Mozilla fixes Firefox zero-day being actively exploited

The US cyber security agency has warned the vulnerability is being used to take control of users’ machines

Graphic concept of a hacker with colour effects, overlaid on words

Mozilla has patched a critical flaw in its Firefox browser that’s being actively exploited by criminals in targeted attacks.

The critical vulnerability, branded CVE-2019-17026, allows an attacker to seize control of an affected computer through a mechanism that leads to ‘type confusion’, according to an advisory released by Mozilla. 

The company confirmed that the critical flaw, which has now been patched, affects users running version 72 of Firefox and version 68.4 of Firefox ESR. The developer added that it’s "aware of targeted attacks in the wild abusing this flaw". 

The severity of the flaw is such that the US Cyber Security and Infrastructure Agency has issued a separate warning urging Firefox users to apply the necessary updates.

The attack works by causing ‘type confusion’, which is a potentially critical error that can lead to data being read from or written to locations of memory normally out of bounds. When triggered, this can lead to an exploitable crash because of issues caused when the browser attempts to manipulate JavaScript objects.

It’s the second time within seven months that Firefox has sustained a critical zero-day vulnerability being actively exploited in the wild.

A previous flaw, discovered in June 2019, gave attackers the tools to execute arbitrary code on flawed machines and in some cases take over users’ devices remotely.

The latest emergency fix follows a round of 11 CVE-rated bug fixes Mozilla has issued, five of which were rated ‘high’ and four rated ‘medium’. Among these highly-rated issues were memory safety bugs in Firefox 72, another type confusion issue, and a memory corruption flaw.

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The second major security scare within a matter of months is a blow to a developer trying to forge a fresh identity for Firefox as a privacy-centric web browser. Mozilla has teased and rolled out a suite of changes to how Firefox functions in the last year, including tools like a virtual private network (VPN).

In September last year, Mozilla also instigated a change in Firefox that would block known third-party tracking cookies and cryptocurrency mining by default as part of its Enhanced Tracking Protection (ETP).

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