server-configs-nginx/h5bp/security/x-xss-protection.conf

# ----------------------------------------------------------------------
# | Reflected Cross-Site Scripting (XSS) attacks                       |
# ----------------------------------------------------------------------

# (1) Try to re-enable the cross-site scripting (XSS) filter built
#     into most web browsers.
#
#     The filter is usually enabled by default, but in some cases it
#     may be disabled by the user. However, in Internet Explorer for
#     example, it can be re-enabled just by sending the
#     `X-XSS-Protection` header with the value of `1`.
#
# (2) Prevent web browsers from rendering the web page if a potential
#     reflected (a.k.a non-persistent) XSS attack is detected by the
#     filter.
#
#     By default, if the filter is enabled and browsers detect a
#     reflected XSS attack, they will attempt to block the attack
#     by making the smallest possible modifications to the returned
#     web page.
#
#     Unfortunately, in some browsers (e.g.: Internet Explorer),
#     this default behavior may allow the XSS filter to be exploited,
#     thereby, it's better to inform browsers to prevent the rendering
#     of the page altogether, instead of attempting to modify it.
#
#     https://hackademix.net/2009/11/21/ies-xss-filter-creates-xss-vulnerabilities
#
# (!) Do not rely on the XSS filter to prevent XSS attacks! Ensure that
#     you are taking all possible measures to prevent XSS attacks, the
#     most obvious being: validating and sanitizing your website's inputs.
#
# https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/X-XSS-Protection
# https://blogs.msdn.microsoft.com/ie/2008/07/02/ie8-security-part-iv-the-xss-filter/
# https://blogs.msdn.microsoft.com/ieinternals/2011/01/31/controlling-the-xss-filter/
# https://www.owasp.org/index.php/Cross-site_Scripting_%28XSS%29

add_header X-XSS-Protection $x_xss_protection always;