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Mozilla Updates Firefox 40 to Patch Two Serious Flaws

Mozilla updated Firefox to version 40.0.3 on Thursday to address a couple of serious vulnerabilities.

The first flaw, a use-after-free triggered when a <canvas> element is resized (CVE-2015-4497), has been rated critical. An attacker can exploit the vulnerability by setting up a malicious webpage that causes Firefox to crash. The weakness can potentially be exploited to execute arbitrary code with the privileges of the attacked Firefox user.

“[The vulnerability] occurs when a resize event is triggered in concert with style changes but the canvas references have been recreated in the meantime, destroying the originally referenced context,” Mozilla wrote in its advisory.

The vulnerability was discovered by Mozilla community member Jean-Max Reymond, and later reported by Georgian researcher Ucha Gobejishvili via HP’s Zero Day Initiative.

The second flaw, rated high-severity, has been described as an add-on notification bypass through data URLs (CVE-2015-4498).

Firefox is designed not to display warning prompts when a user enters a URL that points to an add-on directly in the browser’s address bar. The normal install permission prompt is bypassed because Mozilla considers this a direct user action.

However, researcher Bas Venis discovered that an attacker could manipulate a data: URL on a loaded page to simulate this direct user input and bypass the installation prompt. An attacker can also make the installation prompt appear on top of a different site by triggering a page navigation right after the add-on installation has been initiated.

A malicious actor could exploit this vulnerability to get users to install a rogue add-on by tricking them into thinking that the program is from a trusted source.

There is no evidence that these vulnerabilities have been exploited in the wild. Ubuntu has already released updated packages to address the flaws, and Red Hat is currently working on releasing packages that fix the issues.

With the release of Firefox 40 earlier this month, Mozilla started introducing a new add-on/extension signing policy. The organization wants to ban the installation of unsigned extensions and add-ons in the Release and Beta versions of Firefox. In version 40, users are warned when they are about to install an extension that is not signed. Starting with Firefox 42, unsigned extensions will be banned completely and there will be no override option.

Related: Firefox 40 Patches Vulnerabilities, Expands Malware Protection

Related: Mozilla Patches Firefox Zero-Day Exploited in the Wild

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Eduard Kovacs (@EduardKovacs) is a contributing editor at SecurityWeek. He worked as a high school IT teacher for two years before starting a career in journalism as Softpedia’s security news reporter. Eduard holds a bachelor’s degree in industrial informatics and a master’s degree in computer techniques applied in electrical engineering.