OpenSSH 10.6 turns off LZ77 in ssh and sshd, and OpenBSD 8.0 ships it two weeks later

OpenSSH 10.6 landed October 6 and the line item that matters is one sentence long: the LZ77 dictionary coder is disabled in ssh and sshd, to mitigate the side-channel leaks from the “Crossing the Streams” research. The attack is chosen-plaintext: if an attacker can inject data into one multiplexed channel while a secret travels in another, compressed length leaks the secret. That is CRIME and BREACH wearing a ControlMaster hat. Anyone who has run ssh multiplexing with agent forwarding should read that twice.

The cost is what you expect. Compression gets worse, so Compression yes over a slow link now buys you less. Nobody should be running it on a LAN anyway. I’d rather ship 30 percent more bytes than leak a token, and for once I have no three-paragraph rant to deliver. Fine. Good change.

The rest of the release notes:

• Usernames: $ and \ are now rejected in usernames given on the command line, because usernames from untrusted sources were turning into shell injection. If your tooling builds user@host from a hostile string, congratulations, it was already broken.
• sftp: paths returned by the server are validated harder, so a malicious server can’t steer a recursive copy into writing outside its target directory.
• GSSAPI: the server only stores delegated credentials after authentication succeeds.
• scp -R: remote-to-remote copy starts its deprecation. It still works, with warnings, and will be disabled eventually. Shell quoting differs between the two far ends, which is why it was always a footgun.
• ssh-keygen: a daylight saving time bug in date conversion that could produce wrong certificate expiries is fixed. Check your short-lived certs.
• Crypto: the hybrid ssh-mldsa44-ed25519 signature algorithm is enabled, replacing the experimental @openssh.com name. Same story as the ML-KEM key exchange: fine on your side, a surprise for whatever middlebox or ancient client sits in the path.

OpenBSD 8.0 followed on October 20 with OpenSSH 10.6 and LibreSSL 4.4.0 in base, plus the usual pile of hardening, a 512G amd64 kernel virtual address space, and vmd guests up to 64 vCPUs. Release notes list it as the 61st release. Sixty-one releases, every one on schedule, and a base system that still ships the daemon everyone else copies. See the kernel and security hub for the rest of October’s hardening news, including the host key re-exchange CVE.

Upgrade, then grep your wrappers for scp -R and for scripts that interpolate usernames. Those are the two that will bite.