zwesty
Write-ups / Day 27 / Web
Day 27 · Tue, Oct 27 · Web

Double Spent

Prey slaughtered

Business logic, double spend, state-transition testing

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-21-double-spent (+ /time_trial, /mastery)
Curriculum day
21
Solved
2026-10-27; Single Solve, Time Trial, and Mastery each recorded CORRECT.
Player
Zwesty (solo), with the Homebase agent (GPT-6 Luna). Results below follow the saved target logs and the Game Master's own records.
Timer
All three modes completed; no ranked-time claim is made here.

The challenge

The exact Game Master prompt wasn't kept in our saved start replies; no wording has been reconstructed.

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 The Rails feature route redeemed a single credit twice 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.

Evidence note

The original Game Master prompt and start reply weren't kept in our saved records; the name and behavior are from the assigned page and request log.

What the target showed

There was no downloadable artifact; each mode assigned one or more live targets. The Rails feature route redeemed a single credit twice. The audit route initially reported one remaining credit; two accepted redemption requests produced single_credit_spent_twice and the 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 Rails feature route redeemed a single credit twice. The audit route initially reported one remaining credit; two accepted redemption requests produced single_credit_spent_twice and the proof.

result CORRECT
completion flag (short): `b62354f4…7c41`
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. All three modes completed; no ranked-time claim is made here.

result CORRECT
completion flag (short): `38027034…4f56`
answer: exact proof from the assigned target (value omitted)
proof-1: accepted

Mastery

The Mastery bundle required 4 target-specific answers. The Game Master returned CORRECT after accepting the assigned proofs. Check the full balance transition across repeated operations. A successful action must be atomic with its eligibility check.

result CORRECT
completion flag (short): `3bc65a1e…a2f7`
answer IDs: proof-1 through proof-4
proof values: omitted

Time to solve

The completion records show the Single Solve CORRECT at 09:15:44 ET, the Time Trial at 09:25:52 ET, and the Mastery at 09:27:57 ET on 2026-10-27. We don't have a complete request timeline 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.