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Application & OT Security · Advanced

Limit Overrun

A race condition in which parallel requests exceed a business limit, such as redeeming a coupon more than once.

Definition

The check (has this been used?) and the update (mark as used) are not atomic, so concurrent requests all pass the check. Mitigations include database transactions with proper locking, unique constraints and idempotency keys.

Example

Twenty simultaneous withdraw requests each pass the balance check before any deducts funds.

Exam tip

A web-app form of TOCTOU; fix with atomic operations or locks.

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