GnuTLS before 3.1.0 does not verify that the RSA PKCS #1 signature algorithm matches the signature algorithm in the certificate, which allows remote attackers to conduct downgrade attacks via unspecified vectors.
https://euvd.enisa.europa.eu/vulnerability/EUVD-2015-0295
http://www.securitytracker.com/id/1032148
http://www.securityfocus.com/bid/73119
http://www.gnutls.org/security.html