BTR.sys is Microsoft Defender’s boot-time remediation driver. It is designed to run early in startup so Defender can remove stubborn malware, repair locked files, and clean registry entries that may not be editable once Windows is fully running.
In cyber security, BTR.sys matters because it is trusted, signed code with kernel-level power. If an attacker already has sufficient local privilege, especially SeLoadDriverPrivilege, they may be able to steer this legitimate driver into performing file or registry operations as part of a post-compromise workflow. That makes it an example of trusted code abuse: the component is not malicious by itself, but its authority can be misused. Defenders should restrict driver-loading rights, monitor driver activity, and treat boot-time security tools as high-value attack surfaces.