A new Trojan horse masquerading as a video "codec" required to view content on certain Web sites tries to change key settings on the victim's Internet router so that all of the victim's Web traffic is routed through servers controlled by the attackers.
According to researchers contacted by Security Fix, recent versions of the ubiquitous "Zlob" Trojan (also known as DNSChanger) will check to see if the victim uses a wireless or wired hardware router. If so, it tries to guess the password needed to administer the router by consulting a built-in list of default router username/password combinations. If successful, the malware alters the victim's domain name system (DNS) records so that all future traffic passes through the attacker's network first. DNS can be thought of as the Internet's phone book, translating human-friendly names like example.com into numeric addresses that are easier for networking equipment to handle.
While researchers have long warned that threats against hardware routers could one day be incorporated into malicious software, this appears to be the first time this behavior has been spotted in malware released into the wild.
The type of functionality incorporated into this version of the Zlob Trojan is extremely concerning for a number of reasons. First, Zlob is among the most common type of Trojan downloaded onto Windows machines. According to Microsoft, the company's malicious software removal tool zapped some 14.3 million instances of Zlob-related malware from customer machines in the second half of 2007.
The other, more important reason this shift is scary is that a Windows user with a machine infected with a Zlob/DNSChanger variant may succeed in cleaning the malware off an infected computer completely, but still leave the network compromised. Few regular PC users (or even PC technicians) think to look to the router settings, provided the customer's Internet connection is functioning fine.
Philip Sloss, a software engineer for myNetwatchman.com, said he first observed the activity while examining a Zlob variant distributed on May 22. The DNS hijack occurs, he said, during the installer program, so by the time the user sees the fake codec installer screen, the malware has already attempted to change DNS settings on the victim's router.
I reached out to researchers at Sunbelt Software to check Sloss's data, and Sunbelt was able to confirm that the malware successfully changed the DNS settings on a Linksys router (model BEFSX41), pulled straight out of the factory box (with the default username and password). Another test showed that the Zlob variant successfully changed the DNS settings on a Buffalo router running the DD-WRT open source firmware.
Sunbelt also found that if there are multiple machines using the same router, all of the systems connected to that router will have their traffic hijacked.
"This is definitely something we have not seen before," said Eric Sites, chief technology officer at Sunbelt. Sites said his team is testing the new Zlob variants against multiple routers to see how they fare against the malware. "It was only a matter of time before someone started using this attack."
Sloss said he captured traffic showing the Zlob variant trying to reconfigure different routers by requesting the local Web page for the various "setup wizards" that ship with the devices. Some of the requests he noticed are listed below, with my own research noted next to them:
"/index.asp" (still checking, but I believe this is used on DD-WRT and some Linksys routers);
"/dlink/hwiz.html" (D-Link routers);
"wizard.htm" (appears to be used by several different router manufacturers, including Linksys).
"/home.asp" (no idea)
Relatively few people ever change the default username and password on their wireless routers. I see this often, even among people who have locked down their wireless routers with encryption and all kinds of other security settings: When I confront them about why they haven't changed the default credentials used to administer the router settings, their rationale is that, 'Well, why should I change it? An attacker would need to already have a valid connection on my network in order to reach the router administration page, so what's the difference?'
Obviously, an attack like this illustrates the folly of that reasoning.
What's more, the various components dropped onto victim PCs by this malware are fairly ill-detected by most anti-virus tools out there today. A scan of these three files at Virustotal.com -- which checks submitted files against 31 different anti-virus engines -- indicates that only 11 of the anti-virus products currently detect any of them as malicious.
Specific, manufacturer-based video tutorials on how to secure your wireless router are available at this link here. First and foremost, router users should pick strong router administration passwords, choosing usernames and passwords that are not easily guessed or found in the dictionary. Also, avoid using the username as your password (in any event, try to avoid picking a username and password combo included in the list of those this malware tries).
If your machine has been infected by one of these Zlob/DNSchanger Trojans, and your router settings have been altered, I would strongly recommend that you reset the router to its default configuration. Usually, this can be done by inserting something tiny like a paper clip end or pencil tip into a small hole labeled "reset" located on the back of the router. Press and hold down the small button inside until the lights on the front of the router blink off and then on again (usually about 10 seconds). If you don't know your router's default password, you can look it up at this link here.
It's important to note, however, that if there are other Zlob-infected machines using the same router, they will need to be cleared of the trojan before resetting the router. Otherwise,the malware will simply go back and change the router's DNS settings a few minutes after the reboot, said Sunbelt's Sites.
Bear in mind that you will need to reconfigure any security settings you had in place prior to the reset. Check out this site here for video tutorials on how to properly configure your router's encryption and security settings. In addition, you may also need to consult with your Internet service provider to find out which DNS servers your network should be using.
Updated, 11:47 p.m. ET: Corrected Site's title, and added in the second paragraph a link that lists all of the usernames and passwords this malware tries against each router administration page it finds.
By Brian Krebs | June 11, 2008; 5:54 PM ET Latest Warnings
Previous: Microsoft, Apple Issue Security Updates | Next: Opera 9.5 Offers Anti-Malware Protection