A04 · Cryptographic Failures · Hard

Padding Clerk

A04 · ≈ 55 min · 0 solves

350POINTS

MISSIONDiscover the weakness independently

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

FLAG REQUIREMENTS

What counts as a valid solve

Goal

Use distinguishable decryption failures to recover a protected cookie.

Required interaction

GET /api/session, then OPEN /api/session/check, then POST /api/session/claim.

Completion proof

recover a protected value or produce an accepted synthetic forgery through the application service; the successful workflow returns the current rotating flag.

Session rule

Work only inside your generated lab URL and submit the flag within 55 minutes.

VERIFIED WALKTHROUGH

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 · Padding Clerk

Objective

Use distinguishable decryption failures to recover a protected cookie.

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/session, recover each plaintext block by modifying the preceding block and distinguishing HTTP 422 padding errors at /api/session/check, then POST the recovered recovery value to /api/session/claim.
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 cbc-padding-oracle 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.