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OpenID Connect: Implicit Grant Audience Bypass

G5 Cyber Security

TL;DR

The implicit grant flow in OpenID Connect doesn’t always properly validate the audience (aud) claim. This means a malicious application could potentially use client credentials intended for another application if the authorization server isn’t strict enough. We’ll show how to identify this vulnerability and what you can do about it.

Understanding the Problem

The implicit grant flow is designed for browser-based applications where keeping a client secret secure is difficult. However, because of its nature, it relies heavily on redirect URI validation. If an authorization server doesn’t properly enforce audience restrictions and redirect URI checks, an attacker can trick the server into issuing tokens for their own application using another app’s client ID.

Identifying the Vulnerability

  1. Check Redirect URI Validation: The most common issue is weak redirect URI validation. If the authorization server allows multiple redirect URIs per client, or if it uses a wildcard pattern that’s too broad, this could be exploitable.
  2. Inspect Token Response: Use your browser’s developer tools to examine the token response from the authorization server when using the implicit grant flow. Look for the aud claim in the ID token and access token (if available).
  3. Attempt Audience Manipulation: Try requesting a token with an incorrect or malicious audience value. If the authorization server doesn’t reject this request, it’s likely vulnerable. You can do this by modifying the parameters sent during the authorization request. For example, you might try adding aud=malicious_app to your initial request.

Steps to Bypass (Demonstration)

Disclaimer: This section is for educational purposes only. Do not attempt these steps on systems you do not have permission to test.

  1. Register a Client Application: Register your own client application with the authorization server. Note its Client ID and Redirect URI.
  2. Craft a Malicious Authorization Request: Construct an authorization request URL, including the client_id of the vulnerable application you want to impersonate and your registered redirect URI. Also include the desired scopes. For example: 
    https://authorization-server/authorize?response_type=token&client_id=vulnerable_app_id&redirect_uri=your_registered_redirect_uri&scope=openid profile email aud
  3. Initiate the Authorization Flow: Redirect your browser to the crafted authorization request URL.
  4. Inspect the Token Response: If successful, the authorization server will redirect you back to your registered redirect URI with a token in the fragment (#) part of the URL. Examine the aud claim in the ID token. If it contains the vulnerable application’s Client ID, the bypass is successful.

Mitigation Strategies

  1. Strict Redirect URI Validation: Implement very strict redirect URI validation. Avoid wildcards and ensure that each registered redirect URI is explicitly verified against the incoming request.
  2. Enforce Audience Restriction: The authorization server must validate the aud claim in all token responses. It should reject requests with invalid or missing audience values.
  3. Dynamic Client Registration: Consider using dynamic client registration, which allows for more robust validation and control over client configurations.
  4. Use Proof Key for Code Exchange (PKCE): PKCE adds an extra layer of security to the authorization flow, making it harder for attackers to intercept or manipulate tokens. While not a direct fix for audience bypass, it improves overall security.
  5. Consider Alternative Flows: If possible, use more secure flows like the Authorization Code grant with PKCE instead of the implicit grant flow.

Code Example (Python – Requesting Token)

This example shows how to make an authorization request using Python’s requests library. It’s important to note this is a simplified example and doesn’t include error handling or PKCE.

import requests

authorization_server = "https://authorization-server/authorize"
vulnerable_app_id = "vulnerable_app_id"
your_redirect_uri = "your_registered_redirect_uri"
scope = "openid profile email aud"

url = f"{authorization_server}?response_type=token&client_id={vulnerable_app_id}&redirect_uri={your_redirect_uri}&scope={scope}"

response = requests.get(url)

if response.status_code == 302:
    redirect_url = response.headers['Location']
    print("Redirected to:", redirect_url)
else:
    print("Error:", response.text)

Remember to replace the placeholder values with your actual authorization server details and client IDs.

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