Linksys WRT54G 'restore.cgi' Configuration Modification Design Error
Vulnerability

iDEFENSE Security Advisory 09.13.05
www.idefense.com/application/poi/display?id=306&type=vulnerabilities
September 13, 2005

I. BACKGROUND

The Linksys WRT54G is a combination wireless access point, switch and
router. More information is available at the following URL:

  http://www.linksys.com/products/product.asp?prid=508

II. DESCRIPTION

Remote exploitation of a design error in the 'restore.cgi' component of
Cisco Systems Inc.'s Linksys WRT54G wireless router may allow
unauthenticated modification of the router configuration.

The vulnerability specifically exists in the 'POST' method of
restore.cgi handler. The httpd running on the internal interfaces,
including by default the wireless interface, does not check if
authentication has failed until after data supplied by an external user
has been processed. The restore.cgi handler allows a user to upload a
new configuration into the non-volatile memory of the router. If the
user is authenticated, the router will then restart, and the new
configuration will be loaded.

If the user is not authenticated, they will receive an error page when
they attempt to upload a new configuration without supplying
authentication and the router will not reboot. The settings the user
set will be saved, but will not take effect until the next time the
router restarts.

III. ANALYSIS

Successful exploitation of this vulnerability would allow an
unauthenticated user the ability to modify the configuration of the
affected router, including the password. This could allow firewall
rules to be changed, installation of a new firmware with other
features, or denial of service. Exploitation of this vulnerability
would require that an attacker can connect to the web management port
of the router. The httpd is running by default but is only accessible
via the LAN ports or the WLAN (wireless LAN). A mitigating factor is
that if the firmware settings are saved by a process on the router
before the server is reset, the saved settings will overwrite the
settings uploaded by the attacker.

An attacker who can associate with a network running a vulnerable httpd
could send an exploit from a wireless device to reset the password on
the device and enable the remote management port, allowing continued
access from the Internet.

IV. DETECTION

iDEFENSE has confirmed the existence of this vulnerability in version
3.01.03 of the firmware of the Linksys WRT54G wireless router, and has
identified the same code is present in versions 3.03.6 and 4.00.7. All
versions prior to 4.20.7 may be affected.

V. WORKAROUND

To mitigate exposure of the internal network to outside attackers,
ensure encryption is enabled on the wireless interface. The exact
settings to use are dependent on your wireless deployment policies.

VI. VENDOR RESPONSE

This vulnerability is addressed in firmware version 4.20.7 available for
download at:

http://www.linksys.com/servlet/Satellite?childpagename=US%2FLayout
 
&packedargs=c%3DL_Download_C2%26cid%3D1115417109974%26sku%3D112491680264
5
 &pagename=Linksys%2FCommon%2FVisitorWrapper

VII. CVE INFORMATION

A Mitre Corp. Common Vulnerabilities and Exposures (CVE) number has not
been assigned yet.

VIII. DISCLOSURE TIMELINE

07/05/2005 Initial vendor notification
07/25/2005 Initial vendor response
09/12/2005 Coordinated public disclosure

IX. CREDIT

This vulnerability was discovered by Greg MacManus of iDEFENSE Labs.

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X. LEGAL NOTICES

Copyright (c) 2005 iDEFENSE, Inc.

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Disclaimer: The information in the advisory is believed to be accurate
at the time of publishing based on currently available information. Use
of the information constitutes acceptance for use in an AS IS condition.
There are no warranties with regard to this information. Neither the
author nor the publisher accepts any liability for any direct, indirect,
or consequential loss or damage arising from use of, or reliance on,
this information.