Mozilla plans to more strictly enforce industry best practices for SSL certificates in future versions of Firefox with a new certificate verification system.
The new system will be implemented as a library called "mozilla::pkix" and will start being used by Firefox 31, which is expected to be released in July.
Many of the certificate verification changes in the new library are subtle and are related to technical requirements specified in the "Baseline Requirements for the Issuance and Management of Publicly-Trusted Certificates" issued by the Certification Authority/Browser (CAB) Forum. However, some of the behavior modifications also stem from changes Mozilla made to its own policy for trusting CA certificates.
For example, a document describing mozilla::pkix requirements notes that "end certificates used by servers are not allowed to have basic constraints asserting isCA=TRUE" and "certificates used as trust anchors or intermediates are now required to have the basic constraints extension and assert the isCA bit."
These two requirements are intended to prevent the misuse of subordinate CA (sub-CA) or intermediate certificates, which can be used to issue SSL certificates for any domain on the Internet.
In February 2012, Trustwave, one of the CAs trusted by browsers, publicly admitted that it had issued a sub-CA certificate for use by a third-party company to inspect SSL traffic passing through its corporate network. Mozilla said at the time that the use of sub-CA certificates for man-in-the-middle SSL traffic monitoring, even if performed on closed corporate networks, is unacceptable.
Another incident happened in January 2013, when a certificate with sub-CA status issued by Turktrust, a Turkish certificate authority trusted by browsers, was installed in a firewall appliance with SSL traffic monitoring capabilities by the Municipality of Ankara. Turktrust said the certificate in question was one of two that had been issued with sub-CA status by mistake in August 2011 and were actually supposed to be regular end-user certificates.
A month later Mozilla changed its CA policy to require all sub-CA certificates to be technically constrained to particular domain names using certificate extensions or to be publicly disclosed and audited as regular root CA certificates. The mozilla::pkix certificate verification library will begin enforcing that policy change inside the browser.
While the majority of Firefox users are unlikely to notice anything out of the ordinary as a result of the new certificate verification system, some HTTPS websites might encounter problems.