# CVE-2021-23995: Firefox ESR and 2 others: When Responsive Design Mode was enabled, it used references to objects that were previously freed.

CVE-2021-23995 is fixed in 11 products on 2 platforms, first by Mozilla Thunderbird (en-US) 78.10.0, released April 15, 2021. It is not reported as exploited. Patch Radar priority: High.

- Web page: https://patchblox.com/security/cve/CVE-2021-23995
- Priority: High
- CVSS: 8.8 (Important)
- EPSS: 0.0118
- CISA KEV: not listed
- Exploited (vendor report): no
- Publicly disclosed: no
- Published: Apr 18, 2021
- Vendor advisory: https://nvd.nist.gov/vuln/detail/CVE-2021-23995

## Description

When Responsive Design Mode was enabled, it used references to objects that were previously freed. We presume that with enough effort this could have been exploited to run arbitrary code. This vulnerability affects Firefox ESR < 78.10, Thunderbird < 78.10, and Firefox < 88.

## 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 | 0:78.10.0-1.el8_3 | [RHSA-2021:1360](https://patchblox.com/security/data/fix/linux/rhsa-2021-1360.md) | Apr 26, 2021 |
| Linux | firefox Ubuntu 18.04.6 LTS (bionic) | 88.0+build2-0ubuntu0.18.04.2 | [firefox 88.0+build2-0ubuntu0.18.04.2](https://patchblox.com/security/data/fix/linux/ubuntu-bionic-firefox-88.0-build2-0ubuntu0.18.04.2.md) | Apr 25, 2021 |
| Linux | firefox Ubuntu 20.04 LTS (focal) | 88.0+build2-0ubuntu0.20.04.1 | [firefox 88.0+build2-0ubuntu0.20.04.1](https://patchblox.com/security/data/fix/linux/ubuntu-focal-firefox-88.0-build2-0ubuntu0.20.04.1.md) | Apr 25, 2021 |
| Linux | firefox ol 8 | 0:78.10.0-1.0.1.el8_3 | [ELSA-2021-1360](https://patchblox.com/security/data/fix/linux/elsa-2021-1360.md) | Apr 27, 2021 |
| Linux | firefox-geckodriver Ubuntu 18.04.6 LTS (bionic) | 88.0+build2-0ubuntu0.18.04.2 | [firefox-geckodriver 88.0+build2-0ubuntu0.18.04.2](https://patchblox.com/security/data/fix/linux/ubuntu-bionic-firefox-geckodriver-88.0-build2-0ubuntu0.18.04.2.md) | Apr 25, 2021 |
| Linux | firefox-geckodriver Ubuntu 20.04 LTS (focal) | 88.0+build2-0ubuntu0.20.04.1 | [firefox-geckodriver 88.0+build2-0ubuntu0.20.04.1](https://patchblox.com/security/data/fix/linux/ubuntu-focal-firefox-geckodriver-88.0-build2-0ubuntu0.20.04.1.md) | Apr 25, 2021 |
| Linux | firefox-mozsymbols Ubuntu 18.04.6 LTS (bionic) | 88.0+build2-0ubuntu0.18.04.2 | [firefox-mozsymbols 88.0+build2-0ubuntu0.18.04.2](https://patchblox.com/security/data/fix/linux/ubuntu-bionic-firefox-mozsymbols-88.0-build2-0ubuntu0.18.04.2.md) | Apr 25, 2021 |
| Linux | firefox-mozsymbols Ubuntu 20.04 LTS (focal) | 88.0+build2-0ubuntu0.20.04.1 | [firefox-mozsymbols 88.0+build2-0ubuntu0.20.04.1](https://patchblox.com/security/data/fix/linux/ubuntu-focal-firefox-mozsymbols-88.0-build2-0ubuntu0.20.04.1.md) | Apr 25, 2021 |
| Linux | thunderbird RHEL 8 | 0:78.10.0-1.el8_3 | [RHSA-2021:1353](https://patchblox.com/security/data/fix/linux/rhsa-2021-1353.md) | Apr 26, 2021 |
| Linux | thunderbird Ubuntu 18.04.6 LTS (bionic) | 1:78.11.0+build1-0ubuntu0.18.04.2 | [thunderbird 1:78.11.0+build1-0ubuntu0.18.04.2](https://patchblox.com/security/data/fix/linux/ubuntu-bionic-thunderbird-1-78.11.0-build1-0ubuntu0.18.04.2.md) | Jun 1, 2021 |
| Linux | thunderbird Ubuntu 20.04 LTS (focal) | 1:78.11.0+build1-0ubuntu0.20.04.2 | [thunderbird 1:78.11.0+build1-0ubuntu0.20.04.2](https://patchblox.com/security/data/fix/linux/ubuntu-focal-thunderbird-1-78.11.0-build1-0ubuntu0.20.04.2.md) | Jun 1, 2021 |
| Linux | thunderbird ol 8 | 0:78.10.0-1.0.1.el8_3 | [ELSA-2021-1353](https://patchblox.com/security/data/fix/linux/elsa-2021-1353.md) | Apr 27, 2021 |
| Linux | thunderbird-gnome-support Ubuntu 18.04.6 LTS (bionic) | 1:78.11.0+build1-0ubuntu0.18.04.2 | [thunderbird-gnome-support 1:78.11.0+build1-0ubuntu0.18.04.2](https://patchblox.com/security/data/fix/linux/ubuntu-bionic-thunderbird-gnome-support-1-78.11.0-build1-0ubuntu0.18.04.2.md) | Jun 1, 2021 |
| Linux | thunderbird-gnome-support Ubuntu 20.04 LTS (focal) | 1:78.11.0+build1-0ubuntu0.20.04.2 | [thunderbird-gnome-support 1:78.11.0+build1-0ubuntu0.20.04.2](https://patchblox.com/security/data/fix/linux/ubuntu-focal-thunderbird-gnome-support-1-78.11.0-build1-0ubuntu0.20.04.2.md) | Jun 1, 2021 |
| Linux | thunderbird-mozsymbols Ubuntu 18.04.6 LTS (bionic) | 1:78.11.0+build1-0ubuntu0.18.04.2 | [thunderbird-mozsymbols 1:78.11.0+build1-0ubuntu0.18.04.2](https://patchblox.com/security/data/fix/linux/ubuntu-bionic-thunderbird-mozsymbols-1-78.11.0-build1-0ubuntu0.18.04.2.md) | Jun 1, 2021 |
| Linux | thunderbird-mozsymbols Ubuntu 20.04 LTS (focal) | 1:78.11.0+build1-0ubuntu0.20.04.2 | [thunderbird-mozsymbols 1:78.11.0+build1-0ubuntu0.20.04.2](https://patchblox.com/security/data/fix/linux/ubuntu-focal-thunderbird-mozsymbols-1-78.11.0-build1-0ubuntu0.20.04.2.md) | Jun 1, 2021 |
| Linux | xul-ext-calendar-timezones Ubuntu 18.04.6 LTS (bionic) | 1:78.11.0+build1-0ubuntu0.18.04.2 | [xul-ext-calendar-timezones 1:78.11.0+build1-0ubuntu0.18.04.2](https://patchblox.com/security/data/fix/linux/ubuntu-bionic-xul-ext-calendar-timezones-1-78.11.0-build1-0ubuntu0.18.04.2.md) | Jun 1, 2021 |
| Linux | xul-ext-calendar-timezones Ubuntu 20.04 LTS (focal) | 1:78.11.0+build1-0ubuntu0.20.04.2 | [xul-ext-calendar-timezones 1:78.11.0+build1-0ubuntu0.20.04.2](https://patchblox.com/security/data/fix/linux/ubuntu-focal-xul-ext-calendar-timezones-1-78.11.0-build1-0ubuntu0.20.04.2.md) | Jun 1, 2021 |
| Linux | xul-ext-gdata-provider Ubuntu 18.04.6 LTS (bionic) | 1:78.11.0+build1-0ubuntu0.18.04.2 | [xul-ext-gdata-provider 1:78.11.0+build1-0ubuntu0.18.04.2](https://patchblox.com/security/data/fix/linux/ubuntu-bionic-xul-ext-gdata-provider-1-78.11.0-build1-0ubuntu0.18.04.2.md) | Jun 1, 2021 |
| Linux | xul-ext-gdata-provider Ubuntu 20.04 LTS (focal) | 1:78.11.0+build1-0ubuntu0.20.04.2 | [xul-ext-gdata-provider 1:78.11.0+build1-0ubuntu0.20.04.2](https://patchblox.com/security/data/fix/linux/ubuntu-focal-xul-ext-gdata-provider-1-78.11.0-build1-0ubuntu0.20.04.2.md) | Jun 1, 2021 |
| Linux | xul-ext-lightning Ubuntu 18.04.6 LTS (bionic) | 1:78.11.0+build1-0ubuntu0.18.04.2 | [xul-ext-lightning 1:78.11.0+build1-0ubuntu0.18.04.2](https://patchblox.com/security/data/fix/linux/ubuntu-bionic-xul-ext-lightning-1-78.11.0-build1-0ubuntu0.18.04.2.md) | Jun 1, 2021 |
| Linux | xul-ext-lightning Ubuntu 20.04 LTS (focal) | 1:78.11.0+build1-0ubuntu0.20.04.2 | [xul-ext-lightning 1:78.11.0+build1-0ubuntu0.20.04.2](https://patchblox.com/security/data/fix/linux/ubuntu-focal-xul-ext-lightning-1-78.11.0-build1-0ubuntu0.20.04.2.md) | Jun 1, 2021 |

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