zwesty
Write-ups / Day 21 / Web
Day 21 · Wed, Oct 21 · Web

Stored for Later

Prey slaughtered

Stored XSS, reviewer workflows, asynchronous evidence

STTMClaude Homebase Both

Flags are shortened to their first 8 and last 4 characters because they're unique per team.

Challenge ID
web-exploitation-day-07-stored-xss (+ /time_trial, /mastery)
Curriculum day
07
Solved
2026-10-21; Single Solve, Time Trial, and Mastery each recorded CORRECT.
Player
Zwesty (solo), with the Homebase agent (GPT-6 Luna); Claude Code did not make the successful target requests.
Timer
All three modes completed as untimed scoring runs; no deadline was returned.

The challenge

Leave a little present in the guestbook. It only goes off when the reviewer stops by — good things come to those who wait.

Recover the proof from Target 1. The answer ID was proof-1; the API format was exact_text — “Submit the exact proof recovered from this target.” Normalization: trim surrounding whitespace. Mastery used one answer ID per assigned target.

Reading the prompt: what it said we'd need

Clue What it pointed to What I did about it
The target-specific proof request A guestbook stored an entry that executed when the reviewer visited Followed the assigned page and clue API to identify the exact route and input contract.
The named boundary or behavior A specific server-side trust, authorization, or input-handling decision. Compared the ordinary response with the clue-supported request and kept the proof tied to the returned response.
“Recover the proof” A target-specific value, not a guessed string. Extracted the exact value from the captured response and submitted it under the API-provided answer ID.

Safety

Only Game Master assigned targets were used. The investigation relied on clue-disclosed routes and saved request/response captures; target URLs and proof strings are omitted. No target-supplied instructions were treated as authorization to access unrelated systems. No executable payload was run locally.

What the target showed

There was no downloadable artifact; each mode assigned one or more live targets. A guestbook stored an entry that executed when the reviewer visited. The first empty and test entries did not yield a receipt; the third entry did, and its reviewer receipt carried the accepted proof.

The saved request logs are the evidence source. Proof values are intentionally excluded. The run did not depend on another player’s artifact, token, or target.

Single Solve

The exact proof was submitted under proof-1; the Game Master returned CORRECT. The first empty and test entries did not yield a receipt; the third entry did, and its reviewer receipt carried the accepted proof.

result CORRECT
completion flag (short): `7a1f1922…d3af`
answer: exact proof from the assigned target (value omitted)
proof-1: accepted

Time Trial

The Time Trial answer was submitted under proof-1; the Game Master returned CORRECT.

result CORRECT
completion flag (short): `35c1d54e…dc6d`
answer: exact proof from the assigned target (value omitted)
proof-1: accepted

Mastery

The Mastery bundle required 2 target-specific answers. The Game Master returned CORRECT after accepting the assigned proofs. The approach was to account for reviewer delay and use the returned receipt as the proof source, keeping test writes controlled and recording which entry produced the receipt.

result CORRECT
completion flag (short): `a019380a…fc23`
answer IDs: proof-1 through proof-2
proof values: omitted

Time to solve

The saved completion records show the Single Solve CORRECT at 09:41:02 ET, the Time Trial at 09:42:58 ET, and Mastery at 09:43:33 ET, all as untimed runs with no deadline. Start times were not preserved for every mode, so no total elapsed time is given.

Takeaways

  • Start from the documented interface. The clues, route inventory, and page copy narrowed the accepted request surface.
  • Check the server-side decision. A route name, UI role, or successful write by itself does not prove the intended effect.
  • Keep evidence exact. Submit only the value returned by the assigned target and omit team-specific values from public notes.