Malicious MS Office Macro Creator

Post Syndicated from Bruce Schneier original https://www.schneier.com/blog/archives/2019/05/malicious_ms_of.html

Evil Clippy is a tool for creating malicious Microsoft Office macros:

At BlackHat Asia we released Evil Clippy, a tool which assists red teamers and security testers in creating malicious MS Office documents. Amongst others, Evil Clippy can hide VBA macros, stomp VBA code (via p-code) and confuse popular macro analysis tools. It runs on Linux, OSX and Windows.

The VBA stomping is the most powerful feature, because it gets around antivirus programs:

VBA stomping abuses a feature which is not officially documented: the undocumented PerformanceCache part of each module stream contains compiled pseudo-code (p-code) for the VBA engine. If the MS Office version specified in the _VBA_PROJECT stream matches the MS Office version of the host program (Word or Excel) then the VBA source code in the module stream is ignored and the p-code is executed instead.

In summary: if we know the version of MS Office of a target system (e.g. Office 2016, 32 bit), we can replace our malicious VBA source code with fake code, while the malicious code will still get executed via p-code. In the meantime, any tool analyzing the VBA source code (such as antivirus) is completely fooled.