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CSP

Content Security Policy: a browser security mechanism, delivered via an HTTP header, that lets a site restrict which sources of scripts, styles, and other content the browser will load, sharply reducing the impact of cross-site scripting.

CSP tells the browser which origins are trusted for each content type, so even if an attacker injects a script, the browser refuses to run it unless its source is allowed. A well-crafted policy is one of the strongest defenses against cross-site scripting and related injection, though it must be tuned carefully to avoid breaking legitimate content. It is configured through response headers alongside other browser protections.

Introduced in: Defensive Web Fundamentals

Examples

  • A policy that only allows scripts from the site's own origin, blocking injected ones.
  • Using CSP to disallow inline scripts that XSS often relies on.
  • Tuning a policy so legitimate third-party resources still load.