Wednesday, January 04, 2006

How to pwn a million computers without breaking a sweat

There has been a lot of discussion about the Microsoft WMF vulnerability recently, and I frankly don't feel that I have much to add. You're either already taking preventative measures, you're awaiting the patch, or both. But I came across a particular infection attempt today which, while not unusual, is a good example of how an exploit for this sort of vulnerability could get delivered to a victim's PC even without them necessarily doing anything "risky". It happens to involve a WMF file, but I've seen it in the past with other types of image-related vulnerabilities as well.

IMPORTANT SAFETY NOTE: This information is based on an actual incident. I have obscured the names of most of the systems involved in part to protect the reputations of their owners, but mostly to prevent people from trying to click on them.

The scenario begins as the user (we'll call him Fred) is browsing through a popular website, in this case, but really it it wasn't involved in the attack itself. I only include it here because it's the start of the "web session" the user was involved in, and so it also seemed like a logical place to pick up the narrative.

In case you don't have a teenager, MySpace is a free online community that's insanely popular <old_fart>amongst the youth of today</old_fart>. It's kind of like a cross between a free website host, a blog and IM all rolled into one. The point is that each MySpace user gets their own web page/blog/buddy list/chat board page to do with as they wish. Other registered MySpace users can post messages of their own on the page when they visit, and these are automatically tagged with the visitor's name and personalized photo or icon. Later, when someone views the comments, any of the posters who happen to also be using MySpace at the time will also have a special "online" indicator displayed beside their name.

All these messages, icons and status indicators are user-customizable, and this drives many people to learn some basic HTML. Of course, if you don't feel like learning HTML, you can go to any number of websites that will generate HTML on the fly for you to paste into your own web pages.

So Fred was viewing someone's MySpace page, and apparently one of the posters was online at the time. That caused their "online status" icon to be displayed along with their name, and apparently this was linked back to one of these HTML helper websites, from which it had originally been generated. We'll call the site HTMHelper (not the real name, but the exact site isn't important either).

Here's where it starts to get interesting.

HTMHelper has many pages and is quite extensive. To help support this service, the site owners have made deals to display ads from several ad distribution companies. The HTMHelper page contained a small bit of JavaScript code to generate "pop under" ads (like popups, but they appear under your browser window so you don't see them until later). The ad provider, let's call them, is part of a whole
chain of (often sleazy) ad brokers, and it's quite common for ad brokers to get ads from other ad brokers, who got them from other ad brokers, who get them from... you get the idea. Ultimately, there is an individual who registers with one of these ad distribution services, but there are usually several levels between the person
placing the ads and the service that eventually places it on a web page where you'll see it.

In this case, cash4popupads displayed a popunder window that contained nothing but a redirection to the real ad, hosted on a server operated by a site we'll call Technically, it was an invisible 1 pixel square IFRAME, but you get the idea. cash4popupads was just the conduit, while spf99 served the ad itself.

Spf99 is registered in Herndon, VA, by the way. It doesn't make any difference to this story, but it's an interesting fact. There are a lot of government and government contractors in that area, since it's basically part of the whole Washington DC/Northern VA megalopolis, but that could just be a geographical coincidence. I have no way of knowing, but it did make me wonder.

Spf99 served up the actual infected file, "/tape/XXXXX101.wmf". Their internal tracking number indicates that this file was supplied by customer number 101 ("affiliate=101" was part of the URL). I don't know who affiliate 101 is, and it could very well be another ad distribution company.

In case you missed the implication, ad services don't work for free. Everyone along the way has to take a cut from every ad delivery and/or clickthrough. This means that whoever supplied the file had enough money to pay for it to be widely distributed through the various levels of the ad networks.

If you've followed the security news through 2005, you'll know that the lone hacker is on the way out, and nation states and organized crime are where the serious hacking is going on now. This is a good example of that trend. In the past, someone would get a website with a free hosting provider and then try to get people to visit their site by sending spam, posting to discussion forums or using some similar technique. That's inefficient, so now they just pay ad networks to distribute their exploits for them. They don't do this unless they're expecting a healthy return on their investment, of course.

Anyway, I thought this might be an interesting peek into the seamy side of exploit distribution, and quite timely too, since we've recently been discussing this particular exploit. Hope you enjoyed it.

Update 2006-01-05 06:49:00
I should have mentioned this before, but all this analysis was made possible by sguil. A Snort alert tipped me off to the exploit attempt itself, and I generated an ASCII session transcript from the packet logs to verify it. I was then able to search through all network sessions involving the target machine around that time, and used additional transcripts of those sessions to establish the chain of events leading up to the exploit attempt itself. I even recovered the WMF file from the packet logs and was able to search network sessions for the download server listed within, which was a great way to verify that the exploit had not been successful. Network Security Monitoring to the rescue!

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