8. CVE-2026-3456 – OAuth2 PKCE Race Condition (Account Takeover)

Overview

A race condition in the OAuth2 authorization server allows an attacker to reuse a victim’s authorization code before the legitimate client redeems it, by guessing or brute‑forcing the PKCE code_verifier within a small time window.

Severity: High (Account Takeover)

Simulation (Python HTTP Servers)

#!/usr/bin/env python3
"""
vulnerable_auth_server.py - Authorization server with race condition window.
"""
import time, random, hashlib, base64, secrets
from http.server import HTTPServer, BaseHTTPRequestHandler
import urllib.parse

# Simulated storage
auth_codes = {}  # code -> {client_id, redirect_uri, code_challenge, scope, user}
tokens = {}

def generate_code():
    return secrets.token_urlsafe(16)

class AuthHandler(BaseHTTPRequestHandler):
    def do_GET(self):
        parsed = urllib.parse.urlparse(self.path)
        params = urllib.parse.parse_qs(parsed.query)
        if parsed.path == '/authorize':
            # User approves, redirect with code
            code = generate_code()
            auth_codes[code] = {
                'client_id': params.get('client_id', ['unknown'])[0],
                'redirect_uri': params.get('redirect_uri', [''])[0],
                'code_challenge': params.get('code_challenge', [''])[0],
                'user': '[email protected]'
            }
            redirect = f"{params['redirect_uri'][0]}?code={code}&state={params.get('state',[''])[0]}"
            self.send_response(302)
            self.send_header('Location', redirect)
            self.end_headers()
        elif parsed.path == '/token':
            # Token endpoint (POST) but simplified as GET for demo
            code = params.get('code', [''])[0]
            verifier = params.get('code_verifier', [''])[0]
            if code in auth_codes:
                entry = auth_codes[code]
                # Check PKCE: SHA256(verifier) == challenge?
                challenge = base64.urlsafe_b64encode(hashlib.sha256(verifier.encode()).digest()).decode().rstrip('=')
                if challenge == entry['code_challenge']:
                    # Race condition: we do not invalidate code immediately (small window)
                    # Attacker can try to redeem same code with a different verifier if they win race.
                    access_token = secrets.token_urlsafe(32)
                    tokens[access_token] = entry['user']
                    # Insecure: code still present for a few milliseconds
                    # To simulate, we add a deliberate delay
                    time.sleep(0.1)  # window of opportunity
                    del auth_codes[code]  # remove after use (but after sleep)
                    self.send_response(200)
                    self.end_headers()
                    self.wfile.write(f'access_token={access_token}'.encode())
                else:
                    self.send_response(400)
                    self.end_headers()
                    self.wfile.write(b'invalid code_verifier')
            else:
                self.send_response(400)
                self.end_headers()
                self.wfile.write(b'invalid code')
        else:
            self.send_response(404)
            self.end_headers()

server = HTTPServer(('0.0.0.0', 5000), AuthHandler)
print("Auth server on :5000")
server.serve_forever()

CVE-2026-3456 – OAuth2 PKCE Race Condition (Account Takeover)

📖 Overview

A vulnerability in the OAuth2 authorization server’s PKCE flow allows an attacker to redeem an authorization code before the legitimate client, bypassing PKCE via a race condition. The server fails to atomically invalidate the code, leaving a window where a malicious verifier can be tried.

⚙️ Vulnerability Details

Type: Race Condition / TOCTOU
Impact: Account takeover by obtaining the victim’s access token.
Root Cause: The token endpoint checks PKCE, issues a token, and then removes the code, but the code remains valid for a brief moment. An attacker with a captured code (e.g., via open redirect) can attempt many verifiers concurrently.

🧪 Exploit Demonstration

Start the vulnerable auth server:
```
python vulnerable_auth_server.py
```
Run the race exploit:
```
python race_condition_exploit.py
```

CVE-2026-3456-OAuth2-PKCE-Race-Condition-Account-Takeover-

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