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Firefox Privacy Notice

Mozilla Foundation Security Advisory 2008-17

Privacy issue with SSL Client Authentication

March 25, 2008
Peter Brodersen and Alexander Klink
Firefox, SeaMonkey
Fixed in
  • Firefox
  • SeaMonkey 1.1.9


Peter Brodersen and Alexander Klink independently reported that the default setting for SSL Client Authentication, automatically selecting a client certificate on behalf of the user, creates a potential privacy issue for users by allowing tracking through client certificates. For users who already have certificates some real-world identity information such as an email address or name may be available to web sites depending on the purpose of the certificate and its issuer.

The default preference has been changed to prompt the user each time a website requests a client certificate.


Change the Certificate preference in the Options menu (Windows: Tools|Options, Mac: Firefox|Preferences, Linux: Edit|Preferences). Select the Advanced tab and Encryption sub-tab. Under the Certificates section select the option for "Ask me every time".