‘Simda.AT’ designed to divert Internet traffic to disseminate other types of malware.
Today Interpol and the Dutch National High Tech Crime Unit (DNHTCU) announced the disruption of Simda.AT, a significant malware threat affecting more than 770,000 computers in over 190 countries. The Simda.AT variant first appeared in 2012. It is a widely distributed malware that causes significant damage to users through the manipulation of internet traffic and spread of other malware.
Interpol coordinated the operation and the DNHTCU, with the support of the Federal Bureau of Investigation (FBI), successfully took down Simda.AT’s active command and control infrastructure across four countries including the Netherlands, Luxembourg, Russia and the United States.
MMPC activated the Coordinated Malware Eradication (CME) platform to provide in-depth research, telemetry, samples, and cleaning solutions to law enforcement and our partners. This information helped law enforcement take action against Simda.AT and its infrastructure, while providing easy remediation and recovery options for victim machines around the world.
Since 2009, the Simda malware family has been a dynamic and elusive threat. Simda’s function has ranged from a simple password stealer to a complex banking trojan. To read more about the Simda family, see Win32/Simda.
Simda.AT makes up the vast majority of our current detections for this malware family. We’ve measured approximately 128,000 new cases each month over the last six months with infections occurring around the world. The ‘Top 10’ countries accounted for 54 percent of the detections our customers have experienced from February through March:
Figure 1: Simda.AT machine detections from October 2014 to March 2015
Figure 2: Percentage of Simda.AT machine detections by country from February to March 2015
Figure 3: Simda.AT machine detections heat map from February to March 2015
Over time, the Simda family was distributed in various ways, including:
This gate website is part of the exploit tool chain, which will redirect the browser to the exploit landing page. The “gate” in this Simda.AT example, is detected as Exploit:JS/Fiexp (aka Fiesta Exploit kit). Fiesta can serve several types of exploits. For example, we have observed Fiesta delivering Simda.AT through malicious SWF files (Shockwave Flash), detected as Exploit:SWF/Fiexp, malicious Java applet files, detected as Exploit:Java/Fiexp and malicious Silverlight files, detected as Exploit:MSIL/CVE-2013-0074. More specific details related to the exploits can be found in the following CVEs:
The “gate” contains script that redirects the browser to the Fiesta landing page. From the landing page, Fiesta attempts to deliver one of three exploits to compromise the machine. Figure 5 shows the general Simda.AT payload delivery process:
Figure 5: Fiesta exploit kit in action
Simda.AT provides two primary functionalities:
- Internet traffic re-routing
- Distribution and installation of additional software packages or modules
For years, Simda used anti-sandbox techniques to evade detection. In most cases, the malware will not run properly, or might sleep indefinitely when the malware suspects that it’s being installed into a software security research environment like the one we have at MMPC.
During installation, the binary checks against a list of black-listed programs and running processes. The checks performed might seem standard and predictable, but Simda.AT collects information from machines it deems suspicious to update the list. Then it uses an automatic and sustainable process for releasing a new binary every couple of hours with updates that cannot be detected by the majority of the AV scanners. See the Simda.AT encyclopedia page for details about the dozens of files, processes, and registry keys checked by Simda.AT at the time of installation.
HOSTS file manipulation
During installation, Simda.AT also modifies the file %SYSTEM32%\drivers\etc\hosts by updating the content and changing the file attributes to be read-only and hidden. The specific changes are hard-coded into each binary, and can cause the victim machine’s internet traffic to be routed according to the new instructions for targeted hosts.
After applying the updates, the installer creates a new and empty file %SYSTEM32%\drivers\etc\hosts.txt to further obfuscate the changes made to the system. The most recent samples are targeting network communication from the following URLs:
Older samples were also seen targeting Bing.com hosts for redirection (e.g. u.bing.com, bing.com, ca.bing.com, gb.bing.com, www.bing.com) and a portion of recent Simda.AT samples connecting to Bing.com using the following URL pattern: http://www.bing.com/chrome/report.html?<encoded string>
The malware authors might have intended to use the HOSTS file modifications to relay additional information about victim machines to the servers of their choosing. However, from our research, Simda.AT samples stopped updating the HOSTS file with the Bing.com hosts in early February. As a result, we’ve been able to monitor traffic to this, normally unused, location for the last several days, and we have observed an average of approximately 5,000 unique IPs reach out to us each day.
Software distribution and modules
Based on our research, we believe the primary monetization method for this is through a Pay-Per-Install (PPI) program in which the authors can be compensated for distributing and installing additional software packages or modules. Over time, we have observed the following types of software to be distributed by Simda.AT:
- Click-Fraud / Search Hijacking Malwar
- Crypto-currency Mining Software (Bitcoin and Primecoin)
- Unwanted Software / Adware
The initial infection modifies the system registry to execute during every system start-up. There are no communications outside of the initial program execution.
DGA/Command and Control Infrastructure
The Simda.AT command and control infrastructure is organized differently than similar malware families. Each binary contains up to six hard-coded IPs that dictate the communication infrastructure for each bot. The Domain-Generation-Algorithm (DGA) that’s normally used to define the infrastructure is instead used to generate a seed for the encryption that is used by the host and the command and control servers.
Using RDTSC instruction, the DGA creates a random, 15-19 character long string that’s embedded into a domain in one of the following formats:
These domains are then injected as the ‘Host’ in the associated POST requests issued to the command and control servers.
To decrypt the ‘report’ HTTP request, append the query string to the hostname and use as the key. Then unquote the query value and enumerate each byte and get the decrypted byte with the following python code snippet:
decrypted_string += chr(ord(cipher[i]) – ord(hostname[i % len(hostname)]))
The third, or ‘update’ request, requires an additional step to base64 decode the query string.
Check-In and update
As alluded to earlier, Simda.AT has two primary functions while communicating with the command and control server:
These two functions are differentiated in the POST request sent to the servers, and they are normally issued to different servers through the hard-coded configuration in the binary.
The ‘report’ function acts as a simple check-in and provides the following type of information, from the victim machine, to the command and control server prior to terminating the connection ahead of the server response:
- Adapter information
- Assorted other system and registry information to distinctly identify the computer
- Creation time of the folder “C:\System Volume Information”
- Computer name
- Hard disk information
- MAC address
- Volume serial number
This information is used to provide a unique ID for the bot.
In some situations, the bots can also append information about installed applications and processes that are running that we suspect are used for anti-emulation updates for new samples.
The ‘update’ command is used when downloading modules or additional software packages. Again, a small amount of machine and binary information is packaged from the victim machine and sent to a different, ‘module’, or server. When the module servers receives the request and then responds with an ‘Active’ message, the bot drops an embedded component (TrojanDropper:Win32/Simdown.A) that handles the download and installation of all modules using hard-coded paths.
Both functions are called at the initial infection and at every system restart.
It’s interesting to note that Simda.AT has been using the same user agent strings in its command and control communication since 2012, which can provide a valuable signature for IPS/IDS engines:
“Mozilla/5.0 (Windows NT 6.1; WOW64; rv:2.0b8pre) Gecko/20101114 Firefox/4.0b8pre”
“Mozilla/4.0 (compatible; MSIE 8.0; Trident/4.0; .NET CLR 2.0.50727; .NET CLR 1.1.4322; .NET CLR 3.0.04506.590; .NET CLR 3.0.04506.648; .NET CLR 3.5.21022; .NET CLR 3.0.4506.2152; .NET CLR 3.5.30729”
While the disruption action can disable the ability of existing infections to download or update new software components, it will not disable modules that might have been installed by Simda.AT.
If you have been infected by Simda.AT, run a comprehensive scan of your environment using Microsoft Safety Scanner, Microsoft Security Essentials, Windows Defender, or your preferred Anti-Malware Solution.
As a part of our cleaning solution, we will detect and remove any malware distributed by this family, and return your HOSTS file to the default, blank, state.
As always, we urge Windows users to be vigilant against malware:
- Be cautious when opening emails or social media messages from unknown users.
- Be wary about downloading software from websites other than the program developers.
- Run an antivirus software regularly.
As a reminder to organizations invested in security, if your organization is interested in joining or initiating an eradication campaign, or you are just interested in participating in the CME program, please see the CME program page. You can also reach out to us directly through our contact page for more information.
Tommy Blizard, Rex Plantado, Rodel Finones, and Tanmay Ganacharya