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Firefox Privacy Notice

Mozilla Foundation Security Advisory 2009-42

Compromise of SSL-protected communication

August 1, 2009
Dan Kaminsky
Firefox, NSS, SeaMonkey, Thunderbird
Fixed in
  • Firefox 3.0.13
  • Firefox 3.5
  • NSS 3.12.3
  • SeaMonkey 1.1.18
  • Thunderbird


IOActive security researcher Dan Kaminsky reported a mismatch in the treatment of domain names in SSL certificates between SSL clients and the Certificate Authorities (CA) which issue server certificates. In particular, if a malicious person requested a certificate for a host name with an invalid null character in it most CAs would issue the certificate if the requester owned the domain specified after the null, while most SSL clients (browsers) ignored that part of the name and used the unvalidated part in front of the null. This made it possible for attackers to obtain certificates that would function for any site they wished to target. These certificates could be used to intercept and potentially alter encrypted communication between the client and a server such as sensitive bank account transactions.

This vulnerability was independently reported to us by researcher Moxie Marlinspike who also noted that since Firefox relies on SSL to protect the integrity of security updates this attack could be used to serve malicious updates.

Mozilla would like to thank Dan and the Microsoft Vulnerability Research team for coordinating a multiple-vendor response to this problem.


Users of unfixed versions of Firefox 3.0 who are concerned about the potential for this attack on their network should download the latest Firefox 3.5 from our web site, and on Windows ensure that the installer is signed and that "Mozilla Corporation" is the publisher.