A recent article by zdnet explains how hackers have started using steganography to hide malicious code inside of audio .wav files. We know this type of evasion well, but there were other reports of this type of antivirus evasion published in the last few months, indicating increase use. The files can pass through many firewalls undetected since audio files aren’t normally scanned. Many modern malware threats are multi-stage, meaning a “dropper” file on the victim’s system goes and grabs the actual malicious code. This particular evasion technique does well at hiding the malicious code in a file that isn’t scanned, with the dropper extracting the malware from the audio file and executing it.
Steganography works by hiding data, such as malicious code or a file, inside of another file. In the case of this example, not only does the file extension get changed but the file header as well, which helps it hide from many anti-malware engines. Furthermore, the file compound file containing the malicious code is often still valid, meaning the program meant to open the audio file opens the it correctly, but hidden within the data is the malware. Hiding malicious code within the data of the audio file creates a randomness in the file, making it almost impossible to identify the malware. That said, it also makes identifying a file with steganography applied easy if you know what to look for.
It’s not practical to scan every file in most networks. In this example though, the dropper must already be running on the system for the malicious code to execute. This means you can block this attack at the source by using local and perimeter anti-malware systems to block the original malware that extracts the audio file.