X-XSS-Protection

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.

X-XSS-Protection: 0
X-XSS-Protection: 1
X-XSS-Protection: 1; mode=block
X-XSS-Protection: 1; report=<reporting-uri>

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=<reporting-URI> (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.

Block pages from loading when they detect reflected XSS attacks:

X-XSS-Protection: 1; mode=block

PHP

header("X-XSS-Protection: 1; mode=block");

Apache (.htaccess)

<IfModule mod_headers.c>
  Header set X-XSS-Protection "1; mode=block"
</IfModule>

Nginx

add_header "X-XSS-Protection" "1; mode=block";

Not part of any specifications or drafts.

• Content-Security-Policy
• Controlling the XSS Filter – Microsoft
• Understanding XSS Auditor – Virtue Security

• The misunderstood X-XSS-Protection – blog.innerht.ml