PYSEC-2026-1788

Picklescan has pickle parsing logic flaw that leads to malicious pickle file bypass

### Details There's a parsing logic error in picklescan and modelscan while trying to deal with opcode `STACK_GLOBAL`. Function `_list_globals` when handling `STACK_GLOBAL` at position `n`, it is expected to track two arguments but in wrong range. The loop only consider the range from `1` to `n-1` but forgets to consider the opcode at position `0`. The correct range should be `0` to `n-1`. Attacker can put arg in position `0`, thus the parser can only tract one argument. Then, the exception https://github.com/mmaitre314/picklescan/blob/2a8383cfeb4158567f9770d86597300c9e508d0f/src/picklescan/scanner.py#L281 will be triggered. Thus it can cause detection bypass since the malicious pickle file will trigger unexpected exceptions. Example: ``` 0: S STRING 'os' --> arg 0: STRING (untracked argument due to wrong scanning range) 6: S STRING 'system' --> arg 1: STRING (tracked argument) 16: \x93 STACK_GLOBAL 17: S STRING 'ls' 23: \x85 TUPLE1 24: R REDUCE 25: . STOP ``` ### PoC ``` python import pickle payload = b"S'os'\nS'system'\n\x93S'ls'\n\x85R." with open('bad_pickle.pkl', 'wb') as f: f.write(payload) pickle.load(open('bad_pickle.pkl', 'rb')) ``` ### Impact Detection bypass in both picklescan and modelscan. Note that it also affects the online hugging face pickle scanners, making the malicious pickle file bypass the detection. ### Fix To fix the range here, change `range(1, n)` to `range(1, n+1)` to ensure that `n-offset` stays within the range of `0` to `n`. https://github.com/mmaitre314/picklescan/blob/2a8383cfeb4158567f9770d86597300c9e508d0f/src/picklescan/scanner.py#L255