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Message-ID: <CAJVRA1Tx+SV2i4FNfxbxHPAk0MwYA_Rz_0djiLqJRGQejRYKPg@mail.gmail.com> Date: Fri, 8 Nov 2013 10:47:47 -0800 From: coderman <coderman@...il.com> To: Full Disclosure <full-disclosure@...ts.grok.org.uk> Subject: OpenSSH Security Advisory: gcmrekey.adv surprised not a peep about this one here yet,... hmmm a fun one ;) we are accustomed to old software adding risk; new (secondary effects of combined AUTH+ENC modes) also carries risk! --- 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@...nssh.com or aes256-gcm@...nssh.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; _______________________________________________ Full-Disclosure - We believe in it. Charter: http://lists.grok.org.uk/full-disclosure-charter.html Hosted and sponsored by Secunia - http://secunia.com/
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