GnuTLS has been proven to be vulnerable prior to version 2.2.5.

The vulnerabilities can expose Denial-of-Service (DoS) and buffer overflow conditions. Further investigation is needed but maybe possible for an attacker to execute code on the affected system.

Please see the advisory for more information:
http://www.cert.fi/haavoittuvuudet/advisory-gnutls.html

http://lists.gnu.org/archive/html/gnutls-devel/2008-05/msg00060.html