Simon Willison’s Weblog

26th October 2023 - Link Blog

Oh-Auth - Abusing OAuth to take over millions of accounts (via) Describes an attack against vulnerable implementations of OAuth.

Let’s say your application uses OAuth against Facebook, and then takes the returned Facebook token and gives it access to the user account with the matching email address passed in the token from Facebook.

It’s critical that you also confirm the token was generated for your own application, not something else. Otherwise any secretly malicious app online that uses Facebook login could take on of their stored tokens and use it to hijack an account of your site belonging to that user’s email address.