MojoX::Authentication versions before 0.006 for Perl allow SAML authentication bypass because parseassertion builds Net::SAML2::Binding::POST without a trust anchor.

parseassertion in MojoX::Authentication::Model::SAML2 calls Net::SAML2::Binding::POST->new with no cacert, certtext or anchors argument, then passes the returned XML to Net::SAML2::Protocol::Assertion->newfromxml with the IdP signing certificate as cacert. In Net::SAML2 before 0.86 that certificate guards only encrypted assertions, so the signature on an unencrypted assertion is checked against the certificate the response itself carries.

An attacker starts a SAML login, then posts a response signed with a certificate of their own. The audience, InResponseTo and timestamp checks that follow are all satisfiable by the attacker, so the response authenticates any NameID it carries.