Mozilla Rolling Out ‘Site Isolation’ With Release of Firefox 94

Mozilla this week announced that Firefox 94 is bringing Site Isolation to all users, along with patches for over a dozen vulnerabilities, including seven that feature a high severity rating.

The most severe of the security issues patched in Firefox 94 could lead to code execution, the Multi-State Information Sharing and Analysis Center (MS-ISAC) says in an alert. The attacker could install applications, modify data, or even register new user accounts.

The most important of the newly addressed vulnerabilities is CVE-2021-38503, which exists because iframe sandbox rules were incorrectly applied to XSLT (Extensible Stylesheet Language Transformations) stylesheets, which could result in iframes being able to bypass restrictions and execute scripts or navigate the top-level frame.

Next in line is CVE-2021-38504, a use-after-free vulnerability in file picker dialog, followed by CVE-2021-38505, an issue that exists because Firefox before version 94 did not use specific clipboard formats that would prevent Windows 10 Cloud Clipboard from recording sensitive user data.

Another security error could result in Firefox being “coaxed into going into fullscreen mode without notification or warning.” Tracked as CVE-2021-38506, the vulnerability may lead to spoofing attacks in the browser, including phishing, Mozilla explains in an advisory.

Another high-severity bug addressed with the release of Firefox 94 is CVE-2021-38507, where the Opportunistic Encryption feature of HTTP2 (RFC 8164) could be exploited to bypass Same-Origin-Policy. Mozilla decided to disable the feature, noting that it has low usage.

Mozilla also addressed a high-severity universal cross-site scripting (XSS) issue in Firefox for Android (it exists because URLs scanned from QR codes weren’t properly sanitized), as well as memory safety bugs that affect both Firefox 94 and Firefox ESR 91.3 and which could lead to arbitrary code execution.

Four moderate-severity security holes were addressed with the latest Firefox release. These could lead to the overlay of a form validity message on top of a permission prompt; to web extensions accessing pre-redirect URLs; to the spoofing of JavaScript alert box; and to the bypass of download protections on macOS.

Mozilla also addressed two low-severity vulnerabilities with the new Firefox release.

Firefox 94 will roll out over the next few weeks to all users with Site Isolation in tow, the browser maker says. Previewed since May, the new security feature is expected to improve protections against side-channel attacks such as Spectre.

Related: Mozilla Blocks Malicious Firefox Add-Ons Abusing Proxy API

Related: Firefox 93 Improves Protection Against Tracking, Insecure Downloads

Related: Firefox 91 Brings New Privacy, Security Improvements

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Ionut Arghire is an international correspondent for SecurityWeek.

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