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3/22/2017
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Pieter Arntz
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Malware Explained: Packer, Crypter & Protector

These three techniques can protect malware from analysis. Here's how they work.

In this article we will try to explain the terms packer, crypter, and protector in the context of how they are used in malware. Bear in mind that no definition for these categories are set in stone: they all have overlap and there are exceptions to the rules. But this is the classification that makes sense to me.

What they all have in common is their goal
The payload, which is the actual malware that the threat actor wants to run on the victims’ computers, is protected against reverse engineering and detection by security software. This is done by adding code that is not strictly malicious, but only intended to hide the malicious code. So the goal is to hide the payload from the victim - and from researchers who get their hands on the file.

Packers
This usually is short for “runtime packers” which are also known as "self-extracting archives," software that unpacks itself in memory when the “packed file” is executed. Sometimes this technique is also called “executable compression.” This type of compression was invented to make files smaller so users wouldn't have to unpack them manually before they could be executed. But given the current size of portable media and internet speeds, the need for smaller files is not that urgent anymore. So when you see some packers being used nowadays, it is almost always for malicious purposes - in essence, to make reverse engineering more difficult, with the added benefit of a smaller footprint on the infected machine.

Crypters
The crudest technique for crypters is usually called obfuscation. Obfuscation is also used often in scripts, like javascripts and vbscripts. But most of the time these are not very hard to bypass or de-obfuscate. More complex methods use actual encryption. Most crypters not only encrypt the file, but the crypter software also offers the user many other options to make the hidden executable as hard to detect by security vendors as possible. The same is true for some packers. Another expression you will find in this context is  FUD (Fully Undetectable), which sets the ultimate goal for malware authors; being able to go undetected by any security vendor is the holy grail. But if authors can go undetected for a while, and then easily change their files again once they are detected, they will settle for that.

Protectors
A protector in this context is software that is intended to prevent tampering and reverse engineering of programs. The methods used can, and usually will, include both packing and encrypting. That combination, plus some added features makes what is usually referred to as a protector, which surrounds the payload with protective layers, making reverse engineering difficult.

A completely different approach, which also falls under the umbrella of protectors, is code virtualization, which uses a customized and different virtual instruction set every time you use it to protect your application. Of these protectors there are professional versions that are used in the gaming industry against piracy. But the technique itself has also made its way into malware, more specifically in ransomware, that doesn’t need a C&C server to communicate the encryption key. The protection is so efficient that the encryption key can be hardcoded into the ransomware. An example is Locky Bart that uses WProtect, an open-source code-virtualization project.

Discover more at Malwarebytes Labs.

Was a Microsoft MVP in consumer security for 12 years running. Can speak four languages. Smells of rich mahogany and leather-bound books. View Full Bio
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