# 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)