# CVE-2020-6820: Mozilla Firefox And Thunderbird Use-After-Free Vulnerability CVE-2020-6820 is fixed in 10 products on 2 platforms, first by firefox 74.0.1+build1-0ubuntu0.18.04.1, released April 2, 2020. It is being exploited: CISA added it to the Known Exploited Vulnerabilities catalog on November 3, 2021. Patch Radar priority: Patch now. - Web page: https://patchblox.com/security/cve/CVE-2020-6820 - Priority: Patch now - CVSS: 8.8 (Critical) - EPSS: 0.0706 - CISA KEV: listed Nov 3, 2021, federal due date May 3, 2022 - Exploited (vendor report): yes - Publicly disclosed: no - Published: Apr 2, 2020 - Weakness: CWE-362: Concurrent Execution using Shared Resource with Improper Synchronization ('Race Condition') - Vendor advisory: https://nvd.nist.gov/vuln/detail/CVE-2020-6820 ## Description Under certain conditions, when handling a ReadableStream, a race condition can cause a use-after-free. We are aware of targeted attacks in the wild abusing this flaw. This vulnerability affects Thunderbird < 68.7.0, Firefox < 74.0.1, and Firefox ESR < 68.6.1. ## Where it's fixed | Platform | Product | Fixed in | Fix | Released | |---|---|---|---|---| | Windows apps | Mozilla Firefox ESR (en-US) | 78.0esr | [Mozilla Firefox ESR (en-US) 78.0esr](https://patchblox.com/security/data/fix/windows-app/mozilla.firefox.esr-78.0esr.md) | Jun 25, 2020 | | Linux | firefox RHEL 8 | 0:68.6.1-1.el8_1 | [RHSA-2020:1341](https://patchblox.com/security/data/fix/linux/rhsa-2020-1341.md) | Apr 7, 2020 | | Linux | firefox Ubuntu 18.04.6 LTS (bionic) | 74.0.1+build1-0ubuntu0.18.04.1 | [firefox 74.0.1+build1-0ubuntu0.18.04.1](https://patchblox.com/security/data/fix/linux/ubuntu-bionic-firefox-74.0.1-build1-0ubuntu0.18.04.1.md) | Apr 2, 2020 | | Linux | firefox Ubuntu 20.04 LTS (focal) | 75.0+build3-0ubuntu1 | [firefox 75.0+build3-0ubuntu1](https://patchblox.com/security/data/fix/linux/ubuntu-focal-firefox-75.0-build3-0ubuntu1.md) | Apr 6, 2020 | | Linux | firefox ol 8 | 0:68.6.1-1.0.1.el8_1 | [ELSA-2020-1341](https://patchblox.com/security/data/fix/linux/elsa-2020-1341.md) | Apr 9, 2020 | | Linux | firefox-geckodriver Ubuntu 18.04.6 LTS (bionic) | 74.0.1+build1-0ubuntu0.18.04.1 | [firefox-geckodriver 74.0.1+build1-0ubuntu0.18.04.1](https://patchblox.com/security/data/fix/linux/ubuntu-bionic-firefox-geckodriver-74.0.1-build1-0ubuntu0.18.04.1.md) | Apr 2, 2020 | | Linux | firefox-geckodriver Ubuntu 20.04 LTS (focal) | 75.0+build3-0ubuntu1 | [firefox-geckodriver 75.0+build3-0ubuntu1](https://patchblox.com/security/data/fix/linux/ubuntu-focal-firefox-geckodriver-75.0-build3-0ubuntu1.md) | Apr 6, 2020 | | Linux | firefox-mozsymbols Ubuntu 18.04.6 LTS (bionic) | 74.0.1+build1-0ubuntu0.18.04.1 | [firefox-mozsymbols 74.0.1+build1-0ubuntu0.18.04.1](https://patchblox.com/security/data/fix/linux/ubuntu-bionic-firefox-mozsymbols-74.0.1-build1-0ubuntu0.18.04.1.md) | Apr 2, 2020 | | Linux | firefox-mozsymbols Ubuntu 20.04 LTS (focal) | 75.0+build3-0ubuntu1 | [firefox-mozsymbols 75.0+build3-0ubuntu1](https://patchblox.com/security/data/fix/linux/ubuntu-focal-firefox-mozsymbols-75.0-build3-0ubuntu1.md) | Apr 6, 2020 | | Linux | thunderbird RHEL 8 | 0:68.7.0-1.el8_1 | [RHSA-2020:1495](https://patchblox.com/security/data/fix/linux/rhsa-2020-1495.md) | Apr 16, 2020 | | Linux | thunderbird Ubuntu 18.04.6 LTS (bionic) | 1:68.7.0+build1-0ubuntu0.18.04.1 | [thunderbird 1:68.7.0+build1-0ubuntu0.18.04.1](https://patchblox.com/security/data/fix/linux/ubuntu-bionic-thunderbird-1-68.7.0-build1-0ubuntu0.18.04.1.md) | Apr 6, 2020 | | Linux | thunderbird Ubuntu 20.04 LTS (focal) | 1:68.7.0+build1-0ubuntu1 | [thunderbird 1:68.7.0+build1-0ubuntu1](https://patchblox.com/security/data/fix/linux/ubuntu-focal-thunderbird-1-68.7.0-build1-0ubuntu1.md) | Apr 6, 2020 | | Linux | thunderbird ol 8 | 0:68.7.0-1.0.1.el8_1 | [ELSA-2020-1495](https://patchblox.com/security/data/fix/linux/elsa-2020-1495.md) | Apr 18, 2020 | | Linux | thunderbird-gnome-support Ubuntu 18.04.6 LTS (bionic) | 1:68.7.0+build1-0ubuntu0.18.04.1 | [thunderbird-gnome-support 1:68.7.0+build1-0ubuntu0.18.04.1](https://patchblox.com/security/data/fix/linux/ubuntu-bionic-thunderbird-gnome-support-1-68.7.0-build1-0ubuntu0.18.04.1.md) | Apr 6, 2020 | | Linux | thunderbird-gnome-support Ubuntu 20.04 LTS (focal) | 1:68.7.0+build1-0ubuntu1 | [thunderbird-gnome-support 1:68.7.0+build1-0ubuntu1](https://patchblox.com/security/data/fix/linux/ubuntu-focal-thunderbird-gnome-support-1-68.7.0-build1-0ubuntu1.md) | Apr 6, 2020 | | Linux | thunderbird-mozsymbols Ubuntu 18.04.6 LTS (bionic) | 1:68.7.0+build1-0ubuntu0.18.04.1 | [thunderbird-mozsymbols 1:68.7.0+build1-0ubuntu0.18.04.1](https://patchblox.com/security/data/fix/linux/ubuntu-bionic-thunderbird-mozsymbols-1-68.7.0-build1-0ubuntu0.18.04.1.md) | Apr 6, 2020 | | Linux | thunderbird-mozsymbols Ubuntu 20.04 LTS (focal) | 1:68.7.0+build1-0ubuntu1 | [thunderbird-mozsymbols 1:68.7.0+build1-0ubuntu1](https://patchblox.com/security/data/fix/linux/ubuntu-focal-thunderbird-mozsymbols-1-68.7.0-build1-0ubuntu1.md) | Apr 6, 2020 | | Linux | xul-ext-calendar-timezones Ubuntu 18.04.6 LTS (bionic) | 1:68.7.0+build1-0ubuntu0.18.04.1 | [xul-ext-calendar-timezones 1:68.7.0+build1-0ubuntu0.18.04.1](https://patchblox.com/security/data/fix/linux/ubuntu-bionic-xul-ext-calendar-timezones-1-68.7.0-build1-0ubuntu0.18.04.1.md) | Apr 6, 2020 | | Linux | xul-ext-calendar-timezones Ubuntu 20.04 LTS (focal) | 1:68.7.0+build1-0ubuntu1 | [xul-ext-calendar-timezones 1:68.7.0+build1-0ubuntu1](https://patchblox.com/security/data/fix/linux/ubuntu-focal-xul-ext-calendar-timezones-1-68.7.0-build1-0ubuntu1.md) | Apr 6, 2020 | | Linux | xul-ext-gdata-provider Ubuntu 18.04.6 LTS (bionic) | 1:68.7.0+build1-0ubuntu0.18.04.1 | [xul-ext-gdata-provider 1:68.7.0+build1-0ubuntu0.18.04.1](https://patchblox.com/security/data/fix/linux/ubuntu-bionic-xul-ext-gdata-provider-1-68.7.0-build1-0ubuntu0.18.04.1.md) | Apr 6, 2020 | | Linux | xul-ext-gdata-provider Ubuntu 20.04 LTS (focal) | 1:68.7.0+build1-0ubuntu1 | [xul-ext-gdata-provider 1:68.7.0+build1-0ubuntu1](https://patchblox.com/security/data/fix/linux/ubuntu-focal-xul-ext-gdata-provider-1-68.7.0-build1-0ubuntu1.md) | Apr 6, 2020 | | Linux | xul-ext-lightning Ubuntu 18.04.6 LTS (bionic) | 1:68.7.0+build1-0ubuntu0.18.04.1 | [xul-ext-lightning 1:68.7.0+build1-0ubuntu0.18.04.1](https://patchblox.com/security/data/fix/linux/ubuntu-bionic-xul-ext-lightning-1-68.7.0-build1-0ubuntu0.18.04.1.md) | Apr 6, 2020 | | Linux | xul-ext-lightning Ubuntu 20.04 LTS (focal) | 1:68.7.0+build1-0ubuntu1 | [xul-ext-lightning 1:68.7.0+build1-0ubuntu1](https://patchblox.com/security/data/fix/linux/ubuntu-focal-xul-ext-lightning-1-68.7.0-build1-0ubuntu1.md) | Apr 6, 2020 | --- Data as of 2026-11-08T00:53:09Z UTC. 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