Authentication bypass via OAuth implicit flow

Authentication bypass via OAuth implicit flow

This is a writeup for a challenge from PortSwigger's Web Security Academy. In this one, we will learn how a misconfigured OAuth authentication can be bypassed.


This lab uses an OAuth service to allow users to log in with their social media account. Flawed validation by the client application makes it possible for an attacker to log in to other users' accounts without knowing their password.

To solve the lab, log in to Carlos's account. His email address is

You can log in with your own social media account using the following credentials: wiener:peter.

Solving the challenge

We will start with launching our challenge here.

Give it a few seconds, and your browser will generate something like that:

Make sure that you are capturing your traffic with Burp.

We will navigate to the My account section and login with wiener:peter credentials

Let's authorize this request and see how it looks in Burp:

It starts with a GET request for /auth/<client-id>

And ended up with POST request with some input date in the body:

Interesting. We can replace the email address with the target one, and this POST request will not cause the error message:

To solve the lab, we can right-click on a request and select Request in browser - In the original session:

Copy the URL that Burp will generate for you and paste it to the browser:

Congrats! You successfully bypassed OAuth authentication!