‚ÄčRecently Trojan:Win32/Mevade made news for being the first large botnet to use Tor to anonymize and hide its network traffic. Within a few weeks, starting mid-August, the number of directly connecting Tor users increased by almost 600 percent - from about 500,000 users per day to more than 3,000,000.

Last week we concluded, after further review, that Mevade and Sefnit are the same family and our detections for Mevade have now been moved to join the Sefnit family.

Win32/Sefnit is a well-known family which includes a component capable of performing click fraud. From our observations in the wild, this particular component disappeared near the end of 2011. In June 2013 we discovered a new click fraud component which we originally classified as Mevade.  

Despite its recent notoriety due to the Tor activity, there is still a bit of mystery around how the latest version of Sefnit is spreading and the monetization techniques it uses.

In this blog I’ll be going into a bit more detail on the new stealthy click fraud technique used and how it has contributed to Sefnit being largely undetected by AV vendors for the last couple of years. Additionally, we will discuss a few of the attack vectors used by the Sefnit authors to deliver the latest version of the malware.

Interestingly, TrendLabs now believe they have identified the online identities of the actors behind the threat.

An interconnected threat

The Sefnit threat is composed of multiple components dedicated to different tasks. Among the observed samples, we have identified three distinct components. Figure 1 illustrates what is known currently about how these components interconnect as well as their intended purpose. Figure 2 provides sample references.

The Sefnit malware structure

Figure 1: The Sefnit malware structure

 
 
Component
Sha1 Subset
Service Name
Updater and Installer Service
Trojan:Win32/Sefnit.AU
5451cfa12c9acfae6e91f7c13e4b946038bacef4
942860bedf408cc4c6a1831ef3744a3f9e68b375
Adobe Flash Player Update Service”
Click Fraud Service
Trojan:Win32/Sefnit.AS
014ace48897e81052b9552a5a7ab04d00a8e5227
04bb63c3c71b4d033f49434f59a9225d08b4ea70
05a8fb5e61aad8be003a0ab461b39a86767fda23
0e246f6b95a9fd2d2a0c905be87074f5aadc7be0
0f8be849f287cf705ebc0409527fd06670438470
21bfcc14ac5abc6cb8b6fc802038e66ac4e24686
2d10aaf57c45bde69d8f52e23bdabc10a192da20
5d28316acb73e06a5f4c00858b3bf095cfe6b2bf
72d705af606df58aaaec3cc271f46d3d2e4c0499
7c5091177ea375eb3d1a4c4a2bbd5eb07a4cc5cc
8528769281709abd231a46f13ffdfaaa13232336
89c28f7203f9db0762d1c64e42422a5d89c6a83f
a6b055df9ad3d374acaf2dfacded3ba88d20f5cd
a7a41a0c6998f83839c5c6b58840b62a28714b17
a81b04724ab71e4a71e939204e476bb762adc506
bf4151bece1d94d8304df46b2598c14214d9834e
c5af760e62f230ed0f55ff19d2c2215568e6a199
ccd1fa1bf48665270128700bc94043c5fec39984
Trusted Installer”
 
“Bluetooth LE Services Control Protocol”
Peer-to-peer File Seeding Service and More
Trojan:Win32/Sefnit.AT
Trojan:Win32/Sefnit.gen!D
1aba915c0f75432f788fa672a6c7798af5acc94e
5afaadfe20c4776d12001212dc579f5d3851852b
9378acb5a7b6368e07ac2953459be911a84686cc
9dbca75ff98d49bdd211a2a7c8cac506789d6d29
a1733ba81255104c91e916943bb96875bf39d4d9
a5dd1b1d6105a773d1bdbdf961d36be2bbc56de1
abbd69ddb25b1b95c944b8fdb9531963556ea666
b55051915a2cc1a58284679d7753b55cb11bd9b0
d149bb1c2a4767f538a3de4d72f0a5d21ae46165
d95eb268e489928ed3d4bad8f56c0aa9ba0f0160
e50aa43d2df250ec56c92b4efd8df83e440cb167
edc7a434f18424d73c1403a15ee417fbd59eea95
Windows Internet Name Service”
Software Bundlers
Trojan:Win32/Sefnit.AU
c5758309136cd1e7e804d2003dc5ca27ae743ac3
n/a
 
Figure 2: Known Trojan:Win32/Sefnit Components
 

Sefnit’s stealthy new click fraud methodology

The new Sefnit click fraud method is a departure from the method previously used back in 2011. This new, stealthier methodology is believed to be largely responsible for Sefnit being able to evade AV vendor detection during the last couple of years.

The old version of Sefnit relied on click hijacking for performing click fraud. When an infected user was browsing the internet and clicked on a search engine result (such as from Google), sometimes the clicks would be hijacked to travel through advertising agencies to a similar webpage as the intended destination. These clicks are generally considered quite high-value and are hard to detect from an anti-fraud perspective.

Although this is very stealthy from an advertising agency anti-fraud data analytics perspective, it is not stealthy for the user whose click was hijacked. If detection was missing, some observant users would realize they did not land at the intended website, investigate the cause, and submit samples to antimalware researchers for detection. As a result this always brought attention to the malware.

In 2011, the Sefnit authors were observed to have stopped releasing new versions of the component responsible for this click hijacking and consequently were later believed to no longer be active in the wild. At the end of June 2013, we rediscovered Sefnit using a new click fraud strategy.

The Sefnit click fraud component is now structured as a proxy service based on the open-source 3proxy project. The botnet of Sefnit-hosted proxies are used to relay HTTP traffic to pretend to click on advertisements.

In this way, the new version of Sefnit exhibits no clear visible user symptoms to bring attention to the botnet. This allowed them to evade attention from antimalware researchers for a couple years. The figure below illustrates how the hosted 3proxy servers are used to relay Internet traffic through the botnet clients to perform a fake advertisement click.

The Sefnit botnet uses the hosted 3proxy servers to redirect internet traffic and perform fake advertisement clicks

Figure 3: The Sefnit botnet uses the hosted 3proxy servers to redirect internet traffic and perform fake advertisement clicks

A recorded example of this click fraud path is shown below by using the legitimate affiliate search engine mywebsearch.com to simulate a search for "cat" and fake a click on an advertisement provided by Google to defraud the advertiser Groupon.

The landing page for this click fraud instance

Figure 4: The landing page for a click fraud instance

The end result is Groupon paying a small amount of money for this fake advertisement "click" to Google. Google takes a portion of the money and pays the rest out to the website hosting the advertisement – mywebsearch. The Sefnit authors likely signed up as an affiliate for mywebsearch, resulting in the Sefnit criminals then receiving a commission on the click.

Sefnit authors avoid raising red flags on their advertisement affiliate accounts by preceding each clickfraud incident with a large time-gap and simulated normal user Internet browsing behaviour.

From experience, the interval between click fraud incidents is once per multiple-day period or longer. If the trojan simulates fake advertisement clicks too quickly, the anti-fraud team within the advertising agency would be able to detect the fraud, cancel the payout to the affiliate, and return the money to the defrauded advertisers.

Delivery by File Scout

We have been able to identify some of the infection vectors for the new version of Sefnit. One of the prominent methods is an installer for an application called "File Scout." When this application is installed, it will also install Trojan:Win32/Sefnit silently in the background:

File Scout installer that silently installs Trojan:Win32/Sefnit as the same time

Figure 5:  File Scout installer that silently installs Trojan:Win32/Sefnit as the same time

The installed File Scout application is a tool that replaces the standard "Open with" dialog for unrecognized files with a new dialog:

File Scout replacement for the “Open With” dialog

Figure 6:  File Scout replacement for the "Open with" dialog

There is evidence suggesting that this File Scout application is developed by the Trojan:Win32/Sefnit developers. Specifically, it expects a similar format xml structure for the C&C-download and execute commands, both applications are distributed together, and the two applications were compiled 15 minutes apart with the same compiler.

Similarly, Trojan:Win32/Sefnit bears code similarity to some InstallBrain software bundler installers, such as the same string encryption algorithm and the same packer.

We have also seen Trojan:Win32/Sefnit spread through the eMule peer-to-peer file network.

Downloading and running files from any peer-to-peer network as well as downloading applications from untrusted sources puts you at a high risk of being infected by malware.

This latest version of Sefnit shows they are using multiple attack vectors, even going as far as writing their own bundler installers to achieve the maximum number of infections that make this type of clickfraud a financially viable exercise.

The authors have adapted their click fraud mechanisms in a way that takes user interaction out of the picture while maintaining the effectiveness. This removal of the user-interaction reliance in the click fraud methodology was a large factor in the Sefnit authors being able to stay out of the security-researchers' radars over the last couple of years.

Microsoft is working towards thwarting this type of crime as we describe in another blog, "Another way Microsoft is disrupting the malware ecosystem." The more computers we can protect, the less financially viable this type of malware becomes.

We will continue to monitor the family and keep detection in place to limit further fraud by the criminals.

Geoff McDonald

MMPC