TOC

The moment a system verifies a condition, the first half of a TOCTOU race condition.

Time-of-check is the instant a program validates a condition, such as confirming a file's permissions or that a resource exists, before acting on it. In a time-of-check to time-of-use (TOCTOU) race condition, an attacker changes the resource in the gap between that check and the later use, so the program acts on a state that no longer holds. The defense is to make the check and the operation atomic, for example by operating on a held handle rather than re-resolving a path. On the exam, pair TOC with TOU as the two ends of a TOCTOU race condition.