CVE-2026-4040 – Race Condition in File Upload Leading to RCE

Program Code (Python Flask)

# upload_server.py - File upload with race condition
from flask import Flask, request
import os, tempfile, time, threading

app = Flask(__name__)
UPLOAD_DIR = '/tmp/uploads'
os.makedirs(UPLOAD_DIR, exist_ok=True)

@app.route('/upload', methods=['POST'])
def upload():
    file = request.files['file']
    # Save to a temporary file
    fd, tmp_path = tempfile.mkstemp(dir=UPLOAD_DIR)
    file.save(tmp_path)
    # Simulate validation (check extension)
    if not file.filename.endswith('.txt'):
        os.unlink(tmp_path)
        return "Invalid extension", 400
    # Race window: between save and move, attacker can execute the script
    # In a real server, we'd move to safe name, but we simulate time delay
    time.sleep(0.5)   # vulnerability
    final_path = os.path.join(UPLOAD_DIR, file.filename)
    os.rename(tmp_path, final_path)
    return "Uploaded", 200

if __name__ == '__main__':
    app.run(port=5000)

CVE-2026-4040 – File Upload Race Condition to RCE

Overview

An upload endpoint writes the uploaded file to a temporary path, validates the extension, and then moves it to a safe name after a delay. An attacker can race to access and execute the temporary file before the rename, achieving remote code execution.

Vulnerability Details

Type: TOCTOU Race Condition
Impact: Arbitrary code execution on the server.
Root Cause: The file is saved to a predictable temporary location and remains executable during the validation window.

Exploit Demonstration

Start the vulnerable server:

pip install flask
python upload_server.py

Run the exploit:
```
python exploit_race_upload.py
```

CVE-2026-4040-Race-Condition-in-File-Upload-Leading-to-RCE

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