Coupon Randomness
A04 · ≈ 25 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
Predict the next high-value coupon from a non-cryptographic PRNG.
GET /api/coupon/recent, then POST /api/coupon/redeem.
recover a protected value or produce an accepted synthetic forgery through the application service; the successful workflow returns the current rotating flag.
Work only inside your generated lab URL and submit the flag within 25 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 · A04 · Coupon Randomness
Objective
Predict the next high-value coupon from a non-cryptographic PRNG.
Steps
- Start the isolated CipherBank workspace and inspect its service operation and requests.
- Capture the synthetic cryptographic artifact, token, oracle response, or protocol negotiation output.
- Exploit the cryptographic design or implementation failure: GET
/api/coupon/recent, seed Python MT fromgenerated_at, consume the visible draw, predict the nextVIP-value, and POST it to/api/coupon/redeem. - Confirm the recovered secret, accepted forgery, or bypass returns the rotating instance flag.
- Submit the exact flag before stopping or expiring the instance.
Expected proof
A successful response for insecure-random-coupon 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.