Client-side workaround for a bug in OpenSSH 7.4: this release allows

RSA/SHA2 signatures for public key authentication but fails to advertise
this correctly via SSH2_MSG_EXT_INFO. This causes clients of these
server to incorrectly match PubkeyAcceptedAlgorithms and potentially
refuse to offer valid keys.

Reported by and based on patch from Gordon Messmer via bz3213, thanks
also for additional analysis by Jakub Jelen. ok dtucker

diff --git a/usr.bin/ssh/compat.c b/usr.bin/ssh/compat.c
index 8514e0a..2a67390 100644 (file)
--- a/usr.bin/ssh/compat.c
+++ b/usr.bin/ssh/compat.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: compat.c,v 1.117 2021/01/27 09:26:54 djm Exp $ */
+/* $OpenBSD: compat.c,v 1.118 2021/06/06 03:40:39 djm Exp $ */
 /*
  * Copyright (c) 1999, 2000, 2001, 2002 Markus Friedl.  All rights reserved.
  *
@@ -61,6 +61,8 @@ compat_banner(struct ssh *ssh, const char *version)
                { "OpenSSH_6.5*,"
                  "OpenSSH_6.6*",       SSH_NEW_OPENSSH|SSH_BUG_CURVE25519PAD|
                                        SSH_BUG_SIGTYPE},
+               { "OpenSSH_7.4*",       SSH_NEW_OPENSSH|SSH_BUG_SIGTYPE|
+                                       SSH_BUG_SIGTYPE74},
                { "OpenSSH_7.0*,"
                  "OpenSSH_7.1*,"
                  "OpenSSH_7.2*,"

diff --git a/usr.bin/ssh/compat.h b/usr.bin/ssh/compat.h
index c197faf..167409b 100644 (file)
--- a/usr.bin/ssh/compat.h
+++ b/usr.bin/ssh/compat.h
@@ -1,4 +1,4 @@
-/* $OpenBSD: compat.h,v 1.56 2021/01/27 09:26:54 djm Exp $ */
+/* $OpenBSD: compat.h,v 1.57 2021/06/06 03:40:39 djm Exp $ */
 
 /*
  * Copyright (c) 1999, 2000, 2001 Markus Friedl.  All rights reserved.
@@ -29,7 +29,7 @@
 
 #define SSH_BUG_UTF8TTYMODE    0x00000001
 #define SSH_BUG_SIGTYPE                0x00000002
-/* #define unused              0x00000004 */
+#define SSH_BUG_SIGTYPE74      0x00000004
 /* #define unused              0x00000008 */
 #define SSH_OLD_SESSIONID      0x00000010
 /* #define unused              0x00000020 */

diff --git a/usr.bin/ssh/sshconnect2.c b/usr.bin/ssh/sshconnect2.c
index 974a7c9..f1691bd 100644 (file)
--- a/usr.bin/ssh/sshconnect2.c
+++ b/usr.bin/ssh/sshconnect2.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: sshconnect2.c,v 1.347 2021/04/03 06:18:41 djm Exp $ */
+/* $OpenBSD: sshconnect2.c,v 1.348 2021/06/06 03:40:39 djm Exp $ */
 /*
  * Copyright (c) 2000 Markus Friedl.  All rights reserved.
  * Copyright (c) 2008 Damien Miller.  All rights reserved.
@@ -1168,6 +1168,7 @@ static char *
 key_sig_algorithm(struct ssh *ssh, const struct sshkey *key)
 {
        char *allowed, *oallowed, *cp, *tmp, *alg = NULL;
+       const char *server_sig_algs;
 
        /*
         * The signature algorithm will only differ from the key algorithm
@@ -1182,6 +1183,14 @@ key_sig_algorithm(struct ssh *ssh, const struct sshkey *key)
                    options.pubkey_accepted_algos, NULL);
        }
 
+       /*
+        * Workaround OpenSSH 7.4 bug: this version supports RSA/SHA-2 but
+        * fails to advertise it via SSH2_MSG_EXT_INFO.
+        */
+       server_sig_algs = ssh->kex->server_sig_algs;
+       if (key->type == KEY_RSA && (ssh->compat & SSH_BUG_SIGTYPE74))
+               server_sig_algs = "rsa-sha2-256,rsa-sha2-512";
+
        /*
         * For RSA keys/certs, since these might have a different sig type:
         * find the first entry in PubkeyAcceptedAlgorithms of the right type
@@ -1193,7 +1202,7 @@ key_sig_algorithm(struct ssh *ssh, const struct sshkey *key)
                if (sshkey_type_from_name(cp) != key->type)
                        continue;
                tmp = match_list(sshkey_sigalg_by_name(cp),
-                   ssh->kex->server_sig_algs, NULL);
+                   server_sig_algs, NULL);
                if (tmp != NULL)
                        alg = xstrdup(cp);
                free(tmp);