CVE-2026-11111 – TOCTOU in File Permission Check Before Open

Program Code (Python)

# toctou_server.py - File server that checks permission then opens
import os, time, tempfile
from flask import Flask, request

app = Flask(__name__)
SAFE_DIR = '/tmp/safe'

@app.route('/read')
def read_file():
    filename = request.args.get('file')
    filepath = os.path.join(SAFE_DIR, filename)
    # Check: ensure it's a regular file and owned by user
    if not os.path.isfile(filepath):
        return "Not a file", 403
    # Race window: attacker replaces file with symlink to /etc/shadow
    time.sleep(0.2)  # simulate processing delay
    with open(filepath, 'r') as f:
        return f.read()

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

CVE-2026-11111 – TOCTOU Race Condition in File Read

Overview

A file server checks whether a path is a regular file and then opens it after a delay. An attacker can replace the file with a symbolic link to a sensitive system file (e.g., /etc/shadow) during the race window, bypassing the check and reading protected data.

Vulnerability Details

• Type: Time‑of‑Check Time‑of‑Use (TOCTOU)
• Impact: Privilege escalation, sensitive file disclosure.
• Root Cause: The check (isfile) and the use (open) are not atomic, allowing an attacker to change the filesystem object in between.

Exploit Demonstration

1. Start the server:
   root@kitploit:~

   pip install flask
   python toctou_server.py
   

2. Run the exploit:
   root@kitploit:~

   python exploit_toctou.py
   

If the race succeeds, the response contains the shadow file.