gnutls -- X.509 certificate chain validation vulnerability

SecurityFocus reports:

GnuTLS is prone to a security-bypass vulnerability because the application fails to properly validate chained X.509 certificates. Successfully exploiting this issue allows attackers to perform man-in-the-middle attacks by impersonating trusted servers. Unsuspecting users may be under a false sense of security that can aid attackers in launching further attacks.

[source]