Executive brief
Authen::SASL::Perl::DIGEST_MD5 is a Perl library that handles DIGEST-MD5 authentication, commonly used in email and messaging systems. Versions before 2.2100 fail to validate that the nonce received from a client matches the one sent in the server's challenge, allowing an attacker who observes one successful authentication session to replay the captured response against future sessions and gain unauthorized access without knowing the user's password.
Technical details
The vulnerability is an authentication bypass caused by insufficient nonce validation in the DIGEST_MD5 implementation. The server_start function generates a nonce and sends it in the challenge, but server_step does not verify that the nonce returned by the client matches the original server-issued nonce. Instead, it derives the expected digest solely from client-supplied parameters and the internally stored nonce table, keyed on the client-supplied (attacker-controlled) nonce. An attacker who captures a successful qop=auth exchange can replay that response in a later session for the same service, host, realm, and user, because the nonce validation check and replay counter (nc) check will both pass. The root cause is non-compliance with RFC 2831, which mandates that the server verify the nonce in the response against the value sent in the preceding challenge. No network-level authentication is required; an attacker only needs to have observed a prior successful exchange.
Affected products
- Perl Authen::SASL::Perl before 2.2100
Timeline
- 2026-09-06: disclosed