A new embarrassing attack was recently discovered and exploited on Server Certificates and their validation in many current browser environments.

The discovery is that current deployment of domain name matching between the domain expressed in the certificate and the domain protected by the certificate use string matching which treat character 00 (\0) as end of string.

This means that a certificate issued to the owner of example.com that manages to get a server certificate issued to www.paypal.com\0.example.com would have this certificate recognised as a valid certificate for www.paypal.com by many browsers. The worst example is a certificate issued to *\0example.com which could be accepted as a replacement fro any domain.

More information about this attack is available here: http://thoughtcrime.org/papers/null-prefix-attacks.pdf
Article of interest: http://www.theregister.co.uk/2009/10/05/fraudulent_paypay_certificate_published/

Rumour suggest that the big CAs have blocked issuance of these certificates but that you can still get them from some minor CAs.