@fastify/helmet is a security middleware plugin for Fastify web applications that implements HTTP security headers. The plugin serves as a wrapper around the helmet library, providing protection against common web vulnerabilities by setting appropriate security headers in HTTP responses. Key features include: - Automatic application of basic security headers to all routes - Configurable middleware options with granular control over individual security policies - Global registration capability for application-wide protection - Support for disabling specific security policies while maintaining others - Integration with Fastify's hook system through onRequest hooks - Reply decorator functionality for enhanced security header management The plugin allows developers to configure security headers such as Content Security Policy, X-Frame-Options, and other protective headers that help prevent attacks like XSS, clickjacking, and content type sniffing. Configuration options enable selective enabling or disabling of specific security measures, providing flexibility for different application requirements while maintaining baseline security protections.