The HTTP X-XSS-Protection response header is a feature of Internet Explorer, Chrome and Safari that stops pages from loading when they detect reflected cross-site scripting (XSS) attacks. Although these protections are largely unnecessary in modern browsers when sites implement a strong Content-Security-Policy that disables the use of inline JavaScript ('unsafe-inline'), they can still provide protections for users of older web browsers that don't yet support CSP.

• 0 - Disables XSS filtering.
• 1 - Enables XSS filtering (usually default in browsers). If a cross-site scripting attack is detected, the browser will sanitize the page (remove the unsafe parts).
• 1; mode=block - Enables XSS filtering. Rather than sanitizing the page, the browser will prevent rendering of the page if an attack is detected.
• 1; report= (Chromium only) - Enables XSS filtering. If a cross-site scripting attack is detected, the browser will sanitize the page and report the violation. This uses the functionality of the CSP report-uri directive to send a report.

Specification: None.