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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
This commit is contained in:
djm@openbsd.org 2021-06-06 03:40:39 +00:00 committed by Damien Miller
parent bda270d7fb
commit 4265215d73
3 changed files with 16 additions and 5 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

4
compat.c
View File

@ -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*,"

4
compat.h
View File

@ -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 */

13
sshconnect2.c
View File

@ -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);