I Built a FastAPI Auth System From Scratch. A Hacker Broke It in 4 Minutes.

I shipped a custom FastAPI auth flow that looked clean in code review and passed happy-path tests. It still failed in minutes under adversarial probing. The root causes were predictable: weak token invalidation strategy, unsafe password reset design, and missing abuse controls on sensitive endpoints.

What failed first

The attacker chained small weaknesses instead of exploiting one dramatic bug. They abused token reuse windows, brute-forced weak reset flow assumptions, and bypassed expected guard behavior through edge-case request paths.

How the system was fixed

The hardened version uses short-lived access tokens, rotating refresh tokens with revocation tracking, strict audience and issuer checks, nonce-bound reset links, rate limits on all auth endpoints, and complete audit logging for credential-sensitive actions.

Production lesson

Auth is a security system, not a code feature. If your FastAPI auth design is not threat-modeled and tested against abuse scenarios, it is unfinished regardless of how clean the code looks.