A flaw was found in the way GnuTLS verified X509 certificates using unknown signature algorithm. An X509 certificate issues by a trusted CA and using hash algorithm not supported by GnuTLS, can cause client or server applications using GnuTLS to crash on an attempt to dereference NULL pointer while verifying peer's certificate. This was tracked upstream as GNUTLS-SA-2006-2 and fixed using following commit:
Further GnuTLS-dev mailing list discussion:
This issue was fixed upstream in GnuTLS 1.4.2, gnutls packages version in Red Hat Enterprise Linux 4 are not affected by this flaw.
It should be noted that the impact of this flaw is limited.
For client applications, this fix will resolve the crash, but as the server's hash algorithm is unsupported by GnuTLS, client will still be unable to verify server's certificate and connect to it securely.
For server applications configured to require client certificates, crash can only be triggered by a client certificate from trusted CA. Clients with such certificates using unknown hash algorithm will not be able to connect even with the fix applied, as the server still won't be able to verify certificate.
This issue was fixed in a recently released RHBA-2012:0319 along with other bug fixes, and the update was not correctly tagged as RHSA:
This issue was addressed in Red Hat Enterprise Linux 5 via RHBA-2012:0319: https://rhn.redhat.com/errata/RHBA-2012-0319.html
It did not affect versions of gnutls as shipped with Red Hat Enterprise Linux 4 and 6.