ssh: require host key checking in the ClientConfig

This change breaks existing behavior.

Before, a missing ClientConfig.HostKeyCallback would cause host key
checking to be disabled. In this configuration, establishing a
connection to any host just works, so today, most SSH client code in
the wild does not perform any host key checks.

This makes it easy to perform a MITM attack:

* SSH installations that use keyboard-interactive or password
authentication can be attacked with MITM, thereby stealing
passwords.

* Clients that use public-key authentication with agent forwarding are
also vulnerable: the MITM server could allow the login to succeed, and
then immediately ask the agent to authenticate the login to the real
server.

* Clients that use public-key authentication without agent forwarding
are harder to attack unnoticedly: an attacker cannot authenticate the
login to the real server, so it cannot in general present a convincing
server to the victim.

Now, a missing HostKeyCallback will cause the handshake to fail. This
change also provides InsecureIgnoreHostKey() and FixedHostKey(key) as
ready made host checkers.

A simplistic parser for OpenSSH's known_hosts file is given as an
example.  This change does not provide a full-fledged parser, as it
has complexity (wildcards, revocation, hashed addresses) that will
need further consideration.

When introduced, the host checking feature maintained backward
compatibility at the expense of security. We have decided this is not
the right tradeoff for the SSH library.

Fixes golang/go#19767

Change-Id: I45fc7ba9bd1ea29c31ec23f115cdbab99913e814
Reviewed-on: https://go-review.googlesource.com/38701
Run-TryBot: Han-Wen Nienhuys <hanwen@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

ssh/agent/client_test.go

@@ -233,7 +233,9 @@ func TestAuth(t *testing.T) {
 		conn.Close()
 	}()
 
-	conf := ssh.ClientConfig{}
+	conf := ssh.ClientConfig{
+		HostKeyCallback: ssh.InsecureIgnoreHostKey(),
+	}
 	conf.Auth = append(conf.Auth, ssh.PublicKeysCallback(agent.Signers))
 	conn, _, _, err := ssh.NewClientConn(b, "", &conf)
 	if err != nil {

ssh/agent/example_test.go

@@ -6,20 +6,20 @@ package agent_test
 
 import (
 	"log"
-	"os"
 	"net"
+	"os"
 
-        "golang.org/x/crypto/ssh"
-        "golang.org/x/crypto/ssh/agent"
+	"golang.org/x/crypto/ssh"
+	"golang.org/x/crypto/ssh/agent"
 )
 
 func ExampleClientAgent() {
 	// ssh-agent has a UNIX socket under $SSH_AUTH_SOCK
 	socket := os.Getenv("SSH_AUTH_SOCK")
-        conn, err := net.Dial("unix", socket)
-        if err != nil {
-                log.Fatalf("net.Dial: %v", err)
-        }
+	conn, err := net.Dial("unix", socket)
+	if err != nil {
+		log.Fatalf("net.Dial: %v", err)
+	}
 	agentClient := agent.NewClient(conn)
 	config := &ssh.ClientConfig{
 		User: "username",
@@ -29,6 +29,7 @@ func ExampleClientAgent() {
 			// wants it.
 			ssh.PublicKeysCallback(agentClient.Signers),
 		},
+		HostKeyCallback: ssh.InsecureIgnoreHostKey(),
 	}
 
 	sshc, err := ssh.Dial("tcp", "localhost:22", config)

ssh/agent/server_test.go

@@ -56,7 +56,9 @@ func TestSetupForwardAgent(t *testing.T) {
 		incoming <- conn
 	}()
 
-	conf := ssh.ClientConfig{}
+	conf := ssh.ClientConfig{
+		HostKeyCallback: ssh.InsecureIgnoreHostKey(),
+	}
 	conn, chans, reqs, err := ssh.NewClientConn(b, "", &conf)
 	if err != nil {
 		t.Fatalf("NewClientConn: %v", err)

ssh/certs.go

@@ -268,7 +268,7 @@ type CertChecker struct {
 	// HostKeyFallback is called when CertChecker.CheckHostKey encounters a
 	// public key that is not a certificate. It must implement host key
 	// validation or else, if nil, all such keys are rejected.
-	HostKeyFallback func(addr string, remote net.Addr, key PublicKey) error
+	HostKeyFallback HostKeyCallback
 
 	// IsRevoked is called for each certificate so that revocation checking
 	// can be implemented. It should return true if the given certificate

ssh/client.go

@@ -5,6 +5,7 @@
 package ssh
 
 import (
+	"bytes"
 	"errors"
 	"fmt"
 	"net"
@@ -68,6 +69,11 @@ func NewClient(c Conn, chans <-chan NewChannel, reqs <-chan *Request) *Client {
 func NewClientConn(c net.Conn, addr string, config *ClientConfig) (Conn, <-chan NewChannel, <-chan *Request, error) {
 	fullConf := *config
 	fullConf.SetDefaults()
+	if fullConf.HostKeyCallback == nil {
+		c.Close()
+		return nil, nil, nil, errors.New("ssh: must specify HostKeyCallback")
+	}
+
 	conn := &connection{
 		sshConn: sshConn{conn: c},
 	}
@@ -173,6 +179,13 @@ func Dial(network, addr string, config *ClientConfig) (*Client, error) {
 	return NewClient(c, chans, reqs), nil
 }
 
+// HostKeyCallback is the function type used for verifying server
+// keys.  A HostKeyCallback must return nil if the host key is OK, or
+// an error to reject it. It receives the hostname as passed to Dial
+// or NewClientConn. The remote address is the RemoteAddr of the
+// net.Conn underlying the the SSH connection.
+type HostKeyCallback func(hostname string, remote net.Addr, key PublicKey) 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 {
@@ -188,10 +201,12 @@ type ClientConfig struct {
 	// be used during authentication.
 	Auth []AuthMethod
 
-	// HostKeyCallback, if not nil, is called during the cryptographic
-	// handshake to validate the server's host key. A nil HostKeyCallback
-	// implies that all host keys are accepted.
-	HostKeyCallback func(hostname string, remote net.Addr, key PublicKey) error
+	// HostKeyCallback is called during the cryptographic
+	// handshake to validate the server's host key. The client
+	// configuration must supply this callback for the connection
+	// to succeed. The functions InsecureIgnoreHostKey or
+	// FixedHostKey can be used for simplistic host key checks.
+	HostKeyCallback HostKeyCallback
 
 	// ClientVersion contains the version identification string that will
 	// be used for the connection. If empty, a reasonable default is used.
@@ -209,3 +224,33 @@ type ClientConfig struct {
 	// A Timeout of zero means no timeout.
 	Timeout time.Duration
 }
+
+// InsecureIgnoreHostKey returns a function that can be used for
+// ClientConfig.HostKeyCallback to accept any host key. It should
+// not be used for production code.
+func InsecureIgnoreHostKey() HostKeyCallback {
+	return func(hostname string, remote net.Addr, key PublicKey) error {
+		return nil
+	}
+}
+
+type fixedHostKey struct {
+	key PublicKey
+}
+
+func (f *fixedHostKey) check(hostname string, remote net.Addr, key PublicKey) error {
+	if f.key == nil {
+		return fmt.Errorf("ssh: required host key was nil")
+	}
+	if !bytes.Equal(key.Marshal(), f.key.Marshal()) {
+		return fmt.Errorf("ssh: host key mismatch")
+	}
+	return nil
+}
+
+// FixedHostKey returns a function for use in
+// ClientConfig.HostKeyCallback to accept only a specific host key.
+func FixedHostKey(key PublicKey) HostKeyCallback {
+	hk := &fixedHostKey{key}
+	return hk.check
+}

ssh/client_auth_test.go

@@ -92,6 +92,7 @@ func TestClientAuthPublicKey(t *testing.T) {
 		Auth: []AuthMethod{
 			PublicKeys(testSigners["rsa"]),
 		},
+		HostKeyCallback: InsecureIgnoreHostKey(),
 	}
 	if err := tryAuth(t, config); err != nil {
 		t.Fatalf("unable to dial remote side: %s", err)
@@ -104,6 +105,7 @@ func TestAuthMethodPassword(t *testing.T) {
 		Auth: []AuthMethod{
 			Password(clientPassword),
 		},
+		HostKeyCallback: InsecureIgnoreHostKey(),
 	}
 
 	if err := tryAuth(t, config); err != nil {
@@ -123,6 +125,7 @@ func TestAuthMethodFallback(t *testing.T) {
 					return "WRONG", nil
 				}),
 		},
+		HostKeyCallback: InsecureIgnoreHostKey(),
 	}
 
 	if err := tryAuth(t, config); err != nil {
@@ -141,6 +144,7 @@ func TestAuthMethodWrongPassword(t *testing.T) {
 			Password("wrong"),
 			PublicKeys(testSigners["rsa"]),
 		},
+		HostKeyCallback: InsecureIgnoreHostKey(),
 	}
 
 	if err := tryAuth(t, config); err != nil {
@@ -158,6 +162,7 @@ func TestAuthMethodKeyboardInteractive(t *testing.T) {
 		Auth: []AuthMethod{
 			KeyboardInteractive(answers.Challenge),
 		},
+		HostKeyCallback: InsecureIgnoreHostKey(),
 	}
 
 	if err := tryAuth(t, config); err != nil {
@@ -203,6 +208,7 @@ func TestAuthMethodRSAandDSA(t *testing.T) {
 		Auth: []AuthMethod{
 			PublicKeys(testSigners["dsa"], testSigners["rsa"]),
 		},
+		HostKeyCallback: InsecureIgnoreHostKey(),
 	}
 	if err := tryAuth(t, config); err != nil {
 		t.Fatalf("client could not authenticate with rsa key: %v", err)
@@ -219,6 +225,7 @@ func TestClientHMAC(t *testing.T) {
 			Config: Config{
 				MACs: []string{mac},
 			},
+			HostKeyCallback: InsecureIgnoreHostKey(),
 		}
 		if err := tryAuth(t, config); err != nil {
 			t.Fatalf("client could not authenticate with mac algo %s: %v", mac, err)
@@ -254,6 +261,7 @@ func TestClientUnsupportedKex(t *testing.T) {
 		Config: Config{
 			KeyExchanges: []string{"diffie-hellman-group-exchange-sha256"}, // not currently supported
 		},
+		HostKeyCallback: InsecureIgnoreHostKey(),
 	}
 	if err := tryAuth(t, config); err == nil || !strings.Contains(err.Error(), "common algorithm") {
 		t.Errorf("got %v, expected 'common algorithm'", err)
@@ -273,7 +281,8 @@ func TestClientLoginCert(t *testing.T) {
 	}
 
 	clientConfig := &ClientConfig{
-		User: "user",
+		User:            "user",
+		HostKeyCallback: InsecureIgnoreHostKey(),
 	}
 	clientConfig.Auth = append(clientConfig.Auth, PublicKeys(certSigner))
 
@@ -363,6 +372,7 @@ func testPermissionsPassing(withPermissions bool, t *testing.T) {
 		Auth: []AuthMethod{
 			PublicKeys(testSigners["rsa"]),
 		},
+		HostKeyCallback: InsecureIgnoreHostKey(),
 	}
 	if withPermissions {
 		clientConfig.User = "permissions"
@@ -409,6 +419,7 @@ func TestRetryableAuth(t *testing.T) {
 			}), 2),
 			PublicKeys(testSigners["rsa"]),
 		},
+		HostKeyCallback: InsecureIgnoreHostKey(),
 	}
 
 	if err := tryAuth(t, config); err != nil {
@@ -430,7 +441,8 @@ func ExampleRetryableAuthMethod(t *testing.T) {
 	}
 
 	config := &ClientConfig{
-		User: user,
+		HostKeyCallback: InsecureIgnoreHostKey(),
+		User:            user,
 		Auth: []AuthMethod{
 			RetryableAuthMethod(KeyboardInteractiveChallenge(Cb), NumberOfPrompts),
 		},
@@ -450,7 +462,8 @@ func TestClientAuthNone(t *testing.T) {
 	serverConfig.AddHostKey(testSigners["rsa"])
 
 	clientConfig := &ClientConfig{
-		User: user,
+		User:            user,
+		HostKeyCallback: InsecureIgnoreHostKey(),
 	}
 
 	c1, c2, err := netPipe()

ssh/client_test.go

@@ -6,6 +6,7 @@ package ssh
 
 import (
 	"net"
+	"strings"
 	"testing"
 )
 
@@ -13,6 +14,7 @@ func testClientVersion(t *testing.T, config *ClientConfig, expected string) {
 	clientConn, serverConn := net.Pipe()
 	defer clientConn.Close()
 	receivedVersion := make(chan string, 1)
+	config.HostKeyCallback = InsecureIgnoreHostKey()
 	go func() {
 		version, err := readVersion(serverConn)
 		if err != nil {
@@ -37,3 +39,43 @@ func TestCustomClientVersion(t *testing.T) {
 func TestDefaultClientVersion(t *testing.T) {
 	testClientVersion(t, &ClientConfig{}, packageVersion)
 }
+
+func TestHostKeyCheck(t *testing.T) {
+	for _, tt := range []struct {
+		name      string
+		wantError string
+		key       PublicKey
+	}{
+		{"no callback", "must specify HostKeyCallback", nil},
+		{"correct key", "", testSigners["rsa"].PublicKey()},
+		{"mismatch", "mismatch", testSigners["ecdsa"].PublicKey()},
+	} {
+		c1, c2, err := netPipe()
+		if err != nil {
+			t.Fatalf("netPipe: %v", err)
+		}
+		defer c1.Close()
+		defer c2.Close()
+		serverConf := &ServerConfig{
+			NoClientAuth: true,
+		}
+		serverConf.AddHostKey(testSigners["rsa"])
+
+		go NewServerConn(c1, serverConf)
+		clientConf := ClientConfig{
+			User: "user",
+		}
+		if tt.key != nil {
+			clientConf.HostKeyCallback = FixedHostKey(tt.key)
+		}
+
+		_, _, _, err = NewClientConn(c2, "", &clientConf)
+		if err != nil {
+			if tt.wantError == "" || !strings.Contains(err.Error(), tt.wantError) {
+				t.Errorf("%s: got error %q, missing %q", err.Error(), tt.wantError)
+			}
+		} else if tt.wantError != "" {
+			t.Errorf("%s: succeeded, but want error string %q", tt.name, tt.wantError)
+		}
+	}
+}

ssh/doc.go

@@ -14,5 +14,8 @@ others.
 References:
   [PROTOCOL.certkeys]: http://cvsweb.openbsd.org/cgi-bin/cvsweb/src/usr.bin/ssh/PROTOCOL.certkeys?rev=HEAD
   [SSH-PARAMETERS]:    http://www.iana.org/assignments/ssh-parameters/ssh-parameters.xml#ssh-parameters-1
+
+This package does not fall under the stability promise of the Go language itself,
+so its API may be changed when pressing needs arise.
 */
 package ssh // import "golang.org/x/crypto/ssh"

ssh/example_test.go

@@ -5,12 +5,16 @@
 package ssh_test
 
 import (
+	"bufio"
 	"bytes"
 	"fmt"
 	"io/ioutil"
 	"log"
 	"net"
 	"net/http"
+	"os"
+	"path/filepath"
+	"strings"
 
 	"golang.org/x/crypto/ssh"
 	"golang.org/x/crypto/ssh/terminal"
@@ -90,8 +94,6 @@ func ExampleNewServerConn() {
 	// The incoming Request channel must be serviced.
 	go ssh.DiscardRequests(reqs)
 
-	// Service the incoming Channel channel.
-
 	// Service the incoming Channel channel.
 	for newChannel := range chans {
 		// Channels have a type, depending on the application level
@@ -131,16 +133,59 @@ func ExampleNewServerConn() {
 	}
 }
 
+func ExampleHostKeyCheck() {
+	// Every client must provide a host key check.  Here is a
+	// simple-minded parse of OpenSSH's known_hosts file
+	host := "hostname"
+	file, err := os.Open(filepath.Join(os.Getenv("HOME"), ".ssh", "known_hosts"))
+	if err != nil {
+		log.Fatal(err)
+	}
+	defer file.Close()
+
+	scanner := bufio.NewScanner(file)
+	var hostKey ssh.PublicKey
+	for scanner.Scan() {
+		fields := strings.Split(scanner.Text(), " ")
+		if len(fields) != 3 {
+			continue
+		}
+		if strings.Contains(fields[0], host) {
+			var err error
+			hostKey, _, _, _, err = ssh.ParseAuthorizedKey(scanner.Bytes())
+			if err != nil {
+				log.Fatalf("error parsing %q: %v", fields[2], err)
+			}
+			break
+		}
+	}
+
+	if hostKey == nil {
+		log.Fatalf("no hostkey for %s", host)
+	}
+
+	config := ssh.ClientConfig{
+		User:            os.Getenv("USER"),
+		HostKeyCallback: ssh.FixedHostKey(hostKey),
+	}
+
+	_, err = ssh.Dial("tcp", host+":22", &config)
+	log.Println(err)
+}
+
 func ExampleDial() {
+	var hostKey ssh.PublicKey
 	// An SSH client is represented with a ClientConn.
 	//
 	// To authenticate with the remote server you must pass at least one
-	// implementation of AuthMethod via the Auth field in ClientConfig.
+	// implementation of AuthMethod via the Auth field in ClientConfig,
+	// and provide a HostKeyCallback.
 	config := &ssh.ClientConfig{
 		User: "username",
 		Auth: []ssh.AuthMethod{
 			ssh.Password("yourpassword"),
 		},
+		HostKeyCallback: ssh.FixedHostKey(hostKey),
 	}
 	client, err := ssh.Dial("tcp", "yourserver.com:22", config)
 	if err != nil {
@@ -166,6 +211,7 @@ func ExampleDial() {
 }
 
 func ExamplePublicKeys() {
+	var hostKey ssh.PublicKey
 	// A public key may be used to authenticate against the remote
 	// server by using an unencrypted PEM-encoded private key file.
 	//
@@ -188,6 +234,7 @@ func ExamplePublicKeys() {
 			// Use the PublicKeys method for remote authentication.
 			ssh.PublicKeys(signer),
 		},
+		HostKeyCallback: ssh.FixedHostKey(hostKey),
 	}
 
 	// Connect to the remote server and perform the SSH handshake.
@@ -199,11 +246,13 @@ func ExamplePublicKeys() {
 }
 
 func ExampleClient_Listen() {
+	var hostKey ssh.PublicKey
 	config := &ssh.ClientConfig{
 		User: "username",
 		Auth: []ssh.AuthMethod{
 			ssh.Password("password"),
 		},
+		HostKeyCallback: ssh.FixedHostKey(hostKey),
 	}
 	// Dial your ssh server.
 	conn, err := ssh.Dial("tcp", "localhost:22", config)
@@ -226,12 +275,14 @@ func ExampleClient_Listen() {
 }
 
 func ExampleSession_RequestPty() {
+	var hostKey ssh.PublicKey
 	// Create client config
 	config := &ssh.ClientConfig{
 		User: "username",
 		Auth: []ssh.AuthMethod{
 			ssh.Password("password"),
 		},
+		HostKeyCallback: ssh.FixedHostKey(hostKey),
 	}
 	// Connect to ssh server
 	conn, err := ssh.Dial("tcp", "localhost:22", config)

ssh/handshake.go

@@ -74,7 +74,7 @@ type handshakeTransport struct {
 	startKex chan *pendingKex
 
 	// data for host key checking
-	hostKeyCallback func(hostname string, remote net.Addr, key PublicKey) error
+	hostKeyCallback HostKeyCallback
 	dialAddress     string
 	remoteAddr      net.Addr
 
@@ -614,11 +614,9 @@ func (t *handshakeTransport) client(kex kexAlgorithm, algs *algorithms, magics *
 		return nil, err
 	}
 
-	if t.hostKeyCallback != nil {
-		err = t.hostKeyCallback(t.dialAddress, t.remoteAddr, hostKey)
-		if err != nil {
-			return nil, err
-		}
+	err = t.hostKeyCallback(t.dialAddress, t.remoteAddr, hostKey)
+	if err != nil {
+		return nil, err
 	}
 
 	return result, nil

ssh/handshake_test.go

@@ -436,6 +436,7 @@ func testHandshakeErrorHandlingN(t *testing.T, readLimit, writeLimit int, couple
 	clientConf.SetDefaults()
 	clientConn := newHandshakeTransport(&errorKeyingTransport{b, -1, -1}, &clientConf, []byte{'a'}, []byte{'b'})
 	clientConn.hostKeyAlgorithms = []string{key.PublicKey().Type()}
+	clientConn.hostKeyCallback = InsecureIgnoreHostKey()
 	go clientConn.readLoop()
 	go clientConn.kexLoop()
 

ssh/session_test.go

@@ -59,7 +59,8 @@ func dial(handler serverType, t *testing.T) *Client {
 	}()
 
 	config := &ClientConfig{
-		User: "testuser",
+		User:            "testuser",
+		HostKeyCallback: InsecureIgnoreHostKey(),
 	}
 
 	conn, chans, reqs, err := NewClientConn(c2, "", config)
@@ -641,7 +642,8 @@ func TestSessionID(t *testing.T) {
 	}
 	serverConf.AddHostKey(testSigners["ecdsa"])
 	clientConf := &ClientConfig{
-		User: "user",
+		HostKeyCallback: InsecureIgnoreHostKey(),
+		User:            "user",
 	}
 
 	go func() {
@@ -747,7 +749,9 @@ func TestHostKeyAlgorithms(t *testing.T) {
 
 	// By default, we get the preferred algorithm, which is ECDSA 256.
 
-	clientConf := &ClientConfig{}
+	clientConf := &ClientConfig{
+		HostKeyCallback: InsecureIgnoreHostKey(),
+	}
 	connect(clientConf, KeyAlgoECDSA256)
 
 	// Client asks for RSA explicitly.

ssh/test/cert_test.go

@@ -36,7 +36,8 @@ func TestCertLogin(t *testing.T) {
 	}
 
 	conf := &ssh.ClientConfig{
-		User: username(),
+		User:            username(),
+		HostKeyCallback: ssh.InsecureIgnoreHostKey(),
 	}
 	conf.Auth = append(conf.Auth, ssh.PublicKeys(certSigner))
 	client, err := s.TryDial(conf)