Race Conditions

Make Security-Sensitive State Changes Atomic

Security checks can be individually correct and still fail when two requests run at the same time. A request checks that a recovery code is unused, a withdrawal is within a limit, or an approval is still pending. Before it records the state change, another request performs the same check against the same old state. Both requests then proceed even though the rule was meant to allow only one. This is a race condition: correctness depends on the relative timing of concurrent operations. A common form is a time-of-check to time-of-use problem, often shortened to TOCTOU, where the fact established by a check can become false before the protected action uses it.