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Mozilla Patches Two Firefox Vulnerabilities Exploited in Attacks

Mozilla has released updates for its Firefox web browser to patch two critical use-after-free vulnerabilities that have been exploited in attacks.

One of the flaws, tracked as CVE-2020-6819, has been described as a use-after-free caused by a race condition that is triggered in certain conditions when running the nsDocShell destructor. Researchers at Tenable have analyzed the patch and they believe the issue exists “due to the mContentViewer not being released properly.”

The second vulnerability, identified as CVE-2020-6820, has been described as a use-after-free caused by a race condition triggered by the handling of a ReadableStream.

Both flaws have been addressed with the release of Firefox 74.0.1 and Firefox ESR 68.6.1. Sophos’ Paul Ducklin believes the vulnerabilities may have existed since July 2019.

“Given that the bug needed patching in both the latest and the ESR versions, we can assume either that the vulnerability has been in the Firefox codebase at least since version 68 first appeared, which was back in July 2019, or that it was introduced as a side effect of a security fix that came out after version 68.0 showed up,” Ducklin explained.

Mozilla has credited researchers Francisco Alonso and Javier Marcos for reporting the vulnerabilities. Alonso applauded Mozilla for the way it handled their report, but also suggested that other browsers may be impacted as well.

The U.S. Cybersecurity and Infrastructure Security Agency (CISA) has advised users to update Firefox as soon as possible.

This is not the first round of zero-days fixed by Mozilla this year in Firefox. In January, the organization patched a flaw that has been exploited by an APT actor alongside an Internet Explorer zero-day in attacks aimed at China and Japan. The attacks have been attributed by Chinese cybersecurity firm Qihoo 360 to DarkHotel, a threat group linked by some to South Korea.

Related: Firefox Zero-Day Exploited to Deliver Malware to Cryptocurrency Exchanges

Related: Mozilla Patches Second Firefox Zero-Day Used in Cryptocurrency Attacks

Related: Google Patches Chrome Vulnerability 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.