The GnuTLS team has their announcement at http://www.gnutls.org/security.html#GNUTLS-SA-2014-2 but the announcement with details seems to be Red Hat's at https://rhn.redhat.com/errata/RHSA-2014-0247.html
"It was discovered that GnuTLS did not correctly handle certain errors that could occur during the verification of an X.509 certificate, causing it to incorrectly report a successful verification. An attacker could use this flaw to create a specially crafted certificate that could be accepted by GnuTLS as valid for a site chosen by the attacker."
https://www.gitorious.org/gnutls/gnutls/commit/6aa26f78150cc...
Uninitialized "result" variable? Any time the code hits one of those "cleanup" gotos, it's probably returning nonzero unexpectedly?
Later: @filcab on Twitter points out the much dumber issue, which is that if issuer_version is < 0, the function returns issuer_version and not zero. Ow.
(Who uses GnuTLS?)