| name | web-race-conditions |
| description | 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".
|
| domain | web |
| type | technique |
| stability | learning |
| modes | ["bugbounty"] |
| severity | high |
| owasp | ["A04:2021-Insecure-Design"] |
| cwe | ["CWE-362"] |
| tools | ["burp","turbo-intruder"] |
| schema_version | 1 |
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
- Pick a limited action with observable state (balance, redeemed flag, count).
- Send a burst in parallel aligned to hit the window together: Burp Turbo Intruder
(single-packet attack for HTTP/2, or last-byte sync for HTTP/1) — the point is simultaneity,
not volume.
- Measure the overrun: did the action succeed more times than allowed (2 redemptions, double credit)?
- Variants: multi-endpoint races (state set by A, consumed by B), and MFA/OTP brute windows.
Gotchas
- Sequential fast requests aren't a race — you need true concurrency (single-packet / synced send).
- Effect can be small (2× not ∞) but still valid; quantify the actual overrun.
- Idempotency keys / DB locks defeat it — a single success may just be correct behaviour.
Verify success
The limited action occurs more times than the rules allow (e.g. one coupon applied twice,
balance double-spent), reproduced with the concurrent PoC.
References
PortSwigger race-condition labs; James Kettle "Smashing the state machine"; Turbo Intruder.
