Definition
A security header is a metadata field within an HTTP request or response that provides instructions to the browser on how to handle the content securely. These headers act as directives, enabling web servers to communicate security policies to clients, thereby mitigating common web vulnerabilities. They function by influencing the browser's behavior, such as preventing cross-site scripting attacks or enforcing secure connections. Implementing security headers is crucial for protecting users and sensitive data from various online threats. For instance, the Content Security Policy (CSP) header helps prevent data injection attacks by specifying allowed sources for content. A common point of confusion is mistaking security headers for security protocols like TLS, which operate at a different layer of the network stack. Understanding security headers is essential for domains like web application security and is covered in certifications such as CompTIA Security+ and CySA+.