OpenSSH 5.2 Secured and Tuned

Feb 25, 2009

Even though the OpenSSH project emphasizes that the focus of 5.2 is bug fixes to the 5.1 version, 5.2 does contain some notable enhancements.

Security-wise the new OpenSSH version replaces cipher block chaining (CBC) mode as the default cipher order with the Advanced Encryption Standard (AES) Counter (CTR) mode to remove the susceptibility to "Plaintext Recovery Attack Against SSH." The software also adds other countermeasures to these attacks, as reported in CPNI Vulnerability Advisory SSH 957037. Last November it became clear that many versions of OpenSSH exposed up to 32 bits of plaintext ciphertext to attackers when the default CBC mode was in use. The solution in 5.2 was to read the maximum supported packet length instead of terminating the connection, thereby eliminating the leaks that allowed the plaintext recovery attacks.

Compared to OpenSSH 5.1, the updated version provides further command line options and minor functional enhancements. For example, the ssh -y option redirects logging to syslog and dynamic port forwarding was improved. The release changelog includes the list of fixed bugs.

OpenSSH emanates from the OpenBSD project, where a separate team focuses on OpenSSH's portability to different systems. The mirrors with the portable versions also include diffs against the OpenBSD source.

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