CVE-2026-48859: SSH server timing side-channel in ssh_auth:check_password/3 allows unauthenticated username enumeration

Severity: medium (CVSS 5.3 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.4% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-06-10 · updated 2026-10-26
Weakness: CWE-208

Description

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.

Affected products

Erlang OTP: from 29.0 before 29.0.2 — fixed in 29.0.2
Erlang/Ssh: from 6.0 before 6.0.1 — fixed in 6.0.1

How to fix

Upgrade to a fixed version: Erlang OTP 29.0.2; Erlang/Ssh 6.0.1. Check the vendor advisory for mitigations.

References

Get alerted when CVE-2026-48859 or the next vulnerability affects your stack