[openssh-commits] [openssh] 01/01: upstream: Client-side workaround for a bug in OpenSSH 7.4: this release

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djm pushed a commit to branch master
in repository openssh.

commit 4265215d7300901fd7097061c7517688ade82f8e
Author: djm at openbsd.org <djm at openbsd.org>
Date:   Sun Jun 6 03:40:39 2021 +0000

    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
---
 compat.c      |  4 +++-
 compat.h      |  4 ++--
 sshconnect2.c | 13 +++++++++++--
 3 files changed, 16 insertions(+), 5 deletions(-)

diff --git a/compat.c b/compat.c
index 69befa96..3f153bd4 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 c197fafc..167409b2 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 a53ab95d..9b9a99b9 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);

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