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.
http://rhn.redhat.com/errata/RHSA-2015-1457.html
http://www.debian.org/security/2015/dsa-3191
http://www.gnutls.org/security.html