# CVE-2021-23997: Firefox ESR and 1 other: Due to unexpected data type conversions, a use-after-free could have occurred when interacting with the…

CVE-2021-23997 is fixed in 4 products on 2 platforms, first by firefox 88.0+build2-0ubuntu0.18.04.2, released April 25, 2021. It is not reported as exploited. Patch Radar priority: High.

- Web page: https://patchblox.com/security/cve/CVE-2021-23997
- Priority: High
- CVSS: 8.8 (Medium)
- EPSS: 0.0082
- CISA KEV: not listed
- Exploited (vendor report): no
- Publicly disclosed: no
- Published: Apr 18, 2021
- Vendor advisory: https://nvd.nist.gov/vuln/detail/CVE-2021-23997

## Description

Due to unexpected data type conversions, a use-after-free could have occurred when interacting with the font cache. We presume that with enough effort this could have been exploited to run arbitrary code. This vulnerability affects 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 |
| 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-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 |

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Data as of 2026-11-08T19:12:23Z UTC. Source: Patchblox Patch Radar (https://patchblox.com/security/patch-radar). Patchblox finds out-of-date software on Windows, macOS and Linux and updates it through release rings: https://patchblox.com/demo

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