HSTS (HTTP Strict Transport Security) is a security mechanism that instructs browsers to always use HTTPS when communicating with your server. ASP.NET Core includes `app.UseHsts()` middleware by default, which sends the `Strict-Transport-Security` header. The post explains what HSTS does, its key options (max-age, includeSubdomains, preload), and how to configure `HstsOptions` in ASP.NET Core. It recommends a gradual rollout strategy: start with a 5-minute max-age, test thoroughly, then increase to weeks and eventually 2 years before enabling preload. The preload flag gets your domain added to browser-hardcoded HSTS lists, preventing any HTTP communication even on the first request. HSTS should generally be enabled in production but avoided in development environments without valid certificates.
