One of the most silent yet devastating vulnerabilities in modern web security lies in the improper implementation of OAuth — the widely adopted Single Sign-On (SSO) mechanism. While OAuth makes user authentication easier by allowing third-party apps to access limited user data, it’s also prone to subtle yet dangerous misconfigurations.
Let’s explore how a poorly implemented OAuth flow can allow an attacker to completely bypass authentication and impersonate another user — all without needing their credentials.
Scenario: Exploiting Insecure OAuth Implicit Flow
Objective:
Demonstrate how an attacker can manipulate OAuth token handling to log in as another user (e.g., escalate from a low-privileged user to an admin account) due to misconfigurations in the OAuth implicit flow.
Step-by-Step Breakdown of the Exploit
Initial Login and OAuth Observation
Begin by logging in with a low-privileged user account — for instance, a test user such as wiener. Go through the standard OAuth-based login workflow on the application. While doing so, monitor all requests and responses using an intercepting proxy like Burp Suite.
During the OAuth redirection stages, you’ll notice that the client application ultimately receives a payload containing certain user details — including the registered email address. This data typically arrives via a POST request, along with the OAuth token.
Exploiting the Implicit Flow
Forward this final POST request to Burp Repeater. Now comes the critical test — replace the email address in the payload from the current user (wiener) to that of another user (for example, carlos@example.com, whose email is listed in the lab instructions).
When you resend the request, observe the application’s response. If the server does not return an error or perform any validation, this strongly suggests a fundamental flaw: the application is not verifying that the access token is actually tied to the user whose details are being submitted.
Trigger the Session Hijack
Now, simulate this from a browser. Right-click on the modified POST request in Burp and choose:[Text Wrapping Break]➡️ “Request in browser” > “In original session“
Burp will provide a URL. Copy and paste this into your browser. If the lab is vulnerable, you’ll find yourself logged in as the target user, carlos, without ever knowing his credentials.
Congratulations — the lab is now compromised.
Why This Happens: The Implicit Flow Problem
The OAuth implicit flow is designed to work entirely through the browser, making it susceptible to tampering. In this case, the POST request — containing the OAuth token and user data — is completely exposed to the client (browser). If the backend does not cross-verify that the token is bound to the same user identity as the submitted email, attackers can inject arbitrary user identifiers.
This vulnerability essentially allows an attacker to rebind the OAuth token to a different user, resulting in full account takeover.
Key Takeaways and Mitigations
- Always verify that OAuth tokens correspond to the user identity passed in the request.
- Use the Authorization Code Flow with PKCE instead of the implicit flow, especially for public clients.
- Avoid exposing sensitive operations like token exchange or identity confirmation in browser-facing code.
- Implement strict checks on redirect URIs and scope limitations.
- Monitor and log suspicious token re-use or user-swapping behaviors.
