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Malicious Microsoft Word Files Use New Techniques To Evade Security Sandboxes

Zscaler security researchers uncovered new techniques that malware authors use to evade virtual machines and helps protect their code from analysis by security researchers.

The researchers noticed that a recent wave of malicious Microsoft Word documents were evading automatic analysis by using anti-virtual machine and anti-sandboxing techniques. Security researchers tend to use VMs or other types of sandboxes to protect their systems when analyzing files or malware code.

The attackers that created the malware in the first place would normally want their code to remain undetected, because then they can use it for longer periods of time and on more machines.

Malware Through Microsoft Word Macros

Using Word documents by taking advantage of “macro” scripting capabilities is a popular way to infect targets. Malware authors also obfuscate their code to bypass signature-based antivirus software and have implemented techniques that can evade virtual machines or other analysis tools.

These tests are written in macro scripts inside the download document. However, the user must first enable macros, as many years ago Microsoft disabled Word macros by default for obvious security reasons. Through social engineering techniques, the malware authors can get at least a portion of the users who download the file to activate macros.

Then, the script checks for standard virtual environment strings that may be present on the system. It also checks the Windows Management Instrumentation to see whether any analysis systems, virtual environments or popular analysis tools interact with it.

If any of these checks returns positive, then the macro code execution stops and the final malware payload, which was meant to infect the PC, is no longer downloaded. Otherwise, if the tests fail, then the malware will download on the computer.

Zscaler said that many types of malware families use Word documents as the initial vector for infection, including ransomware and banking and backdoor trojans.

New Anti-Sandboxing Techniques

The Zscaler researchers said that in May, they found that malware authors were using some new techniques to evade sandboxes. For instance, they use the Office “RecentFiles” property with a set value of 3 or larger, which would check to see whether the user had recently accessed three or more documents.

Typically, security researchers would use a clean operating system and Office installation, with no more than one or two Word documents opened for testing. Then, they would save a snapshot of that VM, so they can always revert back to that clean state when they test something new. Now, the malware authors try to take advantage of this typical behavior of security researchers to evade detection.

Another new evasion technique involves having the macro script check the user’s external IP address, and then comparing it to known server locations and data centers of multiple security vendors. If the user’s IP matches one of the vendor’s IP addresses, then the macro would not send the malware payload and it would terminate itself.

Although these sort of anti-sandboxing evasion techniques are becoming increasingly more popular with malware authors, it can also be used against them. For instance, HitManPro.Alert 3, an anti-exploit tool, has added exactly this kind of feature, called “Vaccination.” It gets sandbox-aware malware to self-terminate by fooling it into thinking that it’s inside a VM or that it’s being monitored for reverse engineering.

However, as usual, this will continue to be a cat and mouse game between black hats and white hats, with each trying to exploit each other’s code weaknesses. Malware authors will look for new ways to evade analysis tools, while anti-malware vendors will continue improving their products to catch them.

Lucian Armasu is a Contributing Writer for Tom's Hardware. You can follow him at @lucian_armasu. 

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Lucian Armasu is a Contributing Writer for Tom's Hardware US. He covers software news and the issues surrounding privacy and security.