Affected: Authen-SASL

Authen::SASL::Perl::DIGEST_MD5 versions before 2.2100 for Perl accept replayed authentication responses via unverified nonce in server_step.

server_start generates a fresh nonce and sends it in the challenge, and nothing later compares that value against the nonce the client returns. server_step derives the expected digest from the client's own parameters, so a response verifies whenever its digest matches the nonce it carries. The count table it also checks is keyed on the client-supplied nonce and starts empty in each new server object, so a captured first response, carrying `nc=00000001`, passes that too. RFC 2831 defines the nonce in the response as the value the server sent in the preceding challenge.

An attacker who observes one successful `qop=auth` exchange can replay the captured response against a later session for the same service, host, realm and user, and authenticate as that user without knowing the password.

Affected: Ubuntu:Pro:14.04:LTS: libauthen-sasl-perl, Ubuntu:Pro:16.04:LTS: libauthen-sasl-perl, Ubuntu:Pro:18.04:LTS: libauthen-sasl-perl, Ubuntu:Pro:20.04:LTS: libauthen-sasl-perl, Ubuntu:22.04:LTS: libauthen-sasl-perl, Ubuntu:24.04:LTS: libauthen-sasl-perl, Ubuntu:26.04:LTS: libauthen-sasl-perl

It was discovered that Authen::SASL, a Perl authentication library, did not properly validate login attempts. An attacker could possibly use this issue to gain unauthorized access.