A NULL pointer dereference happens when software tries to use a pointer that does not point to valid memory. In C and other low-level code, this often means the program expected an object, buffer, or structure to exist, but the value was missing or never initialized. The result is usually a crash, which makes it a common denial-of-service bug.
In cyber security, attackers may send malformed input to reach error paths that skip a required check and trigger the dereference. In native components such as PHP extensions, that can take down the worker process and sometimes expose deeper logic flaws nearby. Defenders look for strict null checks, safe error handling, fuzzing, and patching vulnerable parsers and protocol handlers. A NULL dereference is not always exploitable for code execution, but it is still important because it can reveal unstable attack surface and weak input validation.