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Mozilla Foundation Security Advisory 2007-27

Unescaped URIs passed to external programs

July 30, 2007
Jesper Johansson
Firefox, SeaMonkey, Thunderbird
Fixed in
  • Firefox
  • SeaMonkey 1.1.4
  • Thunderbird
  • Thunderbird


Jesper Johansson pointed out that Mozilla did not percent-encode spaces and double-quotes in URIs handed off to external programs for handling, which can cause the receiving program to mistakenly interpret a single URI as multiple arguments. The danger depends on the arguments supported by the specific receiving program, though at the very least we know Firefox (and Thunderbird) and older could be used to run arbitrary script (see MFSA 2007-23). The vast majority of programs do not have dangerous arguments, though many could still be made to do something unexpected.

A similar issue with URIs passed to external handlers was reported by Billy Rios and Nate McFeters. When running Firefox on Windows XP with IE7 installed, URIs for certain common protocols (such as mailto:) that contain a %00 do not launch the protocol handler registered for that scheme but instead launch a file handling program based on the file extension at the end of the URI. Coupled with the issue reported by Jesper Johansson this appears to allow execution of any program installed at a known location and limited argument passing that might be enough to exploit a system. Further investigation by Secunia showed that a % not followed by a valid two-digit hexadecimal number also triggered the problem for the affected protocols. The Firefox and Thunderbird releases contain fixes that prevent the original demonstrations of this variant, but it is still possible to launch a filetype handler based on extension rather than the registered protocol handler. A way to exploit a common handler with a single unexpected URI as an argument may yet be found. Since this handling is a property of the Windows Shell API this variant appears to affect other internet-enabled applications that pass these URIs to the Windows Shell.


By default Firefox will ask before launching external protocol handlers, and these prompts should be denied from sites that are not trustworthy, especially if the requested URL contains spaces and double-quote (") characters. An exception is made for mail-related protocols in Firefox, they do not prompt by default. If the default mail handler is Thunderbird or later there will not be a problem, but if another program or older version of Thunderbird is the default handler then mail URIs can be made to prompt as well. (Similarly, in Thunderbird browser protocols like http: and ftp: do not prompt but instead launch the default browser.) To make mail-related links prompt in Firefox before launching external programs:

  • Enter about:config in the location bar
  • Enter warn-external in the Filter: box
  • Double-click to set the mailto, news, nntp, and snews lines to true