SSH vulnerabilities

The Terrapin vulnerability in the SSH transport protocol affects OpenSSH before 9.6 and many other SSH implementations. A remote attacker can bypass integrity checks by omitting some packets during extension negotiation, causing security features to be downgraded or disabled. This impacts algorithms such as ChaCha20-Poly1305 and CBC with Encrypt-then-MAC.

The source-address critical option in the Permissions returned by an authentication callback was only enforced for the PublicKeyCallback and VerifiedPublicKeyCallback paths, extending the fix for CVE-2026-46595. Permissions returned by the PasswordCallback, KeyboardInteractiveCallback, NoClientAuthCallback, and GSSAPIWithMICConfig.AllowLogin callbacks were not validated against the client's remote address, so a source-address restriction set by those callbacks was silently ignored. The check is now applied to the Permissions returned by any authentication callback.

In SSH, when an authentication callback returned PartialSuccessError with non-nil Permissions, those permissions were silently discarded. This could drop certificate restrictions like force-command after a second factor. Returning non-nil Permissions with PartialSuccessError now results in a connection error.

The default configuration of SSH allows X forwarding, which could allow a remote attacker to control a client's X sessions via a malicious xauth program.

ssh version 2.0.12, and possibly other versions, allows valid usernames to attempt to enter the correct password multiple times, but only prompts for a password once for an invalid username. This allows remote attackers to determine user account names on the server.

SSH version 2.0.11 and earlier allows local users to request remote forwarding from privileged ports without being root.