lib/x509/verify.c in GnuTLS before 3.1.21 and 3.2.x before 3.2.11 treats version 1 X.509 certificates as intermediate CAs, which allows remote attackers to bypass intended restrictions by leveraging a X.509 V1 certificate from a trusted CA to issue new certificates.
http://www.ubuntu.com/usn/USN-2121-1
http://www.securityfocus.com/bid/65559
http://www.gnutls.org/security.html
http://www.debian.org/security/2014/dsa-2866