Firefox 148 introduces the new standardized Sanitizer API, becoming the first browser to implement it. The update marks a major step forward for web security, giving developers a straightforward and effective way to prevent Cross-Site Scripting (XSS) attacks.
XSS is one of the most common and persistent vulnerabilities on the internet. It has ranked among the top three web vulnerabilities for nearly ten years.
These attacks happen when a website accidentally allows bad actors to inject malicious HTML or JavaScript through user-generated content.
Once injected, attackers can monitor users’ activities, manipulate their interactions, and steal sensitive data.
For years, preventing XSS has been difficult. Mozilla previously led efforts with the Content-Security-Policy (CSP) standard in 2009.
While CSP is a strong defense that restricts what resources a browser can load, it requires major changes to a website’s architecture and continuous review by security experts. Because of this, CSP has not been widely adopted by all websites.
The new Sanitizer API fills this security gap. It provides a standardized way to convert harmful HTML into safe, harmless HTML before it is inserted into a webpage.
How the Sanitizer API Works
The core of this new protection is the setHTML() method. It integrates sanitization directly into the HTML insertion process, ensuring websites are safe by default.
For example, if an attacker tries to inject this harmful code:
The Sanitizer API will automatically clean it up. It keeps the safe
The resulting safe code looks like this:
Developers can easily improve their website’s security with minimal effort. By simply replacing the older, risky innerHTML method with the new setHTML() method, they can activate stronger XSS protections.
If the default settings are too strict or not strict enough, developers can create a custom configuration to choose exactly which HTML elements to allow.
Mozilla also offers a Sanitizer API playground where developers can test the tool before using it on a live site.
For maximum protection, the Sanitizer API works perfectly with Trusted Types, another security feature supported in Firefox 148.
Together, they control how HTML is parsed and injected, blocking unsafe methods and preventing future XSS flaws.
According to Mozilla Hacks, Firefox 148 makes XSS prevention easier with the new Sanitizer API, with other browsers expected to adopt it soon.
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