Adversaries may hide malicious Visual Basic for Applications (VBA) payloads embedded within MS Office documents by replacing the VBA source code with benign data.^[1]

MS Office documents with embedded VBA content store source code inside of module streams. Each module stream has a PerformanceCache that stores a separate compiled version of the VBA source code known as p-code. The p-code is executed when the MS Office version specified in the _VBA_PROJECT stream (which contains the version-dependent description of the VBA project) matches the version of the host MS Office application.^[2],[3]

An adversary may hide malicious VBA code by overwriting the VBA source code location with zero’s, benign code, or random bytes while leaving the previously compiled malicious p-code. Tools that scan for malicious VBA source code may be bypassed as the unwanted code is hidden in the compiled p-code. If the VBA source code is removed, some tools might even think that there are no macros present. If there is a version match between the _VBA_PROJECT stream and host MS Office application, the p-code will be executed, otherwise the benign VBA source code will be decompressed and recompiled to p-code, thus removing malicious p-code and potentially bypassing dynamic analysis.^[4],[1],[5]

References

1. Cole, R., Moore, A., Stark, G., Stancill, B. (2020, February 5). STOMP 2 DIS: Brilliance in the (Visual) Basics. Retrieved September 17, 2020.
2. Hegt, S. (2019, May 5). Evil Clippy: MS Office maldoc assistant. Retrieved September 17, 2020.
3. Microsoft. (2020, February 19). 2.3.4.1 _VBA_PROJECT Stream: Version Dependent Project Information. Retrieved September 18, 2020.
4. Sayre, K., Ogden, H., Roberts, C. (2018, October 10). VBA Stomping — Advanced Maldoc Techniques. Retrieved September 17, 2020.
5. Bontchev, V. (2019, July 30). pcodedmp.py - A VBA p-code disassembler. Retrieved September 17, 2020.

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