Modern web security isn't just about encryption; it's about execution control. Security headers are a set of HTTP response instructions that restrict the browser's behavior, effectively shrinking the attack surface of your web application by disabling dangerous features and enforcing strict trust boundaries.
The ultimate firewall for the frontend. CSP prevents Cross-Site Scripting (XSS) by defining exactly which scripts, styles, and images are permitted to load. A well-configured CSP makes it impossible for an attacker to execute injected malicious code.
Enforces a "HTTPS-only" policy at the browser level. Even if a user types http://, the browser internally upgrades the request to https:// before it ever leaves the machine, neutralizing SSL Stripping and man-in-the-middle attacks.
Your defense against Clickjacking. It tells the browser whether your site is allowed to be embedded in an <iframe>. By setting this to DENY or SAMEORIGIN, you prevent attackers from overlaying invisible UI elements on top of your site.
Controls how much information is leaked in the "Referer" header when a user clicks a link leading away from your site. Setting this to strict-origin-when-cross-origin protects user privacy by hiding sensitive URL path data from third-party analytics.
Beyond the major policies, headers like X-Content-Type-Options: nosniff are vital. This forces the browser to strictly follow the Content-Type declared by the server, preventing "MIME-type sniffing" where a browser might accidentally execute a harmless .txt file as a .javascript file.