Threats that can evade detection are among the most dangerous kind we’re facing today. We see these characteristics in the most challenging security issues like targeted attacks and zero-day exploits. Being able to stay hidden can determine the success of an attack, making it something that attackers continuously want to achieve. In this series of blog posts, we will take a look at one of the techniques used by cybercriminals to evade detection and analysis. The Greek word steganos means hidden, and malware loves to hide stuff sneakily. For the bad guys, this is a marriage made in heaven. This is the first of a series of blog posts on steganography and malware. We will explore what steganography is, and how it applies to malicious software today. Of course, you can use steganography in real life. An example is putting secret messages in strange places (the image below shows a fun one), but here we'll be talking about data files and specifically how these can be used and abused by malicious attackers.

Figure 1. Then-California Governor Arnold Schwarzenegger's reply to the California Assembly explaining his veto after a legislator insulted him during a speech. The secret message appears when taking the first letter of each line in the main text.

Not all of the methods I discuss below are traditionally considered as steganography. However, I view the differences as purely semantic. For purposes of this blog series, we will consider "steganography" to be anything that attackers do to hide data in an unexpected channel. Hiding data in an unexpected channel has exactly the same result: to fool security researchers into overlooking an innocuous channel, protocol or container where data exchange is not expected (or at least not the kind of data the stego-attacker sends). On to the examples.A particular variant of ZeuS malware downloaded its settings as a pretty landscape. Yes, a real image. The end of the image contained extraneous data that, when properly decrypted, would become a configuration data file. For all intents and purposes, the image file downloaded is a real image so any security device capturing the communication would only see a beautiful sunset.

Figure 2. Picture with hidden message downloaded by ZeusVM

This particular variant was discussed in a March 2014 blog post titled Sunsets and Cats Can Be Hazardous to Your Online Bank Account This insidious banking Trojan has been observed recently hiding its settings in the icon file of a web site. This favicon.ico image is the one displayed by browsers at the left hand side of a URL. Almost every web site contains a favicon.ico image, so security software seeing such a request would never think twice about its validity. On top of this, Vawtrak's hosting sites are located on the Tor network. This makes them difficult to take down or sinkhole, but that's a story for another day. VAWTRAK's image hides the message with a technique called LSB (for Least Significant Bits). It consists of altering the colors of the image ever so slightly in order to encode bits of information. For instance, say a given pixel has its color encoded as 0,0,0. This is complete lack of color (i.e., pure black). If the encoded color is changed to 0,0,1 then the pixel would contain one bit of information and become a slightly grayer black (which is undetectable by human eyes). Any modified bits can encode the hidden message and anyone with the knowledge that there is a message within the image could retrieve it by performing the reverse operation. Others would simply enjoy the beautiful sunset - or whatever the image happens to show us. You can read a full analysis of the steganographic capabilities of VAWTRAK on the SecurityAffairs site. A more complete description of VAWTRAK is also available in the Threat Encyclopedia Websites are not the only sources of icons with hidden data. With at least one malicious Android app (which we detect as ANDROIDOS_SMSREG.A) the main icon (i.e., the one seen on the phone's screen) - actually contains the encoded info.

Figure 3. Screenshot of Android icon with the hidden info. The icon has been pixelated due to its pornographic nature.

GET g4x6a9k2u.txt HTTP/1.1

HTTP/1.1 400 Site Not Installed Server: .V06 Apache Content-Type: text/html MDEwMTAyMDIwMjAyMDMwMzAzMDMwMzAzMDM=

The spreitzenbarch forensics blog contains a detailed analysis of this particular threat.The last example I'll use today is not steganography through image files, but via network protocols. The VBKlip banking Trojan (a threat very specific to Poland) monitors the infected machine looking for Polish bank accounts that have been entered into the machine. Once it finds a legitimate account, it replaces the 26-digit number with a different one in order to redirect payments. This new account belongs to a money mule and is received from the C&C in a very unusual way. It initiates a non-sensical HTTP connection to the C&C server which looks similar to this:This is a request for a dummy text file nobody cares about. The HTTP response back from the C&C has the meat (note that some of the other HTTP headers have been redacted for brevity):The base64 string in the HTTP headers decodes to a bank account number such as 0101-02020202-03030303030303. (I would like to thank Lukasz Siewierski from the Polish NASK for his explanation of this example.) The Polish victim sees a different bank account and sends money to a money mule, instead of to the real recipient. The attackers abused an unexpected channel which is used by VBKlip to sneak in some configuration data. This is not strictly steganography but it fits my definition above: it misuses/abuses an unexpected information channel to include information that a defender would never look there for. We have (very briefly) covered what steganography is, and what malware uses it for. We can get an idea whether or not it's something useful for malicious attackers - hint: it is. Our next post will cover other kinds of information that malware conceals: binary executable data. Until then, be safe...