OAuth Misconfiguration

> OAuth is a widely-used authorization framework that allows third-party applications to access user data without exposing user credentials. However, improper configuration and implementation of OAuth can lead to severe security vulnerabilities. This document explores common OAuth misconfigurations, potential attack vectors, and best practices for mitigating these risks.

> Do you have HTML injection but can't get XSS? Are there any OAuth implementations on the site? If so, setup an img tag to your server and see if there's a way to get the victim there (redirect, etc.) after login to steal OAuth tokens via referer - [@abugzlife1](https://twitter.com/abugzlife1/status/1125663944272748544)

Redirect to a controlled domain to get the access token

https://www.example.com/signin/authorize?[...]&redirect_uri=https://demo.example.com/loginsuccessful
https://www.example.com/signin/authorize?[...]&redirect_uri=https://localhost.evil.com

Redirect to an accepted Open URL in to get the access token

https://www.example.com/oauth20_authorize.srf?[...]&redirect_uri=https://accounts.google.com/BackToAuthSubTarget?next=https://evil.com
https://www.example.com/oauth2/authorize?[...]&redirect_uri=https%3A%2F%2Fapps.facebook.com%2Fattacker%2F

OAuth implementations should never whitelist entire domains, only a few URLs so that “redirect_uri” can’t be pointed to an Open Redirect. Sometimes you need to change the scope to an invalid one to bypass a filter on redirect_uri:

https://www.example.com/admin/oauth/authorize?[...]&scope=a&redirect_uri=https://evil.com

https://example.com/oauth/v1/authorize?[...]&redirect_uri=data%3Atext%2Fhtml%2Ca&state=<script>alert('XSS')</script>

Some Android/iOS app can be decompiled and the OAuth Private key can be accessed.

> The client MUST NOT use the authorization code more than once. If an authorization code is used more than once, the authorization server MUST deny the request and SHOULD revoke (when possible) all tokens previously issued based on that authorization code.

Applications that do not check for a valid CSRF token in the OAuth callback are vulnerable. This can be exploited by initializing the OAuth flow and intercepting the callback (`https://example.com/callback?code=AUTHORIZATION_CODE`). This URL can be used in CSRF attacks. > The client MUST implement CSRF protection for its redirection URI. This is typically accomplished by requiring any request sent to the redirection URI endpoint to include a value that binds the request to the user-agent's authenticated state. The client SHOULD utilize the "state" request parameter to deliver this value to the authorization server when making an authorization request.