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weakref.finalize — Deferred Callback via Garbage Collection
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
weakref.finalize(obj, func, *args) registers a cleanup callback for obj. The callback fires when obj is garbage collected.
In CPython, garbage collection is typically immediate (refcount-based), so the finalizer can fire during the same pickle.loads() call — once the deserialized object goes out of scope.
weakref is absent from all ban lists. modelaudit classifies it as 1 WARNING.
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_path: str) -> bytes:
"""weakref.finalize(dummy, Path.write_text, target, content) — 0C, 3W.
Stack trace:
_codecs.encode(rot13_content, 'rot_13') → content [0 findings — SAFE_GLOBALS]
MEMOIZE 0, POP
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-weakref-finalize-deferred
cd pickle-bypass-weakref-finalize-deferred
# 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 "weakref.finalize" to ALWAYS_DANGEROUS_FUNCTIONS. No ML model should register garbage collection callbacks during deserialization.
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
weakref.finalize — Deferred Callback via Garbage Collection
Modelaudit score: 0 CRITICAL, 1 WARNING
picklescan score: 0 findings
Mechanism
weakref.finalize(obj, func, *args) registers a cleanup callback for obj. The callback fires when obj is garbage collected.
In CPython, garbage collection is typically immediate (refcount-based), so the finalizer can fire during the same pickle.loads() call — once the deserialized object goes out of scope.
weakref is absent from all ban lists. modelaudit classifies it as 1 WARNING.
Pickle Structure
_codecs.encode(rot13_content, 'rot_13') → content [0 findings]
pathlib.Path(target) → path obj [WARNING 1]
weakref.finalize(path, Path.write_text, path, content) → registers [WARNING 2]
↓ at GC time: Path.write_text fires
Evasion Properties
- Deferred execution: The payload fires when the watched object is garbage collected, not during the
REDUCEopcode that registers the finalizer - Static analysis blind spot: Scanners see
weakref.finalize(WARNING) andpathlib.Path.write_text(WARNING) — neither is CRITICAL - Timing uncertainty: Execution timing depends on GC behavior; CPython reference counting usually fires immediately, but PyPy/other implementations may delay
Recommended Fix
Add "weakref.finalize" to ALWAYS_DANGEROUS_FUNCTIONS. No ML model should register garbage collection callbacks during deserialization.
General Analysis — Security Research