CVE-2026-48859: SSH server timing side-channel in ssh_auth:check_password/3 allows unauthenticated username enumeration
Observable Timing Discrepancy vulnerability in Erlang/OTP ssh (sshauth, sshoptions modules) allows unauthenticated remote username enumeration via timing side-channel in password authentication.
When the SSH daemon is configured with the userpasswords or password option, sshauth:checkpassword/3 performs a PBKDF2-SHA256 computation with 600,000 iterations (~300ms) for valid usernames, but returns immediately (~0ms) for invalid usernames via the sshoptions:getpasswordoption/2 path. This timing difference is detectable in a single authentication attempt and allows an unauthenticated attacker to distinguish valid from invalid usernames.
The userpasswords 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/sshauth.erl and lib/ssh/src/sshoptions.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 Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
- Upgrade
Upgrade
Erlang/OTP ssh (ssh_auth, ssh_options)to a version that resolves this vulnerability.Fixed in 6.0.1 - Configuration
When configuring the SSH daemon, avoid the user_passwords or password option (which route through ssh_options:get_password_option/2 and create a timing side-channel in ssh_auth:check_password/3). Use pwdfun as the recommended alternative (not affected by this vulnerability).
Erlang/OTP ssh_auth module (ssh_auth:check_password/3) user_passwords/password option = Do not use user_passwords or password options; use pwdfun instead - Compensating control
If you cannot immediately upgrade, configure the SSH daemon to use pwdfun instead of user_passwords/password options to eliminate the timing side-channel that enables unauthenticated username enumeration.
Event History
Frequently Asked Questions
What is the severity of CVE-2026-48859?
CVE-2026-48859 has a medium severity rating of 6.3 according to the CVSS scoring system.
How do I fix CVE-2026-48859?
To mitigate CVE-2026-48859, disable the user_passwords or password option in the SSH daemon configuration.
What type of vulnerability is CVE-2026-48859?
CVE-2026-48859 is an observable timing discrepancy vulnerability that allows unauthenticated remote username enumeration.
Which software is affected by CVE-2026-48859?
CVE-2026-48859 affects the Erlang/OTP ssh module, specifically in the ssh_auth and ssh_options components.
What are the potential risks associated with CVE-2026-48859?
The potential risk of CVE-2026-48859 includes unauthorized access and information disclosure through unauthenticated username enumeration.