X-XSS-Protection is a now-deprecated HTTP response header previously used by several major browsers to protect websites against Cross-Site Scripting (XSS) attacks. However, using X-XSS-Protection was found to create additional security vulnerabilities in some cases instead of preventing them.
Here’s what you need to know about this security header and what alternatives you can use to prevent XSS attacks.
What is X-XSS-Protection?
The X-XSS-Protection header was a reflected cross-site scripting detection feature used in Chrome, Safari, and Internet Explorer browsers. It would prevent some categories of XSS attacks, a common JavaScript vulnerability type, by stopping pages from loading if the browser detects a malicious script inserted with the user input.
Like the X-Frame-Options header, X-XSS-Protection has also been deprecated due to vulnerabilities that arose from its use. Here is how the header worked and why it is no longer used.
How does X-XSS-Protection work?
The X-XSS-Protection header was sent by servers in response and then implemented by browsers. The purpose of the header was to prevent cross-site scripting attacks in several ways, depending on the header’s value, as set on the server side.
The security header could be set to have one of the following four values:
- X-XSS-Protection: 0; Disables the header and its filtering functionality
- X-XSS-Protection: 1; Enables the XSS filter and sanitizes the page containing the malicious script
- X-XSS-Protection: 1; mode=block; Enables the XSS filter and completely blocks the rendering of the page hosting the script
- X-XSS-Protection: 1; report=<reporting-uri>; Enables the XSS filter, sanitizes the page, and sends a report to a chosen URI. This was a Chromium function.
Even when the header was still used, it had several limitations. Since only a few significant browsers supported the header, it was useless if a user was using a custom or less popular browser that did not recognize the header. Moreover, it turned out that even as it protected from one set of XSS attacks, the header could introduce XSS vulnerabilities where there were none before. Here’s how that problem arose.
Why does X-XSS-Protection create vulnerabilities?
The problem with the X-XSS-Protection header was that it would not filter scripts effectively, making it easy to bypass the XSS auditor browsers (specifically Chrome). This would create a false sense of security while opening possibilities for cross-site scripting attacks.
This was partly due to the header only applicable to reflected XSS attacks, which do not exhaust all XSS possibilities. The header would also generate false positives, stopping perfectly safe websites from loading.
The following code example from Mozilla demonstrates how the X-XSS-Protection header could create vulnerabilities:
<script>var productionMode = true;</script>
<!-- [...] -->
<script>
if (!window.productionMode) {
// Some vulnerable debug code
}
</script>In the above example, if the filter is not enabled, the code will not lead to any vulnerabilities for the website. However, with the header enabled, a search query that includes the initial script might cause the browser to ignore it and execute the second one, which is unsafe. This filtering of the initial script would be based on the assumption that it was included in the response because it was in the URI. In effect, this would create a vulnerability where there wasn’t one.
Prevention Guide
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How to fix X-XSS-Protection vulnerabilities?
The easiest way to avoid the complications that can occur with this header is to stop using it altogether and set its value to 0.
Using the Content-Security-Policy (CSP) instead of this header, specifically enabling security headers, avoids both the complications associated with the header and prevents XSS attacks from occurring.
By default, the CSP uses the ‘unsafe-inline’ option, which disables the use of inline resources, such as JavaScript. It also features the “reflected-xss” directive that can target reflected XSS attacks specifically. However, the CSP still requires careful setup and configuration for optimal performance.
That said, some legacy browsers do not support the CSP, which means that the X-XSS-Protection response header can still offer them some protection. In this case, the header should be used with the” 1; mode=block” value so that it entirely blocks the rendering of the page.
At the same time, if events align, the header may just as well create vulnerabilities for them, which is why it has been deprecated and is not supported by most modern browsers.
FAQs
Does Chrome prevent cross-site scripting (XSS)?
In 2019 Chrome removed its XSS Auditor module that was used to protect against cross-site scripting attacks. The auditor was easy to bypass and frequently arrived at false positives. Other modern browsers also do not have XSS protection – for example, Firefox never implemented the X-XSS-Protection header.
How does cross-site scripting work?
An XSS attack is the injection of scripts on the client side into an application. These scripts are executed within their browser to steal the user’s session, monitor or alter their actions, and more. This is made possible by a lack of validation and sanitization of user input.
How to prevent cross-site scripting attacks?
To limit the possibility of a cross-site scripting attack, you should implement the Content-Security-Policy. In addition, you should avoid displaying untrusted user input and filter and sanitize user input.
