GnuTLS before 2.9.10 does not verify the activation and expiration dates of CA certificates, which allows man-in-the-middle attackers to spoof servers via a certificate issued by a CA certificate that is (1) not yet valid or (2) no longer valid.
http://rhn.redhat.com/errata/RHSA-2015-1457.html
http://www.securityfocus.com/bid/73317
https://gitlab.com/gnutls/gnutls/commit/897cbce62c0263a498088ac3e465aa5f05f8719c