The host is installed with Mozilla Firefox before 39.0, Firefox ESR 38.x before 38.1, Thunderbird before 31.8 or 38.x before 38.1 and is prone to a security bypass vulnerability. A flaw is present in the applications, which do not enforce key pinning upon encountering an X.509 certificate problem that generates a user dialog. Successful exploitation could allow user-assisted man-in-the-middle attackers to bypass intended access restrictions by triggering a (1) expired certificate or (2) mismatched hostname for a domain with pinning enabled.