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What Does X-XSS-Protection Do? – Technipages

X-XSS-Protection was a security header that’s been around since version 4 of Google Chrome. It was designed to enable a tool that checked the content of the website for reflected cross-site scripting. All major browsers have now retired support for the header as it ended up introducing security flaws. It is highly recommended that you don’t set the header at all and instead configure a strong Content Security Policy.

Tip: Cross-Site Scripting is generally shortened to the acronym “XSS”.

Reflected cross-site scripting is a class of XSS vulnerability where the exploit is directly encoded in the URL and only affects the user that visits the URL. Reflected XSS is a risk when the webpage displays data from the URL. For example, if a web store allows you to search for products it may well have a URL that looks like this “” and include the word “gift” on the page. The problem starts if someone puts JavaScript in the URL, if it’s not properly sanitised, this JavaScript could be executed rather than printed to the screen as it should be. If an attacker could trick a user to click a link with this sort of XSS payload they may be able to do things like take over their session.

X-XSS-Protection was intended to detect and prevent this type of attack. Unfortunately, over time a number of bypasses and even vulnerabilities were found in the way the system worked. These vulnerabilities meant that implementing the X-XSS-Protection header would introduce a cross-site scripting vulnerability in an otherwise secure website.

To protect against this, with the understanding that the Content Security Policy header, generally shortened to “CSP”, includes functionality to replace it, browser developers decided to retire the feature. Most browsers, including Chrome, Opera, and Edge have either removed support or in the case of Firefox, never implemented it. It’s recommended that websites disable the header, to protect those users still using legacy browsers with the feature enabled.

X-XSS-Protection can be replaced with the “unsafe-inline” setting in the CSP header. Being able to enable this setting may take a lot of work depending on the website, as it means all JavaScript must be in external scripts and can’t be included in the HTML directly.

This post was written by Mel Hawthorne and was first posted to Technipages

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