From: djm@openbsd.org <djm@openbsd.org>
Date: Sun, 6 Jun 2021 03:40:39 +0000 (+0000)
Subject: upstream: Client-side workaround for a bug in OpenSSH 7.4: this release
X-Git-Tag: V_8_7_P1~164
X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=4265215d7300901fd7097061c7517688ade82f8e;p=thirdparty%2Fopenssh-portable.git

upstream: 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

OpenBSD-Commit-ID: d6d0b7351d5d44c45f3daaa26efac65847a564f7
---

diff --git a/compat.c b/compat.c
index 69befa96f..3f153bd42 100644
--- a/compat.c
+++ b/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.
  *
@@ -63,6 +63,8 @@ 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/compat.h b/compat.h
index c197fafc5..167409b2b 100644
--- a/compat.h
+++ b/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/sshconnect2.c b/sshconnect2.c
index a53ab95db..9b9a99b98 100644
--- a/sshconnect2.c
+++ b/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.
@@ -1175,6 +1175,7 @@ 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
@@ -1189,6 +1190,14 @@ 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
@@ -1200,7 +1209,7 @@ 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);