Markdown Link
A05 · ≈ 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
Bypass an unsafe Markdown URL sanitizer using parser disagreement.
OPEN /browser-collect, then OPEN /browser-review, then OPEN /browser-result.
make the vulnerable input flow reach its protected server or isolated-browser result; 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 · A05 · Markdown Link
Objective
Bypass an unsafe Markdown URL sanitizer using parser disagreement.
Steps
- Start the isolated lab and open its vulnerable feature from the landing page.
- Confirm the normal request or input works before changing the relevant parser boundary.
- Exploit the real vulnerable flow: Use an entity-obfuscated
javascript:URL whose code POSTsdocument.cookieto/browser-collect; submit the Markdown preview URL to/browser-review, then read/browser-result. - Copy the rotating flag returned by the exploited data flow or rendered browser sink.
- Submit the exact flag before stopping or expiring the instance.
Expected proof
A successful response for markdown-sanitizer-bypass 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.