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Cyber Risks · Cisco Talos

OpenSSH Plaintext Recovery Attack - nothing to panic about

Summary: Cisco Talos examined a CPNI advisory on recovering plaintext from SSH sessions and found the attack was verified only against OpenSSH 4.7p1 on Debian, can recover at most 32 bits (about 4 ASCII characters) from an arbitrary ciphertext block, and succeeds with probability of 2^-18, or roughly 1 in 262,144. The attack requires the default configuration using CBC mode ciphers, which Talos changed in sshd_config and ssh_config before restarting the daemon, and OpenSSH's own advisory reached the same conclusion.

Key Takeaway: The exposure depends on running CBC mode ciphers in the default configuration, so a security leader should confirm whether any fleet still has OpenSSH 4.7p1-era hosts or CBC ciphers left enabled rather than assume the 1-in-262,144 odds make the question moot.

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