RFC 6125 specifies how to match a X.509v3 certificate against a DNS name for application usage.

Otherwise valid certificates using wildcards would be accepted as matching certain hostnames they should not according to RFC 6125. For example a certificate issued for ‘*.example.com’ should match ‘foo.example.com’ but not ‘example.com’ or ‘bar.foo.example.com’. Previously Botan would accept such a certificate as valid for ‘bar.foo.example.com’.

RFC 6125 also requires that when matching a X.509 certificate against a DNS name, the CN entry is only compared if no subjectAlternativeName entry is available. Previously X509_Certificate::matches_dns_name would always check both names.

External references:

http://botan.randombit.net/security.html