Mozilla has announced that it is rolling out a new site isolation technology to its Firefox web browser. Site Isolation is a security feature that isolates each website a user visits into its process. This way, if one website is compromised, other websites will not be affected.

The technology is already available in Google Chrome 67 and Microsoft Edge since 2018, and Mozilla says that it has been working on implementing it in Firefox for the past two years.

Site Isolation is turned on by default in the latest Firefox update, but users can disable it if they wish.

Universal Cross-site Scripting (UXSS)

UXSS, which stands for universal cross-site scripting, is a type of security vulnerability that allows an attacker to inject and execute malicious JavaScript code on any website, regardless of the domain or origin of the website. This type of vulnerability is considered to be particularly dangerous, as it allows the attacker to execute code on any website, and it can potentially be used to steal sensitive information or take control of the user’s browser or computer.

UXSS vulnerabilities typically arise when a web application is not properly validated or sanitized, and it allows the attacker to inject arbitrary JavaScript code into the website. For example, if a website allows users to submit comments or other user-generated content, and it does not properly validate or escape this content before displaying it on the page, an attacker could potentially inject JavaScript code into the website. When other users visit the website, the injected code would be executed in their browsers, and it could potentially steal sensitive information, such as the user’s cookies or other sensitive data.

To prevent UXSS vulnerabilities, it is important to properly validate and sanitize all user-generated content before it is displayed on the page. This can help to ensure that the content is safe and does not contain any malicious code. Additionally, using technologies such as site isolation or content security policies can help to prevent UXSS attacks by limiting the ability of malicious code to interact with or access sensitive data on the website.

Site Isolation Provides Side-channel Protection with Mozilla

As part of Firefox 94, released on Tuesday (November 2), Mozilla has announced a new feature for its Firefox browser that provides user side-channel protection. Site Isolation is a technique that isolates different sites from each other, ensuring that even if one site becomes compromised, the others remain safe.

This is crucial because it implies that even if a malicious actor could access a user’s browsing data, they could only see data from the one site they’ve compromised. The other sites would remain isolated and out of reach.

Mozilla is the first browser to implement this type of side-channel protection, but other browsers will likely follow suit. This is a significant advancement in the fight against online threats and will help keep users safe browsing the web.

Secure Against Universal Cross-site Scripting Vulnerabilities (UXSS)

Similarly, a security enhancement can also insulate a website from vulnerabilities that result from universal cross-site scripting (UXSS). UXSS vulnerabilities can allow an attacker such as Spectre to generate malware code into a web page, which can then be executed by unsuspecting users who visit the page.

By implementing security enhancement technology, web pages can be protected from UXSS attacks, and users can prevent inadvertently executing malicious code.

What is the easiest way to turn on site isolation for Firefox Nightly?

To enable site isolation on Firefox Nightly, go to the Firefox preferences, click on the “Privacy & Security” tab, and then check the “Enable Site Isolation” box. Once you’ve done that, restart Firefox, and you should be good to go.

What is the process for enabling site isolation in Firefox Beta or Release?

To enable site isolation on Firefox, you’ll need to go into the “Advanced” settings for the browser. Once there, you’ll scroll down to the “Performance” section and check the box next to “Enable site isolation”. Once you’ve done that, you’ll need to restart the browser to implement the changes. After restarting, you should notice a difference in how Firefox handles web pages - site isolation is now enabled!

Site Isolation for Developers

To enable site isolation in JavaScript, you can use the SiteIsolation object, which is part of the dom.categories API. This object provides a number of properties and methods that allow you to configure and control site isolation in the browser.

For example, to enable site isolation for a specific website, you can use the SiteIsolation.add method, which takes the URL of the website as an argument. Here is an example of how to use this method to enable site isolation for the website:


Alternatively, you can use the SiteIsolation.addByRegExp method, which allows you to enable site isolation for a group of websites that match a regular expression pattern. Here is an example of how to use this method to enable site isolation for all websites that have the domain:


Once you have enabled site isolation for a website or group of websites, the browser will create a separate process or sandbox for each of these websites, and they will not be able to interact with each other or access each other’s data. This can help to protect against certain types of security vulnerabilities, and it can improve the security of your web applications.

Note that the SiteIsolation object and the dom.categories API are experimental and may not be supported by all browsers. Additionally, these APIs may change in future versions of the browser, so you should check the documentation and use them carefully.

Final Words

Early in 2018, Firefox incorporated security mitigations to protect users from Meltdown and Spectre vulnerabilities. However, a rise in malicious actors on the web demonstrated the need for more security measures. Therefore, Firefox has redesigned its browser to protect users from future variants of these vulnerabilities!