OpenSSH: Memory corruption in AES-GCM support

How it was found

1. Vulnerability

   A memory corruption vulnerability exists in the post- authentication sshd process when an AES-GCM cipher ([email protected] or [email protected]) 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;

This document may be found at: http://www.openssh.com/txt/gcmrekey.adv