Last Seat Twice
A06 · ≈ 45 min · 0 solves
Explore the application as a real target: map the exposed functionality, inspect requests and responses, build a working exploit, and recover the rotating flag. If you get stuck, the verified official solution is available below as an optional spoiler.
Rules of engagement
Stay inside the generated lab URL. Use only the synthetic accounts and data in this instance. The main SecOne4all site and external systems are out of scope.
Challenge brief
What counts as a valid solve
Two parallel reservations succeed because capacity is checked outside the transaction.
POST /reserve.
leave the workflow in the prohibited synthetic state described by the business rule; the successful workflow returns the current rotating flag.
Work only inside your generated lab URL and submit the flag within 45 minutes.
Official solution
Contains the complete exploitation path. Open it only when you want the spoiler.
Reveal official solutionStep-by-step · verified against the live lab
Official solution · A06 · Last Seat Twice
Objective
Two parallel reservations succeed because capacity is checked outside the transaction.
Steps
- Start the isolated lab and read the normal business rule returned by the vulnerable workflow.
- Perform the legitimate first step so the synthetic record, balance, token, or state exists.
- Break the workflow invariant: Send exactly two concurrent POST requests to
/reserve; both observe the final seat before either decrements it. - Confirm the violated rule returns the rotating instance flag.
- Submit that flag before the instance expires or is stopped.
Expected proof
A successful response for reservation-race returns the instance-specific rotating flag. The flag is valid only for the current player and running instance.
Safety boundary
Use only the generated lab URL. No request to the main site, another user, or an external service is part of this solution.