Cylance Discovers WAV Files Can Hide Malware

BlackBerry Cylance threat researchers Anuj Soni, Jordan Barth and Brian Marks recently discovered obfuscated malware code that was embedded within WAV audio files. The technique is called steganography, and has been used by malware operators before. Security Now previously detailedits use in malware-containing PDF files.

The use of audio files for this purpose is relatively new, however. Each WAV file was found by Cylance to be coupled with a loader component for the decoding and execution of malicious content. Some of the WAV files contained code that was associated with the XMRig Monero CPU miner. Others included Metasploit code used to establish a reverse shell.

The researchers found different kinds of WAV file loaders. There were loaders that employ Least Significant Bit (LSB) steganography to decode and execute a Windows Portable Executable (PE) file, as well as a rand()-based decoding algorithm used to decode and execute a file.

Cylance notes that the same techniques were used when malware was found by Symantec in their June discovery of Waterbug/Turla threat actor activity.

For the first loader, code is hidden within the audio file using the Least Significant Bit (LSB) technique, where the right-most bit of an individual byte contains the data of interest. The researchers found that this loader contains hardcoded strings that specify the filename to load ("Song.wav") and, once decoded, the exported function to execute ("Start"). Upon execution, the loader was found to read Song.wav (assuming it is in the same directory), extract a DLL in memory, and execute the "Start" export. What emerges is an XMRig Monero miner.

The second category of loader uses a rand()-based decoding algorithm to hide PE files.

To load a WAV file with this loader the following command line is used: . Different from the WAV file used by the first loader, this audio file has legitimate headers but no music if it is played. The audio will sound like white noise.

This loader reads the file, extract a DLL in memory, and attempts to execute the specified entry point. The extracted file is again a XMRig Monero CPU miner.

The researchers summed up the situation thusly: "The malware authors used a combination of steganography and other encoding techniques to deobfuscate and execute code. These strategies allowed attackers to conceal their executable content, making detection a challenging task. In this case, attackers employed obfuscation to both perform cryptomining activities and establish a reverse connection for command and control. The similarities between these methods and known threat actor TTPs may indicate an association or willingness to emulate adversary activity, perhaps to avoid direct attribution." In short, threat actors are getting smarter about their methods and have no compunction about stealing ideas from others.

— Larry Loeb has written for many of the last century's major "dead tree" computer magazines, having been, among other things, a consulting editor for BYTE magazine and senior editor for the launch of WebWeek.