Zero-Day Vulnerability in Mozilla Firefox Exploited in Targeted Attacks

On January 8, Mozilla Foundation released a security advisory to address a critical zero-day flaw in Mozilla Firefox, which has been exploited in targeted attacks.

Analysis

CVE-2019-17026 is a type confusion vulnerability in IonMonkey, the JavaScript Just-In-Time (JIT) compiler for SpiderMonkey, Mozilla’s JavaScript engine. According to Mozilla’s advisory, the flaw exists in the JIT compiler due to “incorrect alias information for setting array elements,” specifically in StoreElementHole and FallibleStoreElement.

The vulnerability was reported to Mozilla by researchers at Qihoo 360 ATA. Mozilla’s advisory states they are “aware of targeted attacks in the wild abusing this flaw.” Based on this note in the advisory, it appears the vulnerability was exploited in the wild as a zero-day. Further information about the exploitation was not available at the time this blog post was published.

This advisory follows the release of Firefox 72 and Firefox Extended Support Release (ESR) 68.4 on January 7, which included the following security advisories:

• Firefox 72: Mozilla Foundation Security Advisory 2020-01

• Firefox ESR 68.4: Mozilla Foundation Security Advisory 2020-02

Last year, Mozilla patched CVE-2019-11707, another type confusion flaw that was used in conjunction with CVE-2019-11708, a sandbox escape vulnerability in targeted attacks.

Proof of concept

At this time, no proof of concept is available for this vulnerability.

Solution

To address CVE-2019-17026, Mozilla released Firefox 72.0.1 and Firefox ESR 68.4.1. Because this vulnerability has been exploited in targeted attacks, Firefox users are advised to upgrade as soon as possible.