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Researcher hides stealthy malware inside legitimate digitally signed files

Lucian Constantin | Aug. 8, 2016
The technique, which doesn't break the original file's signature, can allow malware to bypass antivirus detection

A new technique allows attackers to hide malicious code inside digitally signed files without breaking their signatures and then to load that code directly into the memory of another process.

The attack method, developed by Tom Nipravsky, a researcher with cybersecurity firm Deep Instinct, might prove to be a valuable tool for criminals and espionage groups in the future, allowing them to get malware past antivirus scanners and other security products.

The first part of Nipravsky's research, which was presented at the Black Hat security conference in Las Vegas this week, has to do with file steganography -- the practice of hiding data inside a legitimate file.

While malware authors have hidden malicious code or malware configuration data inside pictures in the past, Nipravsky's technique stands out because it allows them to do the same thing with digitally signed files. That's significant because the whole point of digitally signing a file is to guarantee that it comes from a particular developer and hasn't been altered en route.

If an executable file is signed, information about its signature is stored in its header, inside a field called the attribute certificate table (ACT) that's excluded when calculating the file's hash -- a unique string that serves as a cryptographic representation of its contents.

This makes sense because the digital certificate information is not part of the original file at the time when it is signed. It's only added later to certify that the file is configured as intended by its creator and has a certain hash.

However, this means that attackers can add data, including another complete file inside the ACT field, without changing the file hash and breaking the signature. Such an addition will modify the overall file size on disk, which includes its header fields, and this file size is checked by Microsoft's Authenticode technology when validating a file signature.

However, the file size is specified in three different places inside the file header and two of those values can be modified by an attacker without breaking the signature. The problem is that Authenticode checks those two modifiable file size entries and doesn't check the third one.

According to Nipravsky, this is a design logic flaw in Authenticode. Had the technology checked the third, unmodifiable file size value, attackers wouldn't be able to pull off this trick and still keep the file signature valid, he said.

The malicious data added to the ACT is not loaded into memory when the modified file itself is executed because it's part of the header, not the file body. However, the ACT can serve as a hiding place to pass a malicious file undetected past antivirus defenses.

 

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