A null pointer dereference is a programming error that occurs when software attempts to access or modify data through a pointer that has not been assigned a valid memory address (i.e., it points to null). This can lead to application crashes, unpredictable behavior, or security vulnerabilities. Attackers may exploit null pointer dereference flaws to cause denial of service or, in rare cases, execute arbitrary code. Preventing this issue involves proper pointer initialization and validation before use, as well as rigorous error handling. Null pointer dereferences are common in languages like C and C++ that use manual memory management.