Home / vulnerabilities OpenSSH 6.3 Memory Corruption
Posted on 09 November 2013
Source : packetstormsecurity.org Link
OpenSSH Security Advisory: gcmrekey.adv
This document may be found at: http://www.openssh.com/txt/gcmrekey.adv
1. Vulnerability
A memory corruption vulnerability exists in the post-
authentication sshd process when an AES-GCM cipher
(aes128-gcm@openssh.com or aes256-gcm@openssh.com) is
selected during kex exchange.
If exploited, this vulnerability might permit code execution
with the privileges of the authenticated user and may
therefore allow bypassing restricted shell/command
configurations.
2. Affected configurations
OpenSSH 6.2 and OpenSSH 6.3 when built against an OpenSSL
that supports AES-GCM.
3. Mitigation
Disable AES-GCM in the server configuration. The following
sshd_config option will disable AES-GCM while leaving other
ciphers active:
Ciphers
aes128-ctr,aes192-ctr,aes256-ctr,aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc,aes256-cbc
4. Details
When using AES-GCM, sshd was not initialising a Message
Authentication Code (MAC) context that is unused when the
cipher mode offers authentication itself. This context
contains some callback pointers, including a cleanup callback
that was still being invoked during a rekeying operation.
As such, the address being called was derived from previous
heap contents.
This vulnerability is mitigated by the difficulty of
pre-loading the heap with a useful callback address and by
any platform address-space layout randomisation applied to
sshd and the shared libraries it depends upon.
5. Credit
This issue was identified by Markus Friedl (an OpenSSH
developer) on November 7th, 2013.
6. Fix
OpenSSH 6.4 contains a fix for this vulnerability. Users who
prefer to continue to use OpenSSH 6.2 or 6.3 may apply this
patch:
Index: monitor_wrap.c
===================================================================
RCS file: /cvs/src/usr.bin/ssh/monitor_wrap.c,v
retrieving revision 1.76
diff -u -p -u -r1.76 monitor_wrap.c
--- monitor_wrap.c 17 May 2013 00:13:13 -0000 1.76
+++ monitor_wrap.c 6 Nov 2013 16:31:26 -0000
@@ -469,7 +469,7 @@ mm_newkeys_from_blob(u_char *blob, int b
buffer_init(&b);
buffer_append(&b, blob, blen);
- newkey = xmalloc(sizeof(*newkey));
+ newkey = xcalloc(1, sizeof(*newkey));
enc = &newkey->enc;
mac = &newkey->mac;
comp = &newkey->comp;
