Nikos Mavrogiannopoulos of Red Hat discovered an X.509 certificate verification issue in GnuTLS, an SSL/TLS library. A certificate validation could be reported sucessfully even in cases were an error would prevent all verification steps to be performed. An attacker doing a man-in-the-middle of a TLS connection could use this vulnerability to present a carefully crafted certificate that would be accepted by GnuTLS as valid even if not signed by one of the trusted authorities.