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-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 =========================================================================== AUSCERT External Security Bulletin Redistribution ESB-2008.1076 -- [Win][UNIX/Linux][Appliance] SSH CBC plaintext recovery vulnerability 26 November 2008 =========================================================================== AusCERT Security Bulletin Summary --------------------------------- Product: SSH Cipher Block Chaining mode Publisher: US-CERT Operating System: UNIX variants (UNIX, Linux, OSX) Windows Network Appliance Impact: Access Confidential Data Access: Remote/Unauthenticated CVE Names: CVE-2008-5161 Original Bulletin: http://www.kb.cert.org/vuls/id/958563 Comment: There are currently no patches for this vulnerability. A potential workaround is to use CTR (counter) mode rather than CBC. - --------------------------BEGIN INCLUDED TEXT-------------------- US-CERT Vulnerability Note VU#958563 SSH CBC vulnerability Overview A vulnerability exists in SSH messages that employ CBC mode that may allow an attacker to recover plaintext from a block of ciphertext. I. Description The Secure Shell (SSH) is a network protocol that creates a secure channel between two networked devices in order to allow data to be exchanged. SSH can create this secure channel by using Cipher Block Chaining (CBC) mode encryption. This mode adds a feedback mechanism to a block cipher that operates in a way that ensures that each block is used to modify the encryption of the next block. SSH contains a vulnerability in the way certain types of errors are handled. Attacks leveraging this vulnerabilty would lead to the loss of the SSH session. According to CPNI Vulnerability Advisory SSH: If exploited, this attack can potentially allow an attacker to recover up to 32 bits of plaintext from an arbitrary block of ciphertext from a connection secured using the SSH protocol in the standard configuration. If OpenSSH is used in the standard configuration, then the attacker's success probability for recovering 32 bits of plaintext is 2^{-18}. A variant of the attack against OpenSSH in the standard configuration can verifiably recover 14 bits of plaintext with probability 2^{-14}. The success probability of the attack for other implementations of SSH is not known. II. Impact An attacker may be able to recover up to 32 bits of plaintext from an arbitrary block of ciphertext. III. Solution We are currently unaware of a practical solution to this problem. Use CTR Mode SSH can be done using Counter (CTR) mode encryption. This mode generates the keystream by encrypting successive values of a "counter" function. For more information see the Block Cipher Modes article on wikipedia. In order to mitigate this vulnerabilty SSH can be setup to use CTR mode rather CBC mode. According to CPNI Vulnerability Advisory SSH: The most straightforward solution is to use CTR mode instead of CBC mode, since this renders SSH resistant to the attack. An RFC already exists to standardise counter mode for use in SSH (RFC 4344) ... Systems Affected Vendor Status Date Notified Date Updated Bitvise Vulnerable 2008-11-07 2008-11-24 FiSSH Vulnerable 2008-11-07 2008-11-24 Icon Labs Vulnerable 2008-11-07 2008-11-24 OpenSSH Vulnerable 2008-11-07 2008-11-24 OSSH Vulnerable 2008-11-07 2008-11-24 PuTTY Vulnerable 2008-11-07 2008-11-24 Redback Networks, Inc. Vulnerable 2008-11-07 2008-11-24 SSH Communications Security Corp Vulnerable 2008-11-07 2008-11-24 TTSSH Vulnerable 2008-11-07 2008-11-24 VanDyke Software Vulnerable 2008-11-07 2008-11-24 Wind River Systems, Inc. Vulnerable 2008-11-07 2008-11-24 References http://www.cpni.gov.uk/Docs/Vulnerability_Advisory_SSH.txt http://isc.sans.org/diary.html?storyid=5366 http://en.wikipedia.org/wiki/Block_cipher_modes_of_operation Credit Thanks to CPNI for reporting this vulnerability. This document was written by Chris Taschner. Other Information Date Public: 2008-11-14 Date First Published: 2008-11-24 Date Last Updated: 2008-11-24 CERT Advisory: CVE-ID(s): NVD-ID(s): US-CERT Technical Alerts: Metric: 0.30 Document Revision: 13 - --------------------------END INCLUDED TEXT-------------------- You have received this e-mail bulletin as a result of your organisation's registration with AusCERT. The mailing list you are subscribed to is maintained within your organisation, so if you do not wish to continue receiving these bulletins you should contact your local IT manager. If you do not know who that is, please send an email to auscert@auscert.org.au and we will forward your request to the appropriate person. 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If you have any questions or need further information, please contact them directly. Previous advisories and external security bulletins can be retrieved from: http://www.auscert.org.au/render.html?cid=1980 If you believe that your computer system has been compromised or attacked in any way, we encourage you to let us know by completing the secure National IT Incident Reporting Form at: http://www.auscert.org.au/render.html?it=3192 =========================================================================== Australian Computer Emergency Response Team The University of Queensland Brisbane Qld 4072 Internet Email: auscert@auscert.org.au Facsimile: (07) 3365 7031 Telephone: (07) 3365 4417 (International: +61 7 3365 4417) AusCERT personnel answer during Queensland business hours which are GMT+10:00 (AEST). On call after hours for member emergencies only. =========================================================================== -----BEGIN PGP SIGNATURE----- Comment: http://www.auscert.org.au/render.html?it=1967 iQCVAwUBSSzkBCh9+71yA2DNAQIFOgP6A0zeoz6KJpLubeLp6ITjdgTBJZ0uKX/R 3ZRIWvZ5lduArS9khhTV4KsPRx4vCReVeXo1hYFSU3OlJ0IIy2RZg/wbFaumQGwL 5NTRoJCKYwkCYxLeRdKapzETR/G8JzeQPDyAFyrWr0xkiPdhFb/Gg34FBzdEg3DT ESGp5O7SK5o= =+4Jk -----END PGP SIGNATURE-----