# CVE-2021-23960: Firefox ESR and 2 others: Performing garbage collection on re-declared JavaScript variables resulted in a user-after-poison, and…

CVE-2021-23960 is fixed in 11 products on 2 platforms, first by firefox 85.0+build1-0ubuntu0.18.04.1, released January 25, 2021. It is not reported as exploited. Patch Radar priority: High.

- Web page: https://patchblox.com/security/cve/CVE-2021-23960
- Priority: High
- CVSS: 8.8 (Important)
- EPSS: 0.0129
- CISA KEV: not listed
- Exploited (vendor report): no
- Publicly disclosed: no
- Published: Jan 25, 2021
- Vendor advisory: https://nvd.nist.gov/vuln/detail/CVE-2021-23960

## Description

Performing garbage collection on re-declared JavaScript variables resulted in a user-after-poison, and a potentially exploitable crash. This vulnerability affects Firefox < 85, Thunderbird < 78.7, and Firefox ESR < 78.7.

## Where it's fixed

| Platform | Product | Fixed in | Fix | Released |
|---|---|---|---|---|
| Windows apps | Mozilla Firefox ESR (en-US) | 91.0 | [Mozilla Firefox ESR (en-US) 91.0](https://patchblox.com/security/data/fix/windows-app/mozilla.firefox.esr-91.0.md) | Aug 4, 2021 |
| Windows apps | Mozilla Thunderbird (en-US) | 78.10.0 | [Mozilla Thunderbird (en-US) 78.10.0](https://patchblox.com/security/data/fix/windows-app/mozilla.thunderbird-78.10.0.md) | Apr 15, 2021 |
| Linux | firefox RHEL 8 | 0:78.7.0-2.el8_3 | [RHSA-2021:0288](https://patchblox.com/security/data/fix/linux/rhsa-2021-0288.md) | Jan 27, 2021 |
| Linux | firefox Ubuntu 18.04.6 LTS (bionic) | 85.0+build1-0ubuntu0.18.04.1 | [firefox 85.0+build1-0ubuntu0.18.04.1](https://patchblox.com/security/data/fix/linux/ubuntu-bionic-firefox-85.0-build1-0ubuntu0.18.04.1.md) | Jan 25, 2021 |
| Linux | firefox Ubuntu 20.04 LTS (focal) | 85.0+build1-0ubuntu0.20.04.1 | [firefox 85.0+build1-0ubuntu0.20.04.1](https://patchblox.com/security/data/fix/linux/ubuntu-focal-firefox-85.0-build1-0ubuntu0.20.04.1.md) | Jan 25, 2021 |
| Linux | firefox ol 8 | 0:78.7.0-2.0.1.el8_3 | [ELSA-2021-0288](https://patchblox.com/security/data/fix/linux/elsa-2021-0288.md) | Jan 28, 2021 |
| Linux | firefox-geckodriver Ubuntu 18.04.6 LTS (bionic) | 85.0+build1-0ubuntu0.18.04.1 | [firefox-geckodriver 85.0+build1-0ubuntu0.18.04.1](https://patchblox.com/security/data/fix/linux/ubuntu-bionic-firefox-geckodriver-85.0-build1-0ubuntu0.18.04.1.md) | Jan 25, 2021 |
| Linux | firefox-geckodriver Ubuntu 20.04 LTS (focal) | 85.0+build1-0ubuntu0.20.04.1 | [firefox-geckodriver 85.0+build1-0ubuntu0.20.04.1](https://patchblox.com/security/data/fix/linux/ubuntu-focal-firefox-geckodriver-85.0-build1-0ubuntu0.20.04.1.md) | Jan 25, 2021 |
| Linux | firefox-mozsymbols Ubuntu 18.04.6 LTS (bionic) | 85.0+build1-0ubuntu0.18.04.1 | [firefox-mozsymbols 85.0+build1-0ubuntu0.18.04.1](https://patchblox.com/security/data/fix/linux/ubuntu-bionic-firefox-mozsymbols-85.0-build1-0ubuntu0.18.04.1.md) | Jan 25, 2021 |
| Linux | firefox-mozsymbols Ubuntu 20.04 LTS (focal) | 85.0+build1-0ubuntu0.20.04.1 | [firefox-mozsymbols 85.0+build1-0ubuntu0.20.04.1](https://patchblox.com/security/data/fix/linux/ubuntu-focal-firefox-mozsymbols-85.0-build1-0ubuntu0.20.04.1.md) | Jan 25, 2021 |
| Linux | thunderbird RHEL 8 | 0:78.7.0-1.el8_3 | [RHSA-2021:0298](https://patchblox.com/security/data/fix/linux/rhsa-2021-0298.md) | Jan 28, 2021 |
| Linux | thunderbird Ubuntu 18.04.6 LTS (bionic) | 1:78.8.1+build1-0ubuntu0.18.04.1 | [thunderbird 1:78.8.1+build1-0ubuntu0.18.04.1](https://patchblox.com/security/data/fix/linux/ubuntu-bionic-thunderbird-1-78.8.1-build1-0ubuntu0.18.04.1.md) | Apr 22, 2021 |
| Linux | thunderbird Ubuntu 20.04 LTS (focal) | 1:78.7.1+build1-0ubuntu0.20.04.1 | [thunderbird 1:78.7.1+build1-0ubuntu0.20.04.1](https://patchblox.com/security/data/fix/linux/ubuntu-focal-thunderbird-1-78.7.1-build1-0ubuntu0.20.04.1.md) | Jan 25, 2021 |
| Linux | thunderbird ol 8 | 0:78.7.0-1.0.1.el8_3 | [ELSA-2021-0298](https://patchblox.com/security/data/fix/linux/elsa-2021-0298.md) | Jan 29, 2021 |
| Linux | thunderbird-gnome-support Ubuntu 18.04.6 LTS (bionic) | 1:78.8.1+build1-0ubuntu0.18.04.1 | [thunderbird-gnome-support 1:78.8.1+build1-0ubuntu0.18.04.1](https://patchblox.com/security/data/fix/linux/ubuntu-bionic-thunderbird-gnome-support-1-78.8.1-build1-0ubuntu0.18.04.1.md) | Apr 22, 2021 |
| Linux | thunderbird-gnome-support Ubuntu 20.04 LTS (focal) | 1:78.7.1+build1-0ubuntu0.20.04.1 | [thunderbird-gnome-support 1:78.7.1+build1-0ubuntu0.20.04.1](https://patchblox.com/security/data/fix/linux/ubuntu-focal-thunderbird-gnome-support-1-78.7.1-build1-0ubuntu0.20.04.1.md) | Jan 25, 2021 |
| Linux | thunderbird-mozsymbols Ubuntu 18.04.6 LTS (bionic) | 1:78.8.1+build1-0ubuntu0.18.04.1 | [thunderbird-mozsymbols 1:78.8.1+build1-0ubuntu0.18.04.1](https://patchblox.com/security/data/fix/linux/ubuntu-bionic-thunderbird-mozsymbols-1-78.8.1-build1-0ubuntu0.18.04.1.md) | Apr 22, 2021 |
| Linux | thunderbird-mozsymbols Ubuntu 20.04 LTS (focal) | 1:78.7.1+build1-0ubuntu0.20.04.1 | [thunderbird-mozsymbols 1:78.7.1+build1-0ubuntu0.20.04.1](https://patchblox.com/security/data/fix/linux/ubuntu-focal-thunderbird-mozsymbols-1-78.7.1-build1-0ubuntu0.20.04.1.md) | Jan 25, 2021 |
| Linux | xul-ext-calendar-timezones Ubuntu 18.04.6 LTS (bionic) | 1:78.8.1+build1-0ubuntu0.18.04.1 | [xul-ext-calendar-timezones 1:78.8.1+build1-0ubuntu0.18.04.1](https://patchblox.com/security/data/fix/linux/ubuntu-bionic-xul-ext-calendar-timezones-1-78.8.1-build1-0ubuntu0.18.04.1.md) | Apr 22, 2021 |
| Linux | xul-ext-calendar-timezones Ubuntu 20.04 LTS (focal) | 1:78.7.1+build1-0ubuntu0.20.04.1 | [xul-ext-calendar-timezones 1:78.7.1+build1-0ubuntu0.20.04.1](https://patchblox.com/security/data/fix/linux/ubuntu-focal-xul-ext-calendar-timezones-1-78.7.1-build1-0ubuntu0.20.04.1.md) | Jan 25, 2021 |
| Linux | xul-ext-gdata-provider Ubuntu 18.04.6 LTS (bionic) | 1:78.8.1+build1-0ubuntu0.18.04.1 | [xul-ext-gdata-provider 1:78.8.1+build1-0ubuntu0.18.04.1](https://patchblox.com/security/data/fix/linux/ubuntu-bionic-xul-ext-gdata-provider-1-78.8.1-build1-0ubuntu0.18.04.1.md) | Apr 22, 2021 |
| Linux | xul-ext-gdata-provider Ubuntu 20.04 LTS (focal) | 1:78.7.1+build1-0ubuntu0.20.04.1 | [xul-ext-gdata-provider 1:78.7.1+build1-0ubuntu0.20.04.1](https://patchblox.com/security/data/fix/linux/ubuntu-focal-xul-ext-gdata-provider-1-78.7.1-build1-0ubuntu0.20.04.1.md) | Jan 25, 2021 |
| Linux | xul-ext-lightning Ubuntu 18.04.6 LTS (bionic) | 1:78.8.1+build1-0ubuntu0.18.04.1 | [xul-ext-lightning 1:78.8.1+build1-0ubuntu0.18.04.1](https://patchblox.com/security/data/fix/linux/ubuntu-bionic-xul-ext-lightning-1-78.8.1-build1-0ubuntu0.18.04.1.md) | Apr 22, 2021 |
| Linux | xul-ext-lightning Ubuntu 20.04 LTS (focal) | 1:78.7.1+build1-0ubuntu0.20.04.1 | [xul-ext-lightning 1:78.7.1+build1-0ubuntu0.20.04.1](https://patchblox.com/security/data/fix/linux/ubuntu-focal-xul-ext-lightning-1-78.7.1-build1-0ubuntu0.20.04.1.md) | Jan 25, 2021 |

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