Vulnerability Note VU#783400

Mozilla Firefox URI filtering vulnerability

Original Release date: 26 Jul 2007 | Last revised: 31 Jul 2007


Mozilla Firefox does not filter input when sending certain URIs to registered protocol handlers. This may allow a remote, authenticated attacker to use Firefox as a vector for executing commands on a vulnerable system.


A Uniform Resource Identifier (URI) is a string of characters that can be used to identify a location, resource, or protocol. Mozilla Firefox will rely on Microsoft Windows to determine the appropriate protocol handler for certain URIs that it does not handle internally.

Firefox does not filter data passed to certain URI protocol handlers. This allows Firefox to be used as an attack vector for vulnerabilities in other applications. For example, Firefox can be used as an attack vector for the vulnerability in how Microsoft Windows itself determines the appropriate protocol handler [VU#403150]. Public exploit code for this vulnerability exists, which uses mailto, news, nntp, snews, and telnet URI handlers.

Note this vulnerability is only present in the Windows version of Firefox.


A remote, unauthenticated attacker may be able to execute arbitrary commands, using Firefox as an attack vector.


Mozilla has addressed this issue in Firefox

Workarounds for users

Using the about:config interface, setting the following options to true will make Firefox display a prompt before sending a URI to an external handler.

Workarounds for administrators

Blocking mailto:%00, nntp:%00, news:%00, snews:%00, telnet:%00 strings inside of HTML pages or other network streams using an application layer firewall or IPS may mitigate this vulnerability. See the xs-sniper blog for more information about known vulnerable URIs. Please note that these filters may only work for the public exploit code that is currently available. Other variations of the exploit code can bypass these restrictions.

Systems Affected (Learn More)

VendorStatusDate NotifiedDate Updated
MozillaAffected-26 Jul 2007
If you are a vendor and your product is affected, let us know.

CVSS Metrics (Learn More)

Group Score Vector
Base N/A N/A
Temporal N/A N/A
Environmental N/A N/A



This vulnerability was disclosed by Billy (BK) Rios.

This document was written by Ryan Giobbi, Jeff Gennari, and Will Dormann..

Other Information

  • CVE IDs: CVE-2007-3845
  • Date Public: 25 Jul 2007
  • Date First Published: 26 Jul 2007
  • Date Last Updated: 31 Jul 2007
  • Severity Metric: 25.51
  • Document Revision: 36


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