Individual skill
web-race-conditions
Exploit race conditions / TOCTOU — fire concurrent requests to break single-use limits. Load on "limit-once" actions: coupon/gift-card redemption, withdrawals, votes, invites, MFA/OTP attempts, balance changes. Signals: check-then-act on shared state, per-user quotas, "already used".
skills/web/web-race-conditions/SKILL.md
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- Last checked
- Nov 11, 2026
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- License
- MIT
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# Race conditions (TOCTOU / limit-overrun)
## When it applies
An action is meant to happen once (or N times) and the server checks a condition, then acts, on
shared state — without atomicity. Concurrent requests slip between check and update.
## Why it works
Between "check if allowed" and "record that it happened" there's a window. Fire many requests in
that window and several pass the check before any updates the state — so a one-time coupon
redeems 10×, a balance is spent twice, a limit is overrun.
## Method
1. **Pick a limited action** with observable state (balance, redeemed flag, count).
2. **Send a burst in parallel** aligned to hit the window together: Burp **Turbo Intruder**