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Open-source firmware vuln exposes wireless routers

Back door to complete control

Internet Security Threat Report 2014

A hacker has discovered a critical vulnerability in open-source firmware available for wireless routers made by Linksys and other manufacturers that allows attackers to remotely penetrate the device and take full control of it.

The remote root vulnerability affects the most recent version of DD-WRT, a piece of firmware many router users install to give their device capabilities not available by default. The bug allows unauthenticated users to remotely gain root access simply by luring someone on the local network to a malicious website.

"This means someone can even post some crafted [img] link on a forum and a dd-wrt router owner visiting the forum will get owned," a user named Leka Vecher "gat3way" wrote in this posting to Milw0rm. "A weird vulnerability you're unlikely to see in 2009 :) Quite embarrassing I would say."

Messages sent through the DD-WRT website to the software designers weren't returned by time of publication, but comments posted to this user forum thread said the vulnerability affected the most recent builds, prompting a user by the name of autobot to declare the vulnerability a "mini code red."

The bug resides in DD-WRT's hyper text transfer protocol daemon, which runs as root. Because the httpd doesn't sanitize user-supplied input, it's vulnerable to remote command injection. While the httpd doesn't listen on the outbound interface, attackers can easily access it using CSRF (cross-site request forgery) techniques.

What's more, exploits need not be part of an authenticated session, making them easy to carry out. Examples of URLs that allow remote takeover include:

http://routerIP/cgi-bin/;command_to_execute

or even:

<img src="http://192.168.0.1/cgi-bin/;CMD" alt="">

DD-WRT is open-source firmware that runs more than 200 different models of wireless routers and embedded devices, including those made Linksys, D-Link, Buffalo, and Netgear. If you don't know whether your device uses it, chances are it does not. Penetration testers using the Metasploit project can download this module to audit whether a particular piece of hardware is vulnerable.

Additional details about the bug are here and here. ®

Update

DD-WRT developer Sebastian Gottschall just emailed to say an interim fix is available here. "Consider that this exploit was released without any Report to us," he added.

A DD-WRT representative also says attacks exploiting the vulnerability can be prevented without updating the firmware by adding a firewall rule. Here's how:

Go to your router, "Administration", "Commands" and enter the follwing text:

insmod ipt_webstr

iptables -I INPUT -p tcp -m tcp -m webstr --url cgi-bin -j REJECT --reject-with tcp-reset

and press "Save Firewall", then reboot your router.

This rule blocks any try to access sth that has "cgi-bin" in the URL.

You can prove that the rule works by entering: http://192.168.1.1/cgi-bin/;reboot in your browser. That should give a "Connection was reset" (Firefox).

Important Note: this does not work, if https management is turned on.

You need to turn off https management. If you don't want to do that, PLEASE UPDATE.

Internet Security Threat Report 2014

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