Date: 09 August 2007
References: AU-2007.0018 ESB-2007.0576
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A U S C E R T A L E R T
AL-2007.0091 -- AUSCERT ALERT
Mozilla Firefox unpatched URI filtering vulnerability allows
9 August 2007
AusCERT Alert Summary
Product: Firefox 220.127.116.11 and prior
Operating System: Windows
Impact: Execute Arbitrary Code/Commands
CVE Names: CVE-2007-4041
Original Bulletin: http://www.kb.cert.org/vuls/id/783400
Comment: This is a newly reported vulnerability distinct from the
URI issue described previously in AusCERT Alert AL-2007.0084.
In this case a user's computer may be remotely compromised when
visiting a malicious web page using Firefox or related browsers on
Windows, if Internet Explorer 7 is also installed on the computer.
This vulnerability remains unfixed in Firefox 18.104.22.168.
Revision History: August 9 2007: Added CVE Name
July 27 2007: Initial Release
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US-CERT Vulnerability Note VU#783400
Mozilla Firefox URI filtering vulnerability
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
A Uniform Resource Identifier (URI) is a string of characters that
can be used to identify a location, resource, or protocol. Mozilla
Firefox passes some URIs to applications that have been registered
to handle them.
Firefox does not filter data passed to certain URIs. An attacker may
be able to construct a malicious URI link that Firefox will pass to
the registered URI handler. Public exploit code exists that uses
mailto, news, nntp, snews, and telnet URI handlers. This public exploit
code appears to rely upon having Internet Explorer 7 installed on a
Microsoft Windows system. Also note that the technique used by the
exploit code may affect applications other than Mozilla Firefox.
A remote, unauthenticated attacker may be able to execute arbitrary
commands, using Firefox as an attack vector.
We are currently unaware of a practical solution to this problem.
Workarounds for users
* Using the about:config interface, setting the
network.protocol-handler.warn-external.snews to true will make
Firefox display a prompt before sending a URI to an external
* Do not click on or follow untrusted links, or links that contain
%00 immediately following the protocol name.
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 will only work for the public
exploit code that is currently available. Other variations of the
exploit code may be able to bypass these restrictions.
Vendor Status Date Updated
Mozilla Vulnerable 26-Jul-2007
This vulnerability was disclosed by Billy (BK) Rios.
This document was written by Ryan Giobbi.
Date Public 07/25/2007
Date First Published 07/26/2007 04:02:37 PM
Date Last Updated 07/26/2007
Document Revision 17
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