Linksys WRT54G Router Remote Administration Fixed Encryption Key
Vulnerability

iDEFENSE Security Advisory 09.13.05
www.idefense.com/application/poi/display?id=304&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 multiple versions of the
firmware for Cisco Systems Inc.'s Linksys WRT54G wireless router may
allow unauthenticated modification of the router configuration.

The vulnerability specifically exists in the 'ezconfig.asp' handler of
the httpd running on the internal interfaces, including by default 
the wireless interface. This handler is used by the 'ezSetup' to perform

the initial setup of the router.

struct mime_handler mime_handlers[] = {
//{ "ezconfig.asp", "text/html", ezc_version, do_apply_ezconfig_post,
  do_ezconfig_asp, do_auth },
/*Modified by Daniel(2004-09-06);*/
{ "ezconfig.asp", "text/html", ezc_version, do_apply_ezconfig_post,
  do_ezconfig_asp, NULL },

The 'auth()' method for this page does not contain an authentication 
initialization function. As the authentication initializer (do_auth) was

removed, no check is made when requesting the page. If the auth_fail
flag  was set for any reason, this call will fail. The code which sets
the  auth_fail flag is shown below. When the httpd starts, the value of 
auth_flag defaults to 0.

 if (handler->auth) {
     handler->auth(auth_userid, auth_passwd, auth_realm);
     auth_fail = 0;
     if (!auth_check(auth_realm, authorization))
         auth_fail = 1;
         
 }

The request returns an encrypted version of the configuration 
information, however the encryption on this data is very weak, it is a 
simple XOR based encryption, with a fixed 256 byte mask. In order to 
change the configuration, this key must be known. Once this key is known

and the new configuration data is encrypted with it, and the new data
can  simply be posted to the httpd, the new configuration will take
effect.

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).

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
internet access.

Authentication credentials may be set if another user has attempted to
view a page since the router was restarted. An attacker may be able to 
crash the httpd using another vulnerability, in which case it will 
restart within 2 minutes, with no authentication details initialized. 
This would then allow them to exploit the httpd with this vulnerability.

IV. DETECTION

iDEFENSE has confirmed the existence of this vulnerability in version 
3.01.03 of the firmware of the Linksys WRT54G, and has identified the 
same code is present in version 3.03.6.

Version 2.04.4 of the WRT54G is confirmed to contain the affected code, 
however by default it initializes the authentication details, and so 
requires a password to set the configuration.

V. WORKAROUND

In order to prevent exposure of this vulnerability from wireless
clients, 
disable wireless access to the web interface:

  * Connect to the web interface, typically at http://192.168.1.1/
  * Go to the Administration page
  * Select 'Disable' next to the 'Wireless Access Web'
  * Click the 'Save Settings' button.

Please note that this will only prevent wireless access, and not access 
from one of the physical ports. Additionally, other vulnerabilities in 
the httpd may allow exploitation of the router, even with this setting 
enabled. 

In order to mitigate exposure of the router and 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

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

06/07/2005 Initial vendor notification
06/07/2005 Initial vendor response
09/13/2005 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.

Permission is granted for the redistribution of this alert
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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.