		{"Y_much_greater_than_P", new(big.Int).Lsh(big.NewInt(1), 20000)},
+	} {
+		t.Run(tc.name, func(t *testing.T) {
+			rawKey := struct {
+				P, Q, G, Y *big.Int
+			}{P: P, Q: Q, G: G, Y: tc.Y}
+
+			_, _, err := parseDSA(Marshal(&rawKey))
+			if err == nil {
+				t.Fatalf("parseDSA accepted a DSA key with Y=%s (P=%s)", tc.Y, P)
+			}
+			expectedError := "DSA public value Y out of range"
+			if !strings.Contains(err.Error(), expectedError) {
+				t.Errorf("unexpected error message: got %q, want substring %q", err.Error(), expectedError)
+			}
+		})
+	}
+}
+
 func TestMarshalPrivateKey(t *testing.T) {
 	tests := []struct {
 		name string
@@ -760,6 +842,296 @@ func TestSKKeys(t *testing.T) {
 	}
 }
 
+// skTestHarness builds SK-formatted signatures over a fixed payload
+// using a caller-supplied signing function, letting tests vary the UP
+// flag byte without duplicating the wire-format scaffolding.
+type skTestHarness struct {
+	format      string
+	application string
+	data        []byte
+	// sign takes the SHA-256 digest of the marshalled SK blob and
+	// returns the value to embed in Signature.Blob. For ECDSA keys the
+	// helper feeds the digest to ecdsa.Sign; for ed25519 the helper
+	// feeds the raw marshalled blob to ed25519.Sign.
+	signDigest func(digest []byte) []byte
+	signBlob   func(blob []byte) []byte
+}
+
+func (h skTestHarness) sign(t *testing.T, flags byte) *Signature {
+	t.Helper()
+	hsh := sha256.New()
+	hsh.Write([]byte(h.application))
+	appDigest := hsh.Sum(nil)
+
+	hsh.Reset()
+	hsh.Write(h.data)
+	dataDigest := hsh.Sum(nil)
+
+	var counter uint32 = 1
+	blob := struct {
+		ApplicationDigest []byte `ssh:"rest"`
+		Flags             byte
+		Counter           uint32
+		MessageDigest     []byte `ssh:"rest"`
+	}{appDigest, flags, counter, dataDigest}
+	marshalled := Marshal(blob)
+
+	var sigBlob []byte
+	if h.signDigest != nil {
+		hsh.Reset()
+		hsh.Write(marshalled)
+		sigBlob = h.signDigest(hsh.Sum(nil))
+	} else {
+		sigBlob = h.signBlob(marshalled)
+	}
+
+	return &Signature{
+		Format: h.format,
+		Blob:   sigBlob,
+		Rest: Marshal(struct {
+			Flags   byte
+			Counter uint32
+		}{flags, counter}),
+	}
+}
+
+func TestSKUserPresence(t *testing.T) {
+	ecKey, err := ecdsa.GenerateKey(elliptic.P256(), rand.Reader)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	h := skTestHarness{
+		format:      "sk-ecdsa-sha2-nistp256@openssh.com",
+		application: "ssh:",
+		data:        []byte("test data"),
+		signDigest: func(digest []byte) []byte {
+			r, s, err := ecdsa.Sign(rand.Reader, ecKey, digest)
+			if err != nil {
+				t.Fatal(err)
+			}
+			return Marshal(struct{ R, S *big.Int }{r, s})
+		},
+	}
+
+	pk := &skECDSAPublicKey{
+		application: h.application,
+		PublicKey:   ecKey.PublicKey,
+	}
+
+	// Valid signature with UP=1 should pass.
+	if err := pk.Verify(h.data, h.sign(t, flagUserPresence)); err != nil {
+		t.Errorf("Verify failed with UP=1: %v", err)
+	}
+
+	// Valid signature with UP=0 should fail with the user-presence sentinel.
+	sigNoUP := h.sign(t, 0)
+	if err := pk.Verify(h.data, sigNoUP); !errors.Is(err, errSKMissingUserPresence) {
+		t.Errorf("expected errSKMissingUserPresence, got: %v", err)
+	}
+
+	// UV set but UP clear must still fail: we only waive UP, never UV-only.
+	if err := pk.Verify(h.data, h.sign(t, 0x04)); !errors.Is(err, errSKMissingUserPresence) {
+		t.Errorf("UV-only (flags=0x04): expected errSKMissingUserPresence, got: %v", err)
+	}
+
+	// With noTouchRequired, UP=0 passes; UP=1+UV=1 also passes.
+	pk.noTouchRequired = true
+	if err := pk.Verify(h.data, sigNoUP); err != nil {
+		t.Errorf("Verify with noTouchRequired failed: %v", err)
+	}
+	if err := pk.Verify(h.data, h.sign(t, flagUserPresence|0x04)); err != nil {
+		t.Errorf("Verify UP|UV with noTouchRequired failed: %v", err)
+	}
+}
+
+func TestSKKeyWithoutUP(t *testing.T) {
+	ecKey, err := ecdsa.GenerateKey(elliptic.P256(), rand.Reader)
+	if err != nil {
+		t.Fatal(err)
+	}
+	edPub, _, err := ed25519.GenerateKey(rand.Reader)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	skEC := &skECDSAPublicKey{PublicKey: ecKey.PublicKey}
+	skED := &skEd25519PublicKey{PublicKey: edPub}
+	certEC := &Certificate{Key: &skECDSAPublicKey{PublicKey: ecKey.PublicKey}}
+	certED := &Certificate{Key: &skEd25519PublicKey{PublicKey: edPub}}
+	rsaKey, err := rsa.GenerateKey(rand.Reader, 2048)
+	if err != nil {
+		t.Fatal(err)
+	}
+	rsaPub, err := NewPublicKey(&rsaKey.PublicKey)
+	if err != nil {
+		t.Fatal(err)
+	}
+	certRSA := &Certificate{Key: rsaPub}
+
+	// SK raw keys: return a clone with the flag set, original untouched.
+	gotEC := skKeyWithoutUP(skEC)
+	if gotEC == skEC {
+		t.Error("skECDSAPublicKey: expected a clone, got the original pointer")
+	}
+	if !gotEC.(*skECDSAPublicKey).noTouchRequired {
+		t.Error("skECDSAPublicKey clone: noTouchRequired not set")
+	}
+	if skEC.noTouchRequired {
+		t.Error("skECDSAPublicKey: original mutated")
+	}
+
+	gotED := skKeyWithoutUP(skED)
+	if gotED == skED {
+		t.Error("skEd25519PublicKey: expected a clone, got the original pointer")
+	}
+	if !gotED.(*skEd25519PublicKey).noTouchRequired {
+		t.Error("skEd25519PublicKey clone: noTouchRequired not set")
+	}
+	if skED.noTouchRequired {
+		t.Error("skEd25519PublicKey: original mutated")
+	}
+
+	// Certificate wrapping SK: return a clone of the cert with a cloned
+	// SK key inside. Neither the original cert nor the original inner
+	// key must be mutated.
+	originalInnerEC := certEC.Key
+	gotCertEC := skKeyWithoutUP(certEC)
+	if gotCertEC == certEC {
+		t.Error("*Certificate(SK ecdsa): expected a clone, got the original pointer")
+	}
+	if got := gotCertEC.(*Certificate).Key.(*skECDSAPublicKey); !got.noTouchRequired {
+		t.Error("*Certificate(SK ecdsa): inner clone missing noTouchRequired")
+	}
+	if certEC.Key != originalInnerEC {
+		t.Error("*Certificate(SK ecdsa): original cert's Key pointer mutated")
+	}
+	if originalInnerEC.(*skECDSAPublicKey).noTouchRequired {
+		t.Error("*Certificate(SK ecdsa): original inner key mutated")
+	}
+
+	gotCertED := skKeyWithoutUP(certED)
+	if gotCertED == certED {
+		t.Error("*Certificate(SK ed25519): expected a clone, got the original pointer")
+	}
+	if got := gotCertED.(*Certificate).Key.(*skEd25519PublicKey); !got.noTouchRequired {
+		t.Error("*Certificate(SK ed25519): inner clone missing noTouchRequired")
+	}
+
+	// Non-SK key inside a cert: return original unchanged (nothing to clone).
+	if got := skKeyWithoutUP(certRSA); got != certRSA {
+		t.Error("*Certificate(RSA): expected the original pointer back")
+	}
+
+	// Plain non-SK key: return original unchanged.
+	if got := skKeyWithoutUP(rsaPub); got != rsaPub {
+		t.Error("rsaPublicKey: expected the original pointer back")
+	}
+
+	// Pathological: *Certificate whose Key is itself a *Certificate.
+	// The SSH cert format forbids this and parseCert rejects it, but
+	// a Go caller can still construct such a value. skKeyWithoutUP
+	// must not recurse into it (or panic); it returns the input
+	// unchanged. This also defends against a hypothetical cycle built
+	// from hand-constructed Certificate pointers.
+	nestedCert := &Certificate{Key: &Certificate{Key: skEC}}
+	if got := skKeyWithoutUP(nestedCert); got != nestedCert {
+		t.Error("*Certificate wrapping *Certificate: expected the original pointer back")
+	}
+	selfCycle := &Certificate{}
+	selfCycle.Key = selfCycle
+	if got := skKeyWithoutUP(selfCycle); got != selfCycle {
+		t.Error("self-referential *Certificate: expected the original pointer back")
+	}
+}
+
+func TestNoTouchAllowed(t *testing.T) {
+	ecKey, err := ecdsa.GenerateKey(elliptic.P256(), rand.Reader)
+	if err != nil {
+		t.Fatal(err)
+	}
+	sk := &skECDSAPublicKey{PublicKey: ecKey.PublicKey}
+	certNoExt := &Certificate{Key: sk}
+	certOptOut := &Certificate{
+		Key:         sk,
+		Permissions: Permissions{Extensions: map[string]string{"no-touch-required": ""}},
+	}
+	// Non-empty value: OpenSSH writes "" but any value must be accepted
+	// because the check is presence-only.
+	certOptOutNonEmpty := &Certificate{
+		Key:         sk,
+		Permissions: Permissions{Extensions: map[string]string{"no-touch-required": "yes"}},
+	}
+	// no-touch-required belongs in Extensions, never CriticalOptions.
+	// Putting it in CriticalOptions must NOT be treated as opt-out.
+	certCritOnly := &Certificate{
+		Key:         sk,
+		Permissions: Permissions{CriticalOptions: map[string]string{"no-touch-required": ""}},
+	}
+
+	permsEmpty := &Permissions{}
+	permsOptOut := &Permissions{Extensions: map[string]string{"no-touch-required": ""}}
+	permsCritOnly := &Permissions{CriticalOptions: map[string]string{"no-touch-required": ""}}
+
+	cases := []struct {
+		name  string
+		pub   PublicKey
+		perms *Permissions
+		want  bool
+	}{
+		{"nil perms, no cert", sk, nil, false},
+		{"empty perms, no cert", sk, permsEmpty, false},
+		{"perms opt-out, raw key", sk, permsOptOut, true},
+		{"nil perms, cert opt-out", certOptOut, nil, true},
+		{"nil perms, cert opt-out non-empty value", certOptOutNonEmpty, nil, true},
+		{"nil perms, cert no ext", certNoExt, nil, false},
+		{"perms opt-out, cert no ext", certNoExt, permsOptOut, true},
+		{"empty perms, cert opt-out", certOptOut, permsEmpty, true},
+		// Negative controls: CriticalOptions must not waive UP.
+		{"critical-options only, raw key", sk, permsCritOnly, false},
+		{"critical-options only, cert", certCritOnly, nil, false},
+	}
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			if got := noTouchAllowed(tc.pub, tc.perms); got != tc.want {
+				t.Errorf("noTouchAllowed = %v, want %v", got, tc.want)
+			}
+		})
+	}
+}
+
+func TestSKUserPresenceEd25519(t *testing.T) {
+	pub, priv, err := ed25519.GenerateKey(rand.Reader)
+	if err != nil {
+		t.Fatal(err)
+	}
+	h := skTestHarness{
+		format:      "sk-ssh-ed25519@openssh.com",
+		application: "ssh:",
+		data:        []byte("test data"),
+		signBlob: func(blob []byte) []byte {
+			return ed25519.Sign(priv, blob)
+		},
+	}
+
+	pk := &skEd25519PublicKey{
+		application: h.application,
+		PublicKey:   pub,
+	}
+
+	if err := pk.Verify(h.data, h.sign(t, flagUserPresence)); err != nil {
+		t.Errorf("Verify failed with UP=1: %v", err)
+	}
+	sigNoUP := h.sign(t, 0)
+	if err := pk.Verify(h.data, sigNoUP); !errors.Is(err, errSKMissingUserPresence) {
+		t.Errorf("expected errSKMissingUserPresence, got: %v", err)
+	}
+	pk.noTouchRequired = true
+	if err := pk.Verify(h.data, sigNoUP); err != nil {
+		t.Errorf("Verify with noTouchRequired failed: %v", err)
+	}
+}
+
 func TestNewSignerWithAlgos(t *testing.T) {
 	algorithSigner, ok := testSigners["rsa"].(AlgorithmSigner)
 	if !ok {

ssh/knownhosts/knownhosts.go

@@ -160,8 +160,13 @@ func (db *hostKeyDB) IsHostAuthority(remote ssh.PublicKey, address string) bool
 
 // IsRevoked can be used as a callback in ssh.CertChecker
 func (db *hostKeyDB) IsRevoked(key *ssh.Certificate) bool {
-	_, ok := db.revoked[string(key.Marshal())]
-	return ok
+	if _, ok := db.revoked[string(key.Marshal())]; ok {
+		return true
+	}
+	if _, ok := db.revoked[string(key.SignatureKey.Marshal())]; ok {
+		return true
+	}
+	return false
 }
 
 const markerCert = "@cert-authority"

ssh/knownhosts/knownhosts_test.go

@@ -6,6 +6,8 @@ package knownhosts
 
 import (
 	"bytes"
+	"crypto/ed25519"
+	"crypto/rand"
 	"fmt"
 	"net"
 	"reflect"
@@ -272,7 +274,45 @@ func TestWildcardMatch(t *testing.T) {
 	}
 }
 
-// TODO(hanwen): test coverage for certificates.
+func TestRevokedCA(t *testing.T) {
+	_, caPriv, err := ed25519.GenerateKey(rand.Reader)
+	if err != nil {
+		t.Fatal(err)
+	}
+	caSigner, err := ssh.NewSignerFromKey(caPriv)
+	if err != nil {
+		t.Fatal(err)
+	}
+	caKey := caSigner.PublicKey()
+
+	_, hostPriv, err := ed25519.GenerateKey(rand.Reader)
+	if err != nil {
+		t.Fatal(err)
+	}
+	hostKey, err := ssh.NewPublicKey(hostPriv.Public())
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	cert := &ssh.Certificate{
+		CertType:        ssh.HostCert,
+		Key:             hostKey,
+		ValidBefore:     ssh.CertTimeInfinity,
+		ValidPrincipals: []string{"server.org"},
+	}
+	if err := cert.SignCert(rand.Reader, caSigner); err != nil {
+		t.Fatal(err)
+	}
+
+	caLine := "ssh-ed25519 " + serialize(caKey)[len("ssh-ed25519 "):]
+	knownHostsData := "@revoked server.org " + caLine + "\n" +
+		"@cert-authority server.org " + caLine + "\n"
+	db := testDB(t, knownHostsData)
+
+	if !db.IsRevoked(cert) {
+		t.Error("IsRevoked returned false for certificate signed by revoked CA")
+	}
+}
 
 const testHostname = "hostname"
 

ssh/mux.go

@@ -91,9 +91,10 @@ type mux struct {
 
 	incomingChannels chan NewChannel
 
-	globalSentMu     sync.Mutex
-	globalResponses  chan interface{}
-	incomingRequests chan *Request
+	globalSentMu      sync.Mutex
+	globalSentPending atomic.Bool
+	globalResponses   chan interface{}
+	incomingRequests  chan *Request
 
 	errCond *sync.Cond
 	err     error
@@ -141,6 +142,24 @@ func (m *mux) SendRequest(name string, wantReply bool, payload []byte) (bool, []
 	if wantReply {
 		m.globalSentMu.Lock()
 		defer m.globalSentMu.Unlock()
+
+		// Open the gate so that responses arriving while this request is in
+		// flight are allowed to reach globalResponses. Any response arriving
+		// while no request is pending is dropped by handleGlobalPacket.
+		m.globalSentPending.Store(true)
+		defer m.globalSentPending.Store(false)
+
+		// Drain any spurious responses that may have been buffered. This prevents
+		// a previously buffered unexpected response from being consumed instead
+		// of the actual response for this request.
+	drain:
+		for {
+			select {
+			case <-m.globalResponses:
+			default:
+				break drain
+			}
+		}
 	}
 
 	if err := m.sendMessage(globalRequestMsg{
@@ -267,7 +286,16 @@ func (m *mux) handleGlobalPacket(packet []byte) error {
 			mux:       m,
 		}
 	case *globalRequestSuccessMsg, *globalRequestFailureMsg:
-		m.globalResponses <- msg
+		// Drop responses that arrive when no SendRequest is waiting, to
+		// prevent a malicious peer from staging responses for a future
+		// caller.
+		if !m.globalSentPending.Load() {
+			return nil
+		}
+		select {
+		case m.globalResponses <- msg:
+		default:
+		}
 	default:
 		panic(fmt.Sprintf("not a global message %#v", msg))
 	}

ssh/mux_test.go

@@ -825,6 +825,50 @@ func TestMuxChannelWindowDeferredUpdates(t *testing.T) {
 	}
 }
 
+func TestMuxChannelRejectRemovesFromMux(t *testing.T) {
+	serverMux, clientMux := muxPair()
+	defer serverMux.Close()
+	defer clientMux.Close()
+
+	var wg sync.WaitGroup
+	t.Cleanup(wg.Wait)
+	wg.Add(1)
+
+	go func() {
+		defer wg.Done()
+
+		// The server waits for the channel creation request
+		newCh, ok := <-serverMux.incomingChannels
+		if !ok {
+			t.Error("failed to accept channel")
+			return
+		}
+		ch := newCh.(*channel)
+
+		if serverMux.chanList.getChan(ch.localId) == nil {
+			t.Errorf("channel %d is not in the chanList before Reject", ch.localId)
+		}
+
+		if err := ch.Reject(Prohibited, "rejecting this channel"); err != nil {
+			t.Errorf("Reject failed: %v", err)
+		}
+
+		if serverMux.chanList.getChan(ch.localId) != nil {
+			t.Errorf("channel %d is still in the chanList after Reject", ch.localId)
+		}
+	}()
+
+	_, _, err := clientMux.OpenChannel("test_leak", nil)
+
+	if err == nil {
+		t.Fatal("expected an error (channel rejected), but got nil")
+	}
+
+	if _, ok := err.(*OpenChannelError); !ok {
+		t.Errorf("expected *OpenChannelError, got: %T", err)
+	}
+}
+
 // Don't ship code with debug=true.
 func TestDebug(t *testing.T) {
 	if debugMux {
@@ -837,3 +881,488 @@ func TestDebug(t *testing.T) {
 		t.Error("transport debug switched on")
 	}
 }
+
+func TestMuxUnexpectedGlobalResponsesDiscarded(t *testing.T) {
+	clientPipe, serverPipe := memPipe()
+	client := newMux(clientPipe)
+	defer serverPipe.Close()
+	defer client.Close()
+
+	done := make(chan error, 1)
+	go func() {
+		// Send multiple unexpected global responses, this should not block the
+		// globalResponses channel.
+		for i := range 5 {
+			err := serverPipe.writePacket(Marshal(globalRequestSuccessMsg{
+				Data: []byte{byte(i)},
+			}))
+			if err != nil {
+				done <- fmt.Errorf("send success msg %d: %w", i, err)
+				return
+			}
+		}
+		for i := range 5 {
+			err := serverPipe.writePacket(Marshal(globalRequestFailureMsg{
+				Data: []byte{byte(i)},
+			}))
+			if err != nil {
+				done <- fmt.Errorf("send failure msg %d: %w", i, err)
+				return
+			}
+		}
+
+		// Now send a global request and wait for the response. This
+		// verifies the mux is still processing packets.
+		err := serverPipe.writePacket(Marshal(globalRequestMsg{
+			Type:      "keepalive@golang.org",
+			WantReply: true,
+			Data:      nil,
+		}))
+		if err != nil {
+			done <- fmt.Errorf("send global request: %w", err)
+			return
+		}
+
+		packet, err := serverPipe.readPacket()
+		if err != nil {
+			done <- fmt.Errorf("read packet: %w", err)
+			return
+		}
+		decoded, err := decode(packet)
+		if err != nil {
+			done <- fmt.Errorf("decode: %w", err)
+			return
+		}
+		switch decoded.(type) {
+		case *globalRequestSuccessMsg, *globalRequestFailureMsg:
+			// Expected response
+		default:
+			done <- fmt.Errorf("unexpected packet type: %T", decoded)
+			return
+		}
+		done <- nil
+	}()
+
+	// Handle the incoming request from the server and reply
+	req, ok := <-client.incomingRequests
+	if !ok {
+		t.Fatal("incomingRequests channel closed unexpectedly")
+	}
+	if req.Type != "keepalive@golang.org" {
+		t.Fatalf("unexpected request type: %s", req.Type)
+	}
+	if err := req.Reply(true, nil); err != nil {
+		t.Fatalf("Reply: %v", err)
+	}
+
+	if err := <-done; err != nil {
+		t.Fatal(err)
+	}
+}
+
+func TestMuxConcurrentGlobalRequests(t *testing.T) {
+	clientMux, serverMux := muxPair()
+	defer serverMux.Close()
+	defer clientMux.Close()
+
+	const numRequests = 50
+
+	serverDone := make(chan struct{})
+	go func() {
+		defer close(serverDone)
+		for r := range serverMux.incomingRequests {
+			if r.WantReply {
+				replyData := append([]byte("reply:"), r.Payload...)
+				r.Reply(true, replyData)
+			}
+		}
+	}()
+
+	var clientWg sync.WaitGroup
+	clientWg.Add(numRequests)
+
+	errCh := make(chan error, numRequests)
+
+	for i := range numRequests {
+		go func(id int) {
+			defer clientWg.Done()
+
+			payloadStr := fmt.Sprintf("req-%d", id)
+			payload := []byte(payloadStr)
+
+			// This call blocks until the globalSentMu is acquired.
+			// The mutex ensures that even with many concurrent attempts,
+			// the "drain" and "send" logic happens atomically per request.
+			ok, data, err := clientMux.SendRequest("echo", true, payload)
+			if err != nil {
+				errCh <- fmt.Errorf("req %d error: %v", id, err)
+				return
+			}
+			if !ok {
+				errCh <- fmt.Errorf("req %d failed (want success)", id)
+				return
+			}
+
+			expected := "reply:" + payloadStr
+			if string(data) != expected {
+				errCh <- fmt.Errorf("req %d mismatch: got %q, want %q", id, string(data), expected)
+			}
+		}(i)
+	}
+
+	clientWg.Wait()
+	close(errCh)
+
+	for err := range errCh {
+		if err != nil {
+			t.Fatal(err)
+		}
+	}
+
+	clientMux.Close()
+	<-serverDone
+}
+
+func TestMuxGlobalResponseDroppedWhenIdle(t *testing.T) {
+	clientPipe, serverPipe := memPipe()
+	clientMux := newMux(clientPipe)
+	defer serverPipe.Close()
+	defer clientMux.Close()
+
+	errCh := make(chan error, 1)
+	go func() {
+		// Send a spurious response while no SendRequest is pending.
+		if err := serverPipe.writePacket(Marshal(globalRequestSuccessMsg{
+			Data: []byte("spurious"),
+		})); err != nil {
+			errCh <- fmt.Errorf("send spurious: %w", err)
+			return
+		}
+		// Follow with a global request; once the client observes this on
+		// incomingRequests, the mux loop has necessarily processed (and
+		// dropped) the prior spurious response.
+		if err := serverPipe.writePacket(Marshal(globalRequestMsg{
+			Type:      "sync@example.com",
+			WantReply: false,
+		})); err != nil {
+			errCh <- fmt.Errorf("send sync request: %w", err)
+			return
+		}
+		errCh <- nil
+	}()
+
+	if err := <-errCh; err != nil {
+		t.Fatal(err)
+	}
+
+	req, ok := <-clientMux.incomingRequests
+	if !ok {
+		t.Fatal("incomingRequests closed unexpectedly")
+	}
+	if req.Type != "sync@example.com" {
+		t.Fatalf("unexpected sync request type %q", req.Type)
+	}
+
+	// The spurious response preceded the sync request, so by now the mux
+	// loop has processed it. The pending-gate must have caused it to be
+	// dropped rather than buffered.
+	if n := len(clientMux.globalResponses); n != 0 {
+		t.Fatalf("globalResponses buffer should be empty after idle drop, has %d entries", n)
+	}
+}
+
+func TestMuxStaleResponseDrained(t *testing.T) {
+	// Simulate a stale response sitting in globalResponses (e.g. a response
+	// that slipped in through the pending-gate on a prior SendRequest that
+	// exited without consuming it). The drain step in the next SendRequest
+	// must discard it so the caller receives the correct reply.
+	clientMux, serverMux := muxPair()
+	defer serverMux.Close()
+	defer clientMux.Close()
+
+	clientMux.globalResponses <- &globalRequestSuccessMsg{Data: []byte("stale")}
+
+	serverDone := make(chan struct{})
+	go func() {
+		defer close(serverDone)
+		for req := range serverMux.incomingRequests {
+			if req.WantReply {
+				req.Reply(true, append([]byte("reply:"), req.Payload...))
+			}
+		}
+	}()
+
+	ok, data, err := clientMux.SendRequest("test", true, []byte("hello"))
+	if err != nil {
+		t.Fatalf("request failed: %v", err)
+	}
+	if !ok {
+		t.Fatal("expected success response")
+	}
+	if string(data) != "reply:hello" {
+		t.Fatalf("got %q, want %q (drain did not remove stale response)", data, "reply:hello")
+	}
+
+	clientMux.Close()
+	<-serverDone
+}
+
+func TestMuxGlobalResponseAcceptedWhilePending(t *testing.T) {
+	// Positive control: when a SendRequest is actually pending, the
+	// response must be delivered (the gate is open).
+	clientMux, serverMux := muxPair()
+	defer serverMux.Close()
+	defer clientMux.Close()
+
+	serverDone := make(chan struct{})
+	go func() {
+		defer close(serverDone)
+		for req := range serverMux.incomingRequests {
+			if req.WantReply {
+				req.Reply(true, []byte("pong"))
+			}
+		}
+	}()
+
+	ok, data, err := clientMux.SendRequest("ping", true, nil)
+	if err != nil {
+		t.Fatalf("SendRequest: %v", err)
+	}
+	if !ok || string(data) != "pong" {
+		t.Fatalf("unexpected response: ok=%v data=%q", ok, data)
+	}
+
+	clientMux.Close()
+	<-serverDone
+}
+
+func TestChannelUnexpectedResponsesDiscarded(t *testing.T) {
+	// A malicious peer that spams channelRequestSuccess/Failure messages
+	// for an open, idle channel must not be able to stall the mux read
+	// loop by filling ch.msg. After the flood, the channel must still be
+	// usable: a subsequent legitimate SendRequest receives its reply.
+	clientMux, serverMux := muxPair()
+	defer serverMux.Close()
+	defer clientMux.Close()
+
+	serverRes := make(chan *channel, 1)
+	go func() {
+		newCh, ok := <-serverMux.incomingChannels
+		if !ok {
+			close(serverRes)
+			return
+		}
+		c, _, err := newCh.Accept()
+		if err != nil {
+			close(serverRes)
+			return
+		}
+		serverRes <- c.(*channel)
+	}()
+
+	clientCh, err := clientMux.openChannel("chan", nil)
+	if err != nil {
+		t.Fatalf("openChannel: %v", err)
+	}
+	serverCh := <-serverRes
+	if serverCh == nil {
+		t.Fatal("server did not accept channel")
+	}
+
+	// Spam many unsolicited success/failure responses. More than chanSize
+	// to ensure ch.msg would overflow without the pending-gate.
+	const spam = chanSize * 4
+	done := make(chan error, 1)
+	go func() {
+		for i := range spam {
+			if err := serverCh.ackRequest(i%2 == 0); err != nil {
+				done <- fmt.Errorf("ackRequest %d: %w", i, err)
+				return
+			}
+		}
+		// Echo any legitimate request back.
+		for req := range serverCh.incomingRequests {
+			if req.WantReply {
+				if err := req.Reply(true, append([]byte("reply:"), req.Payload...)); err != nil {
+					done <- fmt.Errorf("reply: %w", err)
+					return
+				}
+			}
+		}
+		done <- nil
+	}()
+
+	// If the flood had wedged the mux loop, this SendRequest would never
+	// receive a reply.
+	ok, err := clientCh.SendRequest("ping", true, []byte("hello"))
+	if err != nil {
+		t.Fatalf("SendRequest: %v", err)
+	}
+	if !ok {
+		t.Fatal("expected success reply")
+	}
+
+	// Clean up so the server goroutine can exit.
+	clientCh.Close()
+	serverCh.Close()
+	if err := <-done; err != nil {
+		if !errors.Is(err, io.EOF) {
+			t.Fatal(err)
+		}
+	}
+}
+
+func TestChannelConcurrentRequests(t *testing.T) {
+	writer, reader, mux := channelPair(t)
+	defer writer.Close()
+	defer reader.Close()
+	defer mux.Close()
+
+	serverDone := make(chan struct{})
+	go func() {
+		defer close(serverDone)
+		for req := range writer.incomingRequests {
+			if req.WantReply {
+				req.Reply(true, append([]byte("reply:"), req.Payload...))
+			}
+		}
+	}()
+
+	const numRequests = 50
+	var wg sync.WaitGroup
+	wg.Add(numRequests)
+	errCh := make(chan error, numRequests)
+
+	for i := 0; i < numRequests; i++ {
+		go func(id int) {
+			defer wg.Done()
+			payload := []byte(fmt.Sprintf("req-%d", id))
+			ok, err := reader.SendRequest("echo", true, payload)
+			if err != nil {
+				errCh <- fmt.Errorf("req %d: %v", id, err)
+				return
+			}
+			if !ok {
+				errCh <- fmt.Errorf("req %d: expected success", id)
+			}
+		}(i)
+	}
+
+	wg.Wait()
+	close(errCh)
+
+	for err := range errCh {
+		if err != nil {
+			t.Fatal(err)
+		}
+	}
+
+	reader.Close()
+	writer.Close()
+	<-serverDone
+}
+
+func TestChannelResponseDroppedWhenIdle(t *testing.T) {
+	// A spurious response arriving while no SendRequest is pending must
+	// be dropped rather than buffered in ch.msg.
+	writer, reader, mux := channelPair(t)
+	defer writer.Close()
+	defer reader.Close()
+	defer mux.Close()
+
+	// Server sends an unsolicited reply, then a request so we can
+	// synchronise: once the client observes the request, the mux loop has
+	// necessarily processed (and dropped) the prior spurious reply.
+	errCh := make(chan error, 1)
+	go func() {
+		if err := writer.ackRequest(true); err != nil {
+			errCh <- err
+			return
+		}
+		if _, err := writer.SendRequest("sync", false, nil); err != nil {
+			errCh <- err
+			return
+		}
+		errCh <- nil
+	}()
+
+	req := <-reader.incomingRequests
+	if req.Type != "sync" {
+		t.Fatalf("unexpected request type %q", req.Type)
+	}
+
+	if n := len(reader.msg); n != 0 {
+		t.Fatalf("ch.msg should be empty after idle drop, has %d entries", n)
+	}
+
+	if err := <-errCh; err != nil {
+		t.Fatal(err)
+	}
+}
+
+func TestChannelStaleResponseDrained(t *testing.T) {
+	// Simulate a stale response sitting in ch.msg (e.g. a response that
+	// slipped through the pending-gate on a prior SendRequest that exited
+	// without consuming it). The drain step in the next SendRequest must
+	// discard it so the caller receives the correct reply.
+	writer, reader, mux := channelPair(t)
+	defer writer.Close()
+	defer reader.Close()
+	defer mux.Close()
+
+	reader.msg <- &channelRequestSuccessMsg{PeersID: reader.remoteId}
+
+	serverDone := make(chan struct{})
+	go func() {
+		defer close(serverDone)
+		for req := range writer.incomingRequests {
+			if req.WantReply {
+				req.Reply(false, append([]byte("nack:"), req.Payload...))
+			}
+		}
+	}()
+
+	ok, err := reader.SendRequest("test", true, []byte("hello"))
+	if err != nil {
+		t.Fatalf("SendRequest: %v", err)
+	}
+	// If the stale success had been consumed, ok would be true.
+	if ok {
+		t.Fatal("got stale success response; drain did not remove it")
+	}
+
+	reader.Close()
+	writer.Close()
+	<-serverDone
+}
+
+func TestChannelResponseAcceptedWhilePending(t *testing.T) {
+	// Positive control: when a SendRequest is actually pending, the
+	// response must be delivered (the gate is open).
+	writer, reader, mux := channelPair(t)
+	defer writer.Close()
+	defer reader.Close()
+	defer mux.Close()
+
+	serverDone := make(chan struct{})
+	go func() {
+		defer close(serverDone)
+		for req := range writer.incomingRequests {
+			if req.WantReply {
+				req.Reply(true, nil)
+			}
+		}
+	}()
+
+	ok, err := reader.SendRequest("ping", true, nil)
+	if err != nil {
+		t.Fatalf("SendRequest: %v", err)
+	}
+	if !ok {
+		t.Fatal("expected success")
+	}
+
+	reader.Close()
+	writer.Close()
+	<-serverDone
+}

ssh/server.go

@@ -34,15 +34,20 @@ type Permissions struct {
 	// or not supported.
 	CriticalOptions map[string]string
 
-	// Extensions are extra functionality that the server may
-	// offer on authenticated connections. Lack of support for an
-	// extension does not preclude authenticating a user. Common
-	// extensions are "permit-agent-forwarding",
-	// "permit-X11-forwarding". The Go SSH library currently does
-	// not act on any extension, and it is up to server
-	// implementations to honor them. Extensions can be used to
-	// pass data from the authentication callbacks to the server
-	// application layer.
+	// Extensions are extra functionality that the server may offer on
+	// authenticated connections. Lack of support for an extension does not
+	// preclude authenticating a user. Common extensions are
+	// "permit-agent-forwarding", "permit-X11-forwarding". In general the Go
+	// SSH library does not act on extensions and it is up to server
+	// implementations to honor them; extensions can also be used to pass data
+	// from the authentication callbacks to the server application layer.
+	//
+	// The one extension acted upon by this library is "no-touch-required",
+	// which applies only to security-key public keys
+	// (sk-ecdsa-sha2-nistp256@openssh.com and sk-ssh-ed25519@openssh.com).
+	// When present, it waives the default requirement that SK signatures
+	// assert user presence (i.e. a physical touch of the authenticator)
+	// during signature verification.
 	Extensions map[string]string
 
 	// ExtraData allows to store user defined data.
@@ -84,6 +89,79 @@ type ServerPreAuthConn interface {
 	SendAuthBanner(string) error
 }
 
+// noTouchRequiredExtension is the extension name used by OpenSSH in
+// authorized_keys options and certificate extensions to mark keys
+// whose signatures do not need to assert user presence (touch). See
+// ssh-keygen(1) and sshd(8).
+const noTouchRequiredExtension = "no-touch-required"
+
+// noTouchAllowed reports whether the user presence requirement on
+// SK signatures should be waived for this authentication attempt. The
+// requirement is waived when the "no-touch-required" extension is
+// present either in the Permissions returned by the auth callback
+// (authorized_keys-level opt-out) or in the certificate's own
+// Extensions (CA-level opt-out), matching OpenSSH behavior. OpenSSH
+// reads the per-key opt-out only from cert Extensions and
+// authorized_keys options (never from CriticalOptions); we follow the
+// same rule.
+func noTouchAllowed(pubKey PublicKey, perms *Permissions) bool {
+	if perms != nil {
+		if _, ok := perms.Extensions[noTouchRequiredExtension]; ok {
+			return true
+		}
+	}
+	if cert, ok := pubKey.(*Certificate); ok {
+		if _, ok := cert.Extensions[noTouchRequiredExtension]; ok {
+			return true
+		}
+	}
+	return false
+}
+
+// skKeyWithoutUP returns a PublicKey equivalent to pubKey but whose
+// Verify accepts SK signatures with the user-presence flag clear. If
+// pubKey is not (and does not wrap) an SK key, pubKey is returned
+// unchanged. The returned value never mutates pubKey: for SK keys a
+// shallow copy is made so that the noTouchRequired flag is set only on
+// the clone.
+//
+// The implementation is iterative rather than recursive. When pubKey
+// is a *Certificate we unwrap exactly one level to look at the inner
+// key. The SSH cert format forbids Certificate.Key from being another
+// Certificate (parseCert rejects it), but nothing stops callers from
+// constructing such a value directly in Go; a recursive descent could
+// otherwise be driven to unbounded depth by a hand-crafted or cyclic
+// Certificate. A malformed input of that shape simply returns
+// unchanged here.
+func skKeyWithoutUP(pubKey PublicKey) PublicKey {
+	cert, isCert := pubKey.(*Certificate)
+	target := pubKey
+	if isCert {
+		target = cert.Key
+	}
+	var cloned PublicKey
+	switch k := target.(type) {
+	case *skECDSAPublicKey:
+		c := *k
+		c.noTouchRequired = true
+		cloned = &c
+	case *skEd25519PublicKey:
+		c := *k
+		c.noTouchRequired = true
+		cloned = &c
+	default:
+		// Not an SK key (or a pathological *Certificate wrapping
+		// another *Certificate): pubKey is already usable for Verify.
+		return pubKey
+	}
+	if !isCert {
+		return cloned
+	}
+	c := *cert
+	c.Key = cloned
+	return &c
+}
+
 // ServerConfig holds server specific configuration data.
 type ServerConfig struct {
 	// Config contains configuration shared between client and server.
@@ -242,8 +320,10 @@ func (c *pubKeyCache) add(candidate cachedPubKey) {
 type ServerConn struct {
 	Conn
 
-	// If the succeeding authentication callback returned a
-	// non-nil Permissions pointer, it is stored here.
+	// If the succeeding authentication callback returned a non-nil Permissions
+	// pointer, it is stored here. These are the permissions from the final,
+	// successful authentication method. Permissions returned by callbacks that
+	// return PartialSuccessError are not preserved and must be nil.
 	Permissions *Permissions
 }
 
@@ -737,8 +817,15 @@ userAuthLoop:
 				}
 
 				signedData := buildDataSignedForAuth(sessionID, userAuthReq, algo, pubKeyData)
-
-				if err := pubKey.Verify(signedData, sig); err != nil {
+				// pubKey is reused below for VerifiedPublicKeyCallback and
+				// must remain the key as presented by the client; derive a
+				// separate value for Verify that carries any applicable
+				// no-touch-required opt-out.
+				pubKeyForVerify := pubKey
+				if noTouchAllowed(pubKey, candidate.perms) {
+					pubKeyForVerify = skKeyWithoutUP(pubKey)
+				}
+				if err := pubKeyForVerify.Verify(signedData, sig); err != nil {
 					return nil, err
 				}
 
@@ -750,6 +837,13 @@ userAuthLoop:
 					// considered verified and the callback must not run.
 					perms, authErr = config.VerifiedPublicKeyCallback(s, pubKey, perms, algo)
 				}
+				if authErr == nil && perms != nil && perms.CriticalOptions != nil {
+					if saco := perms.CriticalOptions[sourceAddressCriticalOption]; saco != "" {
+						if err := checkSourceAddress(s.RemoteAddr(), saco); err != nil {
+							authErr = err
+						}
+					}
+				}
 			}
 		case "gssapi-with-mic":
 			if authConfig.GSSAPIWithMICConfig == nil {
@@ -824,6 +918,13 @@ userAuthLoop:
 		var failureMsg userAuthFailureMsg
 
 		if partialSuccess, ok := authErr.(*PartialSuccessError); ok {
+			// Permissions are not preserved between authentication steps. To
+			// avoid confusion about the final state of the connection, we
+			// disallow returning non-nil Permissions combined with
+			// PartialSuccessError.
+			if perms != nil {
+				return nil, errors.New("ssh: permissions must be nil when returning PartialSuccessError")
+			}
 			// After a partial success error we don't allow changing the user
 			// name and execute the NoClientAuthCallback.
 			partialSuccessReturned = true

ssh/server_test.go

@@ -6,9 +6,14 @@ package ssh
 
 import (
 	"bytes"
+	"crypto/ecdsa"
+	"crypto/elliptic"
+	"crypto/rand"
+	"crypto/sha256"
 	"errors"
 	"fmt"
 	"io"
+	"math/big"
 	"net"
 	"reflect"
 	"strings"
@@ -664,6 +669,44 @@ func TestVerifiedPubKeyCallbackError(t *testing.T) {
 	}
 }
 
+func TestVerifiedPubKeyCallbackSourceAddress(t *testing.T) {
+	c1, c2, err := netPipe()
+	if err != nil {
+		t.Fatalf("netPipe: %v", err)
+	}
+	defer c1.Close()
+	defer c2.Close()
+
+	serverConf := &ServerConfig{
+		PublicKeyCallback: func(conn ConnMetadata, key PublicKey) (*Permissions, error) {
+			return nil, nil
+		},
+		VerifiedPublicKeyCallback: func(conn ConnMetadata, key PublicKey, permissions *Permissions, signatureAlgorithm string) (*Permissions, error) {
+			return &Permissions{
+				CriticalOptions: map[string]string{
+					sourceAddressCriticalOption: "192.168.99.99",
+				},
+			}, nil
+		},
+	}
+	serverConf.AddHostKey(testSigners["rsa"])
+
+	clientConf := ClientConfig{
+		User: "user",
+		Auth: []AuthMethod{
+			PublicKeys(testSigners["rsa"]),
+		},
+		HostKeyCallback: InsecureIgnoreHostKey(),
+	}
+
+	go NewServerConn(c1, serverConf)
+
+	_, _, _, err = NewClientConn(c2, "", &clientConf)
+	if err == nil {
+		t.Fatal("client login succeed with VerifiedPublicKeyCallback returning mismatching source-address")
+	}
+}
+
 func TestVerifiedPublicCallbackPartialSuccessBadUsage(t *testing.T) {
 	c1, c2, err := netPipe()
 	if err != nil {
@@ -808,6 +851,60 @@ func TestVerifiedPublicKeyCallbackOnly(t *testing.T) {
 	<-done
 }
 
+func TestPartialSuccessWithNonNilPerms(t *testing.T) {
+	c1, c2, err := netPipe()
+	if err != nil {
+		t.Fatalf("netPipe: %v", err)
+	}
+	defer c1.Close()
+	defer c2.Close()
+
+	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()) {
+				// Intentionally return non-nil Permissions along with a
+				// PartialSuccessError. Since permissions are reset between
+				// authentication steps, this constitutes invalid library usage
+				// and the server is expected to reject the connection.
+				return &Permissions{Extensions: map[string]string{"permit-port-forwarding": ""}}, &PartialSuccessError{
+					Next: ServerAuthCallbacks{
+						PasswordCallback: func(conn ConnMetadata, password []byte) (*Permissions, error) {
+							if string(password) == clientPassword {
+								return nil, nil
+							}
+							return nil, nil
+						},
+					},
+				}
+			}
+			return nil, errors.New("invalid credentials")
+		},
+	}
+	serverConf.AddHostKey(testSigners["rsa"])
+
+	clientConf := ClientConfig{
+		User: "user",
+		Auth: []AuthMethod{
+			PublicKeys(testSigners["rsa"]),
+			Password(clientPassword),
+		},
+		HostKeyCallback: InsecureIgnoreHostKey(),
+	}
+
+	go NewServerConn(c1, serverConf)
+
+	_, _, _, err = NewClientConn(c2, "", &clientConf)
+	if err == nil {
+		t.Fatal("authentication succeeded unexpectedly; server should have rejected non-nil Permissions combined with PartialSuccessError")
+	}
+}
+
 type markerConn struct {
 	closed uint32
 	used   uint32
@@ -850,3 +947,126 @@ func (*markerConn) RemoteAddr() net.Addr { return nil }
 func (*markerConn) SetDeadline(t time.Time) error      { return nil }
 func (*markerConn) SetReadDeadline(t time.Time) error  { return nil }
 func (*markerConn) SetWriteDeadline(t time.Time) error { return nil }
+
+// skTestSigner is a Signer that produces SK-ECDSA signatures over
+// user-auth data, simulating a FIDO/U2F authenticator. The flags
+// byte (UP and other bits) is caller-controlled so tests can exercise
+// the server's user-presence enforcement and its opt-out paths. This
+// is test-only: the real Signer is hardware-backed.
+type skTestSigner struct {
+	priv        *ecdsa.PrivateKey
+	pub         PublicKey
+	flags       byte
+	application string
+}
+
+func (s *skTestSigner) PublicKey() PublicKey { return s.pub }
+
+func (s *skTestSigner) Sign(r io.Reader, data []byte) (*Signature, error) {
+	h := sha256.New()
+	h.Write([]byte(s.application))
+	appDigest := h.Sum(nil)
+	h.Reset()
+	h.Write(data)
+	dataDigest := h.Sum(nil)
+	var counter uint32 = 1
+	blob := struct {
+		ApplicationDigest []byte `ssh:"rest"`
+		Flags             byte
+		Counter           uint32
+		MessageDigest     []byte `ssh:"rest"`
+	}{appDigest, s.flags, counter, dataDigest}
+	h.Reset()
+	h.Write(Marshal(blob))
+	digest := h.Sum(nil)
+	x, y, err := ecdsa.Sign(r, s.priv, digest)
+	if err != nil {
+		return nil, err
+	}
+	return &Signature{
+		Format: KeyAlgoSKECDSA256,
+		Blob:   Marshal(struct{ R, S *big.Int }{x, y}),
+		Rest: Marshal(struct {
+			Flags   byte
+			Counter uint32
+		}{s.flags, counter}),
+	}, nil
+}
+
+// TestServerAuthSKUserPresence drives the full userAuthLoop with an SK
+// public-key client and verifies the server's wiring of the UP check
+// and its two opt-out paths: the per-key Permissions.Extensions route
+// and (via cert) the cert-level route. It also confirms that non-SK
+// clients are unaffected by the new code path.
+func TestServerAuthSKUserPresence(t *testing.T) {
+	userKey, err := ecdsa.GenerateKey(elliptic.P256(), rand.Reader)
+	if err != nil {
+		t.Fatal(err)
+	}
+	skPub := &skECDSAPublicKey{application: "ssh:", PublicKey: userKey.PublicKey}
+
+	runAuth := func(t *testing.T, signer Signer, perms *Permissions) error {
+		t.Helper()
+		c1, c2, err := netPipe()
+		if err != nil {
+			t.Fatal(err)
+		}
+		defer c1.Close()
+		defer c2.Close()
+
+		serverConf := &ServerConfig{
+			PublicKeyCallback: func(ConnMetadata, PublicKey) (*Permissions, error) {
+				return perms, nil
+			},
+		}
+		serverConf.AddHostKey(testSigners["ecdsa"])
+
+		serverErr := make(chan error, 1)
+		go func() {
+			_, _, _, err := NewServerConn(c1, serverConf)
+			serverErr <- err
+		}()
+
+		clientConf := &ClientConfig{
+			User:            "user",
+			Auth:            []AuthMethod{PublicKeys(signer)},
+			HostKeyCallback: InsecureIgnoreHostKey(),
+		}
+		_, _, _, clientErr := NewClientConn(c2, "", clientConf)
+		<-serverErr
+		return clientErr
+	}
+
+	optOut := &Permissions{Extensions: map[string]string{noTouchRequiredExtension: ""}}
+
+	t.Run("UP=1, default perms accepts", func(t *testing.T) {
+		s := &skTestSigner{priv: userKey, pub: skPub, flags: flagUserPresence, application: "ssh:"}
+		if err := runAuth(t, s, nil); err != nil {
+			t.Errorf("expected auth to succeed: %v", err)
+		}
+	})
+	t.Run("UP=0, default perms rejects", func(t *testing.T) {
+		s := &skTestSigner{priv: userKey, pub: skPub, flags: 0, application: "ssh:"}
+		if err := runAuth(t, s, nil); err == nil {
+			t.Error("expected auth to fail with UP=0")
+		}
+	})
+	t.Run("UP=0, perms opt-out accepts", func(t *testing.T) {
+		s := &skTestSigner{priv: userKey, pub: skPub, flags: 0, application: "ssh:"}
+		if err := runAuth(t, s, optOut); err != nil {
+			t.Errorf("expected auth to succeed with opt-out: %v", err)
+		}
+	})
+	t.Run("UP=0, perms CriticalOptions does NOT opt out", func(t *testing.T) {
+		s := &skTestSigner{priv: userKey, pub: skPub, flags: 0, application: "ssh:"}
+		critOnly := &Permissions{CriticalOptions: map[string]string{noTouchRequiredExtension: ""}}
+		if err := runAuth(t, s, critOnly); err == nil {
+			t.Error("no-touch-required in CriticalOptions must not waive UP")
+		}
+	})
+	t.Run("non-SK RSA signer unaffected", func(t *testing.T) {
+		if err := runAuth(t, testSigners["rsa"], nil); err != nil {
+			t.Errorf("plain RSA auth must still work: %v", err)
+		}
+	})
+}

ssh/test/agent_unix_test.go

@@ -24,9 +24,8 @@ func TestAgentForward(t *testing.T) {
 		t.Fatalf("Error adding key: %s", err)
 	}
 	if err := keyring.Add(agent.AddedKey{
-		PrivateKey:       testPrivateKeys["ecdsa"],
-		ConfirmBeforeUse: true,
-		LifetimeSecs:     3600,
+		PrivateKey:   testPrivateKeys["ecdsa"],
+		LifetimeSecs: 3600,
 	}); err != nil {
 		t.Fatalf("Error adding key with constraints: %s", err)
 	}