Restrict ssh-agent from signing web challenges for FIDO keys.

When signing messages in ssh-agent using a FIDO key that has an
application string that does not start with "ssh:", ensure that the
message being signed is one of the forms expected for the SSH protocol
(currently pubkey authentication and sshsig signatures).

This prevents ssh-agent forwarding on a host that has FIDO keys
attached granting the ability for the remote side to sign challenges
for web authentication using those keys too.

Note that the converse case of web browsers signing SSH challenges is
already precluded because no web RP can have the "ssh:" prefix in the
application string that we require.

ok markus@

1 files changed, 100 insertions, 10 deletions

diff --git a/usr.bin/ssh/ssh-agent.c b/usr.bin/ssh/ssh-agent.c
index ea73ed272a0..97d70f9199c 100644
--- a/usr.bin/ssh/ssh-agent.c
+++ b/usr.bin/ssh/ssh-agent.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: ssh-agent.c,v 1.257 2020/03/06 18:28:27 markus Exp $ */
+/* $OpenBSD: ssh-agent.c,v 1.258 2020/05/26 01:26:58 djm Exp $ */
 /*
  * Author: Tatu Ylonen <ylo@cs.hut.fi>
  * Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
@@ -63,6 +63,7 @@
 
 #include "xmalloc.h"
 #include "ssh.h"
+#include "ssh2.h"
 #include "sshbuf.h"
 #include "sshkey.h"
 #include "authfd.h"
@@ -153,6 +154,9 @@ static long lifetime = 0;
 
 static int fingerprint_hash = SSH_FP_HASH_DEFAULT;
 
+/* Refuse signing of non-SSH messages for web-origin FIDO keys */
+static int restrict_websafe = 1;
+
 static void
 close_socket(SocketEntry *e)
 {
@@ -268,6 +272,80 @@ agent_decode_alg(struct sshkey *key, u_int flags)
 	return NULL;
 }
 
+/*
+ * This function inspects a message to be signed by a FIDO key that has a
+ * web-like application string (i.e. one that does not begin with "ssh:".
+ * It checks that the message is one of those expected for SSH operations
+ * (pubkey userauth, sshsig, CA key signing) to exclude signing challenges
+ * for the web.
+ */
+static int
+check_websafe_message_contents(struct sshkey *key,
+    const u_char *msg, size_t len)
+{
+	int matched = 0;
+	struct sshbuf *b;
+	u_char m, n;
+	char *cp1 = NULL, *cp2 = NULL;
+	int r;
+	struct sshkey *mkey = NULL;
+
+	if ((b = sshbuf_from(msg, len)) == NULL)
+		fatal("%s: sshbuf_new", __func__);
+
+	/* SSH userauth request */
+	if ((r = sshbuf_get_string_direct(b, NULL, NULL)) == 0 && /* sess_id */
+	    (r = sshbuf_get_u8(b, &m)) == 0 && /* SSH2_MSG_USERAUTH_REQUEST */
+	    (r = sshbuf_get_cstring(b, NULL, NULL)) == 0 && /* server user */
+	    (r = sshbuf_get_cstring(b, &cp1, NULL)) == 0 && /* service */
+	    (r = sshbuf_get_cstring(b, &cp2, NULL)) == 0 && /* method */
+	    (r = sshbuf_get_u8(b, &n)) == 0 && /* sig-follows */
+	    (r = sshbuf_get_cstring(b, NULL, NULL)) == 0 && /* alg */
+	    (r = sshkey_froms(b, &mkey)) == 0 && /* key */
+	    sshbuf_len(b) == 0) {
+		debug("%s: parsed userauth", __func__);
+		if (m == SSH2_MSG_USERAUTH_REQUEST && n == 1 &&
+		    strcmp(cp1, "ssh-connection") == 0 &&
+		    strcmp(cp2, "publickey") == 0 &&
+		    sshkey_equal(key, mkey)) {
+			debug("%s: well formed userauth", __func__);
+			matched = 1;
+		}
+	}
+	free(cp1);
+	free(cp2);
+	sshkey_free(mkey);
+	sshbuf_free(b);
+	if (matched)
+		return 1;
+
+	if ((b = sshbuf_from(msg, len)) == NULL)
+		fatal("%s: sshbuf_new", __func__);
+	cp1 = cp2 = NULL;
+	mkey = NULL;
+
+	/* SSHSIG */
+	if ((r = sshbuf_cmp(b, 0, "SSHSIG", 6)) == 0 &&
+	    (r = sshbuf_consume(b, 6)) == 0 &&
+	    (r = sshbuf_get_cstring(b, NULL, NULL)) == 0 && /* namespace */
+	    (r = sshbuf_get_string_direct(b, NULL, NULL)) == 0 && /* reserved */
+	    (r = sshbuf_get_cstring(b, NULL, NULL)) == 0 && /* hashalg */
+	    (r = sshbuf_get_string_direct(b, NULL, NULL)) == 0 && /* H(msg) */
+	    sshbuf_len(b) == 0) {
+		debug("%s: parsed sshsig", __func__);
+		matched = 1;
+	}
+
+	sshbuf_free(b);
+	if (matched)
+		return 1;
+
+	/* XXX CA signature operation */
+
+	error("web-origin key attempting to sign non-SSH message");
+	return 0;
+}
+
 /* ssh2 only */
 static void
 process_sign_request2(SocketEntry *e)
@@ -300,14 +378,20 @@ process_sign_request2(SocketEntry *e)
 		verbose("%s: user refused key", __func__);
 		goto send;
 	}
-	if (sshkey_is_sk(id->key) &&
-	    (id->key->sk_flags & SSH_SK_USER_PRESENCE_REQD)) {
-		if ((fp = sshkey_fingerprint(key, SSH_FP_HASH_DEFAULT,
-		    SSH_FP_DEFAULT)) == NULL)
-			fatal("%s: fingerprint failed", __func__);
-		notifier = notify_start(0,
-		    "Confirm user presence for key %s %s",
-		    sshkey_type(id->key), fp);
+	if (sshkey_is_sk(id->key)) {
+		if (strncmp(id->key->sk_application, "ssh:", 4) != 0 &&
+		    !check_websafe_message_contents(key, data, dlen)) {
+			/* error already logged */
+			goto send;
+		}
+		if ((id->key->sk_flags & SSH_SK_USER_PRESENCE_REQD)) {
+			if ((fp = sshkey_fingerprint(key, SSH_FP_HASH_DEFAULT,
+			    SSH_FP_DEFAULT)) == NULL)
+				fatal("%s: fingerprint failed", __func__);
+			notifier = notify_start(0,
+			    "Confirm user presence for key %s %s",
+			    sshkey_type(id->key), fp);
+		}
 	}
 	if ((r = sshkey_sign(id->key, &signature, &slen,
 	    data, dlen, agent_decode_alg(key, flags),
@@ -1193,7 +1277,7 @@ main(int ac, char **av)
 	OpenSSL_add_all_algorithms();
 #endif
 
-	while ((ch = getopt(ac, av, "cDdksE:a:P:t:")) != -1) {
+	while ((ch = getopt(ac, av, "cDdksE:a:O:P:t:")) != -1) {
 		switch (ch) {
 		case 'E':
 			fingerprint_hash = ssh_digest_alg_by_name(optarg);
@@ -1208,6 +1292,12 @@ main(int ac, char **av)
 		case 'k':
 			k_flag++;
 			break;
+		case 'O':
+			if (strcmp(optarg, "no-restrict-websafe") == 0)
+				restrict_websafe  = 0;
+			else
+				fatal("Unknown -O option");
+			break;
 		case 'P':
 			if (provider_whitelist != NULL)
 				fatal("-P option already specified");