Detecting py2exe Executables: YARA Rule

Following the release of the tool to decompile EXE files generated with py2exe, we release a YARA rule to detect such EXE files.

Imagine you receive an executable for analysis. If you go for static analysis, it’s useful to know how the executable was produced. For example, if it was “converted” from Python to EXE, decompiling it with a tool like Hex-Rays decompiler will not help you. Python converters like PyInstaller and py2exe don’t actually convert the Python code to machine instructions to create the executable, rather they generate an executable that contains Python bytecode and deploy a Python runtime environment to execute this bytecode. As such, you need to extract and decompile the bytecode to know what the executable does.

How do you know the executable was produced with py2exe? A good indicator is the presence of a resource named PYTHONSCRIPT. Using YARA rules it is possible to automate this detection:  for this purpose we created YARA rule py2exe.

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The idea is that you build a set of YARA rules to classify executables (another good rule to include in this set is a rule to detect PyInstaller generated executables). Then you let these rules run on your executable, and hopefully some rules will trigger and help you identify the type of executable you’re dealing with.

This rule is not an indicator of malware, it just identifies that the executable was generated with py2exe.

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