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>

1 files changed, 16 insertions, 3 deletions

diff --git a/ssh/client_auth_test.go b/ssh/client_auth_test.go
index e384c79..b526f66 100644
--- a/ssh/client_auth_test.go
+++ b/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()