picklescan before 0.0.23 fails to detect malicious pickle files inside PyTorch model archives when certain ZIP file flag bits are modified. By flipping specific bits in the ZIP file headers, an attacker can embed malicious pickle files that remain undetected by PickleScan while still being successfully loaded by PyTorch's torch.load(). This can lead to arbitrary code execution when loading a compromised model.

Affected: PyPI: picklescan

### Summary

PickleScan fails to detect malicious pickle files inside PyTorch model archives when certain ZIP file flag bits are modified. By flipping specific bits in the ZIP file headers, an attacker can embed malicious pickle files that remain undetected by PickleScan while still being successfully loaded by PyTorch's torch.load(). This can lead to arbitrary code execution when loading a compromised model.

### Details

PickleScan relies on Python’s zipfile module to extract and scan files within ZIP-based model archives. However, certain flag bits in ZIP headers affect how files are interpreted, and some of these bits cause PickleScan to fail while leaving PyTorch’s loading mechanism unaffected.

By modifying the flag_bits field in the ZIP file entry, an attacker can:

- Embed a malicious pickle file (bad_file.pkl) in a PyTorch model archive. - Flip specific bits (e.g., 0x1, 0x20, 0x40) in the ZIP metadata. - Prevent PickleScan from scanning the archive due to errors raised by zipfile. - Successfully load the model with torch.load(), which ignores the flag modifications.

This technique effectively bypasses PickleScan's security checks while maintaining model functionality.

### PoC ``` import os import zipfile import torch from picklescan import cli

def can_scan(zip_file): try: cli.print_summary(False, cli.scan_file_path(zip_file)) return True except Exception: return False

bit_to_flip = 0x1 # Change to 0x20 or 0x40 to test different f…

Affected: PyPI: picklescan

picklescan before 0.0.23 fails to detect malicious pickle files inside PyTorch model archives when certain ZIP file flag bits are modified. By flipping specific bits in the ZIP file headers, an attacker can embed malicious pickle files that remain undetected by PickleScan while still being successfully loaded by PyTorch's torch.load(). This can lead to arbitrary code execution when loading a compromised model.