How do you implement CORS correctly in a Go HTTP server?

Cross-Origin Resource Sharing (CORS) is required when a browser-based frontend on one domain calls an API on a different domain. Go has no built-in CORS support — you implement it as middleware or use a library like rs/cors.

// Manual CORS middleware (for learning — use a library in production)
func corsMiddleware(allowedOrigins []string) func(http.Handler) http.Handler {
    originSet := make(map[string]bool)
    for _, o := range allowedOrigins { originSet[o] = true }

    return func(next http.Handler) http.Handler {
        return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
            origin := r.Header.Get("Origin")

            if originSet[origin] {
                w.Header().Set("Access-Control-Allow-Origin", origin)
                w.Header().Set("Access-Control-Allow-Methods",
                    "GET, POST, PUT, DELETE, OPTIONS")
                w.Header().Set("Access-Control-Allow-Headers",
                    "Content-Type, Authorization, X-Request-ID")
                w.Header().Set("Access-Control-Allow-Credentials", "true")
                w.Header().Set("Access-Control-Max-Age", "86400") // 24h preflight cache
            }

            // Handle preflight OPTIONS request
            if r.Method == http.MethodOptions {
                w.WriteHeader(http.StatusNoContent)
                return
            }

            next.ServeHTTP(w, r)
        })
    }
}

// Usage
handler := corsMiddleware([]string{
    "https://app.example.com",
    "https://admin.example.com",
})(mux)

// Production: use github.com/rs/cors
// c := cors.New(cors.Options{
//     AllowedOrigins:   []string{"https://app.example.com"},
//     AllowedMethods:   []string{"GET", "POST", "PUT", "DELETE"},
//     AllowedHeaders:   []string{"Authorization", "Content-Type"},
//     AllowCredentials: true,
// })
// handler = c.Handler(mux)

Security note: never use Access-Control-Allow-Origin: * together with Access-Control-Allow-Credentials: true — browsers reject this combination. Always whitelist specific origins when credentials are involved.

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