A BOOT_START driver is a Windows kernel driver configured to load very early in the startup process, before most user-mode services and security tools are running. Because it comes online before normal endpoint defenses, it can access files, registry keys, and system settings at a point when the machine is only partly protected.
That timing makes BOOT_START drivers important in cyber security. Defenders use them for trusted low-level functions such as storage, security, and remediation, but attackers try to abuse the same early-boot trust to tamper with protections, hide activity, or disable antivirus and EDR before they fully initialize. In practice, the risk is not just the driver itself, but what a signed, privileged component can do during the brief window when the operating system is still waking up.