OpenSSH 10.6, released Tuesday, makes compression less effective and rejects some usernames that previously worked, and the maintainers knew both changes would break things when they shipped them.

One change disables the LZ77 dictionary coder used for SSH compression after researchers found that sharing compression state across multiple channels could expose plaintext. The other blocks $ and \ in command-line usernames to prevent them from being interpreted by the shell through directives such as ProxyCommand and Match exec.

The release comes as OpenSSH sees more security research conducted with AI models, while those models get better at finding exploitable flaws. Other researchers later found some of those bugs independently. OpenSSH warned that adversaries who don’t report what they find “are likely to be able to discover these bugs too,” and for now, the project plans to release fixes more often rather than wait for its usual release schedule.

OpenSSH warned that adversaries who don’t report what they find “are likely to be able to discover these bugs too,” and for now, the project plans to release fixes more often rather than wait for its usual release schedule.

SSH can carry several things over one encrypted connection, including an interactive shell, port forwarding, or a dynamic SOCKS proxy. When compression is enabled, those channels share the same compression state, although OpenSSH leaves compression off by default, so the attack only works when someone has turned it on.

” An attacker who can feed chosen plaintext into one channel and watch the resulting encrypted traffic can use the compression to recover secrets moving through another channel in the same session.

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