CVE-2026-86219: CWE-294 Authentication Bypass by Capture-replay

Authen::SASL::Perl::DIGEST_MD5 versions prior to 2.2100 do not properly verify the nonce value returned by the client during the DIGEST_MD5 SASL authentication process. The server generates a fresh nonce in server_start and sends it in the challenge, but server_step does not compare the client's returned nonce against this original value. Instead, it derives the expected digest from the client's parameters, allowing any response with a matching digest and nonce to be accepted. The count table used to prevent replay attacks is keyed on the client-supplied nonce and resets with each new server object, allowing replay of a previously captured valid response with the same nonce count. This violates RFC 2831 which requires the nonce in the response to match the server's challenge nonce. Consequently, an attacker who captures a successful qop=auth exchange can replay it later to authenticate as the same user without the password.

An attacker can bypass authentication by replaying a previously captured valid DIGEST_MD5 authentication response. This leads to full compromise of the affected service as the attacker gains unauthorized access with the privileges of the targeted user. The vulnerability has a CVSS score of 9.8, indicating critical impact on confidentiality, integrity, and availability.

No official patch or fix is currently confirmed. Patch status is not yet confirmed — check the vendor advisory for current remediation guidance. Until a fix is available, avoid using vulnerable versions of Authen::SASL::Perl::DIGEST_MD5 in sensitive environments or implement additional compensating controls to detect and block replayed authentication attempts.