EEF-CVE-2026-48859

SSH server timing side-channel in ssh_auth:check_password/3 allows unauthenticated username enumeration

## Summary Observable Timing Discrepancy vulnerability in Erlang/OTP ssh (ssh_auth, ssh_options modules) allows unauthenticated remote username enumeration via timing side-channel in password authentication. When the SSH daemon is configured with the `user_passwords` or `password` option, `ssh_auth:check_password/3` performs a PBKDF2-SHA256 computation with 600,000 iterations (~300ms) for valid usernames, but returns immediately (~0ms) for invalid usernames via the `ssh_options:get_password_option/2` path. This timing difference is detectable in a single authentication attempt and allows an unauthenticated attacker to distinguish valid from invalid usernames. The `user_passwords` and `password` options are documented as intended for test purposes; the recommended alternative is `pwdfun`, which is not affected by this vulnerability. This vulnerability is associated with program files `lib/ssh/src/ssh_auth.erl` and `lib/ssh/src/ssh_options.erl`. This issue affects OTP from OTP 29.0 before OTP 29.0.2, corresponding to ssh from 6.0 before 6.0.1. ## Workarounds * Use the `pwdfun` option instead of `user_passwords` for password authentication. The `pwdfun` callback gives full control over timing behavior and is not affected by this vulnerability. Implementations should take care to execute in approximately constant time regardless of username validity. * Restrict SSH port access to trusted networks only via firewall rules, reducing the set of potential attackers who can perform timing measurements. ## Configurations The SSH daemon must be configured with the `user_passwords` or `password` option for password authentication. Systems using the `pwdfun` option instead are not affected.