Linksys WRT54g router authentication bypass

From Ginsu Rabbit, bugtraq mailing list
An authentication bypass vulnerabiity is reported in Linksys WRT54g router. This advisory is published by Ginsu Rabbit in bugtraq mailing list.
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I. DESCRIPTION

Tested product: Linksys WRT54g home router, firmware revision 1.00.9 (VxWorks based V5 router).

Problem #1: No password validation for configuration settings.

The WRT54g does not attempt to verify a username and password when configuration settings are being changed. If you wish to read configuration settings, you must provide the administrator ID and password via HTTP basic authentication. No similar check is done for configuration changes.

This request results in a user-id and password prompt:
GET /wireless.htm

This request disables wireless security on the router, with no password prompt:
POST /Security.tri
Content-Length: 24

SecurityMode=0&layout=en

Problem #2: Cross-site request forgery

The web administration console does not verify that the request to change the router configuration is being made with the consent of the administrator. Any web site can force a browser to send a request to the linksys router, and the router will accept the request.

II. Exploitation

The combination of these two bugs means that any internet web site can change the configuration of your router. Recently published techniques for port-scanning and web server finger printing via java and javascript make this even easier.

The attack scenario is as follows:
- intranet user visits a malicious web site
- malicious web site returns specially crafted HTML page
- intranet user's browser automatically sends a request to the router that enables the remote administration interface
- the owner of the malicious web site now has complete access to your router

I'm not going to share the "specially crafted HTML page" at this time, but it isn't all that special.

III. DETECTION

If your router is vulnerable, the following curl command will disable wireless security on your router. Tests for other router models and firmware revisions may be different:

curl -d "SecurityMode=0&layout=en" http://192.168.0.1/Security.tri

IV. MITIGATION

1) Make sure you've disabled the remote administration feature of your router. If you have this "feature" enabled, anybody on the internet can take control of the router.

2) Change the IP address of the router to a random value, preferably in the range assigned to private networks. For example, change the IP address to 10.x.y.z, where x, y, and z are numbers between 0 and 255 inclusive. This makes it more difficult for an attacker to forge the request necessary to change the router configuration. This mitigation technique might not help
much if you have a java-enabled browser, because of recently published techniques for determining gateway addresses via java applets.

3) Disable HTTP access to the administration interface of the router, allowing only HTTPS access. Under most circumstances, this will cause the browser to show a certificate warning before the configuration is changed.

V. VENDOR NOTIFICATION

Linksys customer support was notified on June 24, 2006.
Full disclosure on August 4, 2006.
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