CVE-2026-86219: Authen::SASL::Perl::DIGEST_MD5 versions before 2.2100 for Perl accept replayed authentication responses via unverified nonce in server_step

Versions before 2.2100 of the Authen::SASL::Perl::DIGEST_MD5 Perl module (SASL DIGEST-MD5 mechanism per RFC 2831) fail to verify that the nonce returned by the client matches the nonce the server generated in its challenge, allowing replayed authentication responses. Because server_step derives the expected digest from the client's own parameters and its nonce-count table is keyed on the client-supplied nonce and starts empty for each new server object, a captured first response (nc=00000001) passes all checks. An attacker who observes one successful qop=auth exchange can replay it against a later session for the same service, host, realm and user and authenticate as that user without knowing the password. Only deployments that use this module on the server side of a SASL DIGEST-MD5 handshake are affected; client-only use is not. No exploitation is known: there is no public PoC, the flaw is not in CISA KEV, and EPSS estimates a 0.5% probability of exploitation in the next 30 days.

Do: Upgrade the Authen::SASL distribution to the fixed version, 2.2100 or later. Until then, disable server-side DIGEST-MD5 in affected Perl services or restrict it to TLS-protected channels and trusted networks, since exploitation requires on-path capture of a successful authentication exchange. Operators should check whether their Perl applications act as SASL servers using this module; client-only deployments are not impacted.