X-XSS-Protection
Non-standard: This feature is non-standard and is not on a standards track. Do not use it on production sites facing the Web: it will not work for every user. There may also be large incompatibilities between implementations and the behavior may change in the future.
The HTTP X-XSS-Protection
response header was a feature of Internet Explorer, Chrome and Safari that stopped pages from loading when they detected reflected cross-site scripting (XSS) attacks. 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'
).
Warning: Even though this feature can protect users of older web browsers that don't yet support CSP, in some cases, XSS protection can create XSS vulnerabilities in otherwise safe websites. See the section below for more information.
Note:
- Chrome has removed their XSS Auditor
- Firefox has not, and will not implement
X-XSS-Protection
- Edge has retired their XSS filter
This means that if you do not need to support legacy browsers, it is recommended that you use Content-Security-Policy
without allowing unsafe-inline
scripts instead.
Header type | Response header |
---|---|
Forbidden header name | no |
Syntax
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.
Vulnerabilities caused by XSS filtering
Consider the following excerpt of HTML code for a webpage:
<script>
var productionMode = true;
</script>
<!-- [...] -->
<script>
if (!window.productionMode) {
// Some vulnerable debug code
}
</script>
This code is completely safe if the browser doesn't perform XSS filtering. However, if it does and the search query is ?something=%3Cscript%3Evar%20productionMode%20%3D%20true%3B%3C%2Fscript%3E
, the browser might execute the scripts in the page ignoring <script>var productionMode = true;</script>
(thinking the server included it in the response because it was in the URI), causing window.productionMode
to be evaluated to undefined
and executing the unsafe debug code.
Setting the X-XSS-Protection
header to either 0
or 1; mode=block
prevents vulnerabilities like the one described above. The former would make the browser run all scripts and the latter would prevent the page from being processed at all (though this approach might be vulnerable to side-channel attacks if the website is embeddable in an <iframe>
).
Example
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";
Specifications
Not part of any specifications or drafts.
Browser compatibility
BCD tables only load in the browser