- traced the login flow between the app, IdP, and backend.
- saw weird automated call to reauth endpoint. was authenticated call.
- the xx-char hex UID being sent as the password. It’s an account ID, not a UUID or secret.
- the backend never checked the UID signature.
- replayed another test account’s UID with no cookies, OTP, session, or password.
- got a valid access token for that account.
- confirmed profile access and read/write access.
- possible cause: the backend trusted the identifier as a password. or bad code, didn't look into the code.