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Firefox Privacy Notice

Mozilla Foundation Security Advisory 2007-36

URIs with invalid %-encoding mishandled by Windows

October 18, 2007
Billy Rios, Nate McFeters, Secunia
Firefox, SeaMonkey, Thunderbird
Fixed in
  • Firefox
  • SeaMonkey 1.1.5
  • Thunderbird


On Windows XP with Internet Explorer 7 installed several "web related" URI schemes do not launch the registered protocol-handler if the URI contains an invalid %-encoded sequence. This was initially reported by Billy Rios and Nate McFeters with additional investigation by Secunia. A patch that mitigated the known exploits was shipped with Firefox as described at MFSA 2007-27.

That mitigation did not prevent the incorrect file-handling programs from launching which left some risk. An additional fix has been applied to Firefox that detects when Windows would mishandle these URIs so that the wrong program does not get launched.