Tuesday, August 16, 2011

Inside an APT Covert Communications Channel

Note: I shortened the title of the post from "Inside an APT “Comment Crew” Covert Communications Channel" to "Inside an APT Covert Communications Channel". To be clear, multiple threat groups are using HTML comments as a means of COVCOM. Thus, this should be considered a general technique as opposed to attribution on a specific group. Both Shady RAT and "Comment Crew", as well as others with additional codenames, have been associated with the use of HTML comments as a means of COVCOM.

For many years, hackers operating out of China have been attacking a myriad of commercial and government systems here in the US and abroad. The term “APT” or Advanced Persistent Threat has often been used to describe these attackers. While HBGary is primarily a product company selling an enterprise incident response product, the team has been deep into APT analysis for over five years. Most of the analysis work is in direct support of Digital DNA – an automated system for detection of unknown malware and APT intrusions. I presented a technical description of how this attribution works, what is solves and what it doesn’t, at the BlackHat Conference last year. The work is about tracking threat groups – that is, tracking the humans and the human factors behind the digital artifacts we see. There are many hacking groups involved in these intrusions. One such group has often been called “Comment Crew” for their use of HTML comments as a means of command and control. This group has been associated with the recent “Shady RAT” intrusion revealed by McAfee. For this article I am going to give you a technical in-depth tour of how such a group operates.

For starters, the attackers will gain access to the network via spear-phishing. In almost all cases we have investigated, spear-phishing was the initial point of infection. These phishing emails are full of very specific project names, names of associates, official sounding documents, etc. It is very clear that the hacking group is using stolen email to learn about their targets before crafting a very convincing email. This underscores why the recent spate of SQLi attacks over the last few months pose a far greater threat than most people realize.

Exploit and Dropper

Once access is gained into the network, the hacking group places remote access tools into the environment. These are backdoor programs that are downloaded automatically by the exploit email – we called these “droppers”. In the diagram, point A shows the exploit email ‘detonating’ after being viewed by the victim, point ‘B’ is a server where a ‘dropper’ is stored, and point ‘C’ is the dropper backdoor being placed onto the compromised computer.

Once the dropper has established a beachhead into the network, a hacker will access the host and uninstall the original backdoor, replacing it with a new and more powerful backdoor. These backdoors, especially the secondary and more powerful one, are called “RAT”s – for Remote Access Tool. Many of these RATs are custom written and that can be the basis for a great deal of attribution, allowing us to detect the malware in physical memory.

Interaction with the Host

Remember that most networks are firewalled. This means the attacker can’t just make a TCP connection into the RAT program. The RAT program is within the internal network so it must first make an outbound connection to the attacker. The RAT is designed to connect outbound over port 80 or 443, a port that is allowed outbound by almost all firewall policies. Once the outbound connection is made, the attacker can use the established TCP session to interact with the host, download tools, run command line programs, and laterally move about the network. In the diagram, point A is where the RAT makes an outbound connection to a server on the Internet, point B is a server under the hacker’s control, and point C is where the hacker uses the established TCP connection to interact with the RAT program and subsequently the host environment, potentially exploiting additional machines nearby in the network.

One of the greatest challenges for an incident response team is discerning the difference between ‘normal’ malware and an APT attack. As we can see in this example, an APT attack involves a real human at the other end of the keyboard performing actions on the host. We call this ‘interaction with the host’ and we recommend that an IR team pull a timeline of last-access times from the MFT (master file table), browsing history from index.DAT, event log, and other sources to determine if such interaction is occurring. This is a fast and easy way to discern the difference between a non-targeted external threat (which over 80% of all adverse events will fall into this category) and external targeted attacks (of which APT is included, probably less than 2% of all adverse events).

The RAT program doesn’t contain any fancy stealth or anti-forensics measures. In fact, we rarely even see packers in use (a packer is a method of obfuscating a program after compilation and is a low-cost way for a hacker to add anti-forensics to his malware). It seems the most of the covert methods are applied to the way to RAT communicates with the hacker. This makes sense. Consider that most of the intrusion detection capability lies at the perimeter of the network, and this is what the hacker is trying to defeat. Thus, the HTML comment method of configuring and controlling the RAT programs.

Hidden Comments for Covert Communication (COVCOM)

Instead of letting the RAT connect directly to his personal server, the hacker will first exploit a webserver somewhere on the Internet. This exploited webserver will then be used as the ‘middleman’ to communicate with the RAT. The hacker will place a hidden comment on an otherwise normal webpage and have the RAT connect outbound to this page. Using the hidden comment, the hacker will be able to give commands to the RAT. The RAT will make periodic outbound connections, sometimes waiting days before checking the page. The hidden comment will contain an encoded message that the RAT knows how to decipher. In this case example, the hidden data is base64 encoded. In this diagram, point A is the RAT program making a periodic outbound connection, point B is a compromised webserver somewhere on the Internet, point C is the hidden comment on the webpage, and point D is where said comment is decoded into actual instructions for the RAT. An example of such a comment is shown in the next image. It is interesting to note that the hacker has attempted to make the page look like a 404 HTML error page if viewed in a normal web browser.

Example of BASE64 Encoded Hidden Comment

Once the RAT decodes the message, the data becomes a configuration file for the malware. The file has many features, such as the ability to specify which server addresses to use on the Internet, including backup servers, configuration of the check-in times, and even has the ability to completely update the RAT binary in the field (shown in the diagram as a .bmp file – this is actually a normal PE header executable).

The Decoded Configuration File

All of the above technical information can be detected on a host after intrusion. The RAT program itself is near trivial to detect once you know what you are looking for. But beyond that, because the RAT program has certain outbound connection characteristics, sleep timers, and built-in “host interaction” capabilities, HBGary’s Digital DNA lights it up like a Christmas Tree (example shown in image).

Digital DNA Detects Unknown Malware

Even if you had no prior knowledge about this specific RAT, you would have detected it with HBGary. Beyond that, the decoded configuration file can also be found in physical memory – the primary search method used by Active Defense. Regardless of the configuration values, the option headers shown in the example above have a specific pattern that can be detected quite easily, even if fragmented over multiple buffers. This is exactly the kind of information I am referring to when I talk about “actionable threat intelligence”. Once you know about the attackers TTP’s (tactics, techniques, and procedures) you can encode this into an enterprise-wide scan. We call it ‘continuous protection’ when you adopt continual scanning while also updating the threat intelligence as you learn more about the attacker. In essence, you are applying attrition against the attacker’s presence in your network. For example, if you know how to detect the above configuration file, then the attacker has to change the way that configuration file looks to defeat you – something that also requires them to recode their parser in the malware. Hence, you cost the attacker time and money. That is a Good Thing.

I hope this gave you a somewhat concrete tour of how a real APT covert communication (COVCOM) channel works. Also, I hope it has illustrated some of the threat intelligence that you access on the host. Using enterprise-wide scans, your IR or security team can put a severe dent in the APT presence in your network. As far as product solutions to enable you, obviously we build HBGary’s Active Defense. If you are interested in continuous protection and threat intelligence, we offer 50-node evaluations of Active Defense that can be installed on a laptop. We also offer a deploy-on-demand license for incident response teams (our 500-node pack has been quite popular), as well as the perpetual node model for full enterprise proactive deployments.