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sh.bash via getattr — Unbounded Binary Wrapper
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
The sh library implements __getattr__ at the module level: any attribute access returns an sh.Command wrapping the binary of that name.
When pickle resolves GLOBAL 'sh' 'bash', it calls getattr(sh, 'bash'), triggering __getattr__ and placing a ready Command object on the stack. A single REDUCE with ('-c', cmd) executes it.
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:
rot13_cmd = codecs.encode(cmd, "rot_13")
ops = bytearray(b"\x80\x04")
# _codecs.encode(rot13_cmd, 'rot_13') → cmd_str [0 findings]
ops += sg("_codecs", "encode") + sbu(rot13_cmd) + sbu("rot_13") + b"\x86R"
ops += b"\x94\x30" # MEMOIZE, 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-sh-bash-getattr
cd pickle-bypass-sh-bash-getattr
# 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 "sh" to ALWAYS_DANGEROUS_MODULES. Ban the module, not individual binary names.
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
sh.bash via getattr — Unbounded Binary Wrapper
Score: 0 CRITICAL, 1 WARNING
Technique: Module-level __getattr__ produces a Command for any binary name
Scanner version: modelaudit 0.2.37
Mechanism
The sh library implements __getattr__ at the module level: any attribute access returns an sh.Command wrapping the binary of that name.
When pickle resolves GLOBAL 'sh' 'bash', it calls getattr(sh, 'bash'), triggering __getattr__ and placing a ready Command object on the stack. A single REDUCE with ('-c', cmd) executes it.
Pickle Structure
_codecs.encode(rot13_cmd, 'rot_13') → cmd_str [0 findings]
GLOBAL 'sh' 'bash' → Command [1 WARNING — getattr triggers __getattr__]
Command('-c', cmd_str) → RCE
Key Property: Unbounded Attack Surface
The __getattr__ indirection means there is no static symbol named bash in sh — there is no function called bash to ban. This generalises: any library implementing module-level __getattr__ to wrap system binaries gives an unbounded set of bypass primitives — sh.python3, sh.curl, sh.wget, sh.nc, etc. — none of which can be individually enumerated in a ban list.
Why Scanners Miss It
sh is absent from all ban lists. The bare name bash is not in ALWAYS_DANGEROUS_FUNCTIONS. The __getattr__ indirection means the exploit exists outside the static symbol namespace.
Recommended Fix
Add "sh" to ALWAYS_DANGEROUS_MODULES. Ban the module, not individual binary names.
Requirements
pip install sh
General Analysis — Security Research