If you're a penetration tester or bug bounty hunter, never skip SSO in your tests. It's one of those features that everyone assumes is safe, but I keep finding critical flaws in it.
I decided to write this because I've bumped into the same kind of bug more than once, and it keeps surprising me that it still exists. People often think that once an application has Single Sign-On, especially with something like Microsoft, then authentication is automatically safe. But that assumption is dangerous.
During a recent test, I found that the login process looked perfectly fine on the surface Microsoft SSO, Authenticator prompt, everything in place. But when I dug deeper into the backend, the whole security model fell apart. The server was relying on a predictable from the client to decide which account was active. Change that value, and suddenly you're logged in as someone else.
I'm writing this because this pattern keeps repeating. If you're on the dev side recheck your assumptions. If you're a tester don't stop at the happy path. If you're on the product side insist your team prove that SSO is validated server side not just shown in the UI.