A null byte is a zero-value character, often written as \0, used to mark the end of a string in many low-level languages and parsers. If software trusts a null byte in attacker-controlled input, it may stop reading early or treat the rest of the data differently than intended.
This matters in cyber security because parsing mismatches can break validation. A file path, filename, or parameter may look safe to one component but be truncated by another, allowing path traversal, filter bypass, or unexpected file access. Defenders look for null-byte handling bugs in gateways, web apps, and legacy C or C-like code, then fix them by using length-aware parsing, strict input normalization, and consistent validation before any file or security decision is made.