A stack overflow is a crash condition that happens when a program uses more call stack memory than the process has available. The usual causes are deep or unbounded recursion, or functions that keep calling one another without a safe stopping point. Each call adds a stack frame for local data and return information, and eventually the stack runs out.
In cyber security, stack overflows matter because they can turn a simple input into a denial-of-service crash. Attackers often trigger them by sending data that forces a parser, resolver, or archive handler into repeated recursion. Defenders watch for this by setting recursion limits, validating input before processing, and patching code paths that can loop unexpectedly. Unlike a memory-corruption bug, a stack overflow is often “just” a crash, but in exposed services that is still a serious availability risk.