Firefox 95 boosts protection against zero-day attacks

The Mozilla Firefox logo on a laptop
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Mozilla is shipping a security technology with Firefox 95 that it hopes will stop zero-day attacks targeting users of the browser.

Called RLBox, the new feature will take a more granular approach to sandboxing, a security approach that runs website code in its own walled-off section of memory to avoid malicious components affecting the rest of the system.

In a blog post announcing the feature, Mozilla distinguished engineer Bobby Holley highlighted a problem with existing browser sandboxing technology, which isolates web site processes in individual sandboxes, and is vulnerable to chained attacks.

Malware developers can compromise the process first, and then escape the sandbox, it warns. It also limits the extent to which code can be separated into different sandboxed processes because of the memory overhead involved.

Developed in conjunction with the University of California San Diego and the University of Texas, RLBox complements process-based isolation with a new approach. It compiles code as native code via WebAssembly, which is a portable compilation format.

This makes each separately compiled piece of native code safer, because it can't access memory outside of a specified region and can't make any unexpected jumps. The new approach makes it possible to run different pieces of trusted and untrusted code in the same process without them affecting each other.


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"RLBox is a big win for us on several fronts: it protects our users from accidental defects as well as supply-chain attacks, and it reduces the need for us to scramble when such issues are disclosed upstream," said Holley.

RLBox is a stand-alone project that Mozilla first trialed on macOS and Linux users last year. It is now deploying it across all Firefox platforms, including mobile systems.

Mozilla will begin with RLBox support for the Graphite font rendering system, the Hunspell spell checker, and the Ogg multimedia container format in Firefox 95. It will follow this up in Firefox 96 with support for the Expat XML parser and Woff2, the font compression technology used in the browser.

"Going forward, we can treat these modules as untrusted code, and — assuming we did it right — even a zero-day vulnerability in any of them should pose no threat to Firefox," Holley said. He also hoped that other browsers would adopt the open-source technology.

Mozilla has updated its bug bounty program to reward researchers for escaping the sandbox technology without exploiting vulnerabilities in an isolated component.

Danny Bradbury

Danny Bradbury has been a print journalist specialising in technology since 1989 and a freelance writer since 1994. He has written for national publications on both sides of the Atlantic and has won awards for his investigative cybersecurity journalism work and his arts and culture writing. 

Danny writes about many different technology issues for audiences ranging from consumers through to software developers and CIOs. He also ghostwrites articles for many C-suite business executives in the technology sector and has worked as a presenter for multiple webinars and podcasts.