# CVE-2021-29991: Firefox ESR and 3 others: Firefox incorrectly accepted a newline in a HTTP/3 header, interpretting it as two separate headers.

CVE-2021-29991 is fixed in 11 products on 2 platforms, first by firefox 91.0.1+build1-0ubuntu0.18.04.1, released August 17, 2021. It is not reported as exploited. Patch Radar priority: High.

- Web page: https://patchblox.com/security/cve/CVE-2021-29991
- Priority: High
- CVSS: 8.1 (Medium)
- EPSS: 0.0091
- CISA KEV: not listed
- Exploited (vendor report): no
- Publicly disclosed: no
- Published: Aug 17, 2021
- Vendor advisory: https://nvd.nist.gov/vuln/detail/CVE-2021-29991

## Description

Firefox incorrectly accepted a newline in a HTTP/3 header, interpretting it as two separate headers. This allowed for a header splitting attack against servers using HTTP/3. This vulnerability affects Firefox < 91.0.1 and Thunderbird < 91.0.1.

## Where it's fixed

| Platform | Product | Fixed in | Fix | Released |
|---|---|---|---|---|
| Windows apps | Mozilla Firefox ESR (en-US) | 91.3.0 | [Mozilla Firefox ESR (en-US) 91.3.0](https://patchblox.com/security/data/fix/windows-app/mozilla.firefox.esr-91.3.0.md) | Oct 28, 2021 |
| Windows apps | Mozilla Thunderbird (en-US) | 91.0.3 | [Mozilla Thunderbird (en-US) 91.0.3](https://patchblox.com/security/data/fix/windows-app/mozilla.thunderbird-91.0.3.md) | Aug 24, 2021 |
| Linux | firefox Ubuntu 18.04.6 LTS (bionic) | 91.0.1+build1-0ubuntu0.18.04.1 | [firefox 91.0.1+build1-0ubuntu0.18.04.1](https://patchblox.com/security/data/fix/linux/ubuntu-bionic-firefox-91.0.1-build1-0ubuntu0.18.04.1.md) | Aug 17, 2021 |
| Linux | firefox Ubuntu 20.04 LTS (focal) | 91.0.1+build1-0ubuntu0.20.04.1 | [firefox 91.0.1+build1-0ubuntu0.20.04.1](https://patchblox.com/security/data/fix/linux/ubuntu-focal-firefox-91.0.1-build1-0ubuntu0.20.04.1.md) | Aug 17, 2021 |
| Linux | firefox-geckodriver Ubuntu 18.04.6 LTS (bionic) | 91.0.1+build1-0ubuntu0.18.04.1 | [firefox-geckodriver 91.0.1+build1-0ubuntu0.18.04.1](https://patchblox.com/security/data/fix/linux/ubuntu-bionic-firefox-geckodriver-91.0.1-build1-0ubuntu0.18.04.1.md) | Aug 17, 2021 |
| Linux | firefox-geckodriver Ubuntu 20.04 LTS (focal) | 91.0.1+build1-0ubuntu0.20.04.1 | [firefox-geckodriver 91.0.1+build1-0ubuntu0.20.04.1](https://patchblox.com/security/data/fix/linux/ubuntu-focal-firefox-geckodriver-91.0.1-build1-0ubuntu0.20.04.1.md) | Aug 17, 2021 |
| Linux | firefox-mozsymbols Ubuntu 18.04.6 LTS (bionic) | 91.0.1+build1-0ubuntu0.18.04.1 | [firefox-mozsymbols 91.0.1+build1-0ubuntu0.18.04.1](https://patchblox.com/security/data/fix/linux/ubuntu-bionic-firefox-mozsymbols-91.0.1-build1-0ubuntu0.18.04.1.md) | Aug 17, 2021 |
| Linux | firefox-mozsymbols Ubuntu 20.04 LTS (focal) | 91.0.1+build1-0ubuntu0.20.04.1 | [firefox-mozsymbols 91.0.1+build1-0ubuntu0.20.04.1](https://patchblox.com/security/data/fix/linux/ubuntu-focal-firefox-mozsymbols-91.0.1-build1-0ubuntu0.20.04.1.md) | Aug 17, 2021 |
| Linux | thunderbird Ubuntu 18.04.6 LTS (bionic) | 1:91.5.0+build1-0ubuntu0.18.04.1 | [thunderbird 1:91.5.0+build1-0ubuntu0.18.04.1](https://patchblox.com/security/data/fix/linux/ubuntu-bionic-thunderbird-1-91.5.0-build1-0ubuntu0.18.04.1.md) | Feb 16, 2023 |
| Linux | thunderbird Ubuntu 20.04 LTS (focal) | 1:91.5.0+build1-0ubuntu0.20.04.1 | [thunderbird 1:91.5.0+build1-0ubuntu0.20.04.1](https://patchblox.com/security/data/fix/linux/ubuntu-focal-thunderbird-1-91.5.0-build1-0ubuntu0.20.04.1.md) | Feb 16, 2023 |
| Linux | thunderbird-gnome-support Ubuntu 18.04.6 LTS (bionic) | 1:91.5.0+build1-0ubuntu0.18.04.1 | [thunderbird-gnome-support 1:91.5.0+build1-0ubuntu0.18.04.1](https://patchblox.com/security/data/fix/linux/ubuntu-bionic-thunderbird-gnome-support-1-91.5.0-build1-0ubuntu0.18.04.1.md) | Feb 16, 2023 |
| Linux | thunderbird-gnome-support Ubuntu 20.04 LTS (focal) | 1:91.5.0+build1-0ubuntu0.20.04.1 | [thunderbird-gnome-support 1:91.5.0+build1-0ubuntu0.20.04.1](https://patchblox.com/security/data/fix/linux/ubuntu-focal-thunderbird-gnome-support-1-91.5.0-build1-0ubuntu0.20.04.1.md) | Feb 16, 2023 |
| Linux | thunderbird-mozsymbols Ubuntu 18.04.6 LTS (bionic) | 1:91.5.0+build1-0ubuntu0.18.04.1 | [thunderbird-mozsymbols 1:91.5.0+build1-0ubuntu0.18.04.1](https://patchblox.com/security/data/fix/linux/ubuntu-bionic-thunderbird-mozsymbols-1-91.5.0-build1-0ubuntu0.18.04.1.md) | Feb 16, 2023 |
| Linux | thunderbird-mozsymbols Ubuntu 20.04 LTS (focal) | 1:91.5.0+build1-0ubuntu0.20.04.1 | [thunderbird-mozsymbols 1:91.5.0+build1-0ubuntu0.20.04.1](https://patchblox.com/security/data/fix/linux/ubuntu-focal-thunderbird-mozsymbols-1-91.5.0-build1-0ubuntu0.20.04.1.md) | Feb 16, 2023 |
| Linux | xul-ext-calendar-timezones Ubuntu 18.04.6 LTS (bionic) | 1:91.5.0+build1-0ubuntu0.18.04.1 | [xul-ext-calendar-timezones 1:91.5.0+build1-0ubuntu0.18.04.1](https://patchblox.com/security/data/fix/linux/ubuntu-bionic-xul-ext-calendar-timezones-1-91.5.0-build1-0ubuntu0.18.04.1.md) | Feb 16, 2023 |
| Linux | xul-ext-calendar-timezones Ubuntu 20.04 LTS (focal) | 1:91.5.0+build1-0ubuntu0.20.04.1 | [xul-ext-calendar-timezones 1:91.5.0+build1-0ubuntu0.20.04.1](https://patchblox.com/security/data/fix/linux/ubuntu-focal-xul-ext-calendar-timezones-1-91.5.0-build1-0ubuntu0.20.04.1.md) | Feb 16, 2023 |
| Linux | xul-ext-gdata-provider Ubuntu 18.04.6 LTS (bionic) | 1:91.5.0+build1-0ubuntu0.18.04.1 | [xul-ext-gdata-provider 1:91.5.0+build1-0ubuntu0.18.04.1](https://patchblox.com/security/data/fix/linux/ubuntu-bionic-xul-ext-gdata-provider-1-91.5.0-build1-0ubuntu0.18.04.1.md) | Feb 16, 2023 |
| Linux | xul-ext-gdata-provider Ubuntu 20.04 LTS (focal) | 1:91.5.0+build1-0ubuntu0.20.04.1 | [xul-ext-gdata-provider 1:91.5.0+build1-0ubuntu0.20.04.1](https://patchblox.com/security/data/fix/linux/ubuntu-focal-xul-ext-gdata-provider-1-91.5.0-build1-0ubuntu0.20.04.1.md) | Feb 16, 2023 |
| Linux | xul-ext-lightning Ubuntu 18.04.6 LTS (bionic) | 1:91.5.0+build1-0ubuntu0.18.04.1 | [xul-ext-lightning 1:91.5.0+build1-0ubuntu0.18.04.1](https://patchblox.com/security/data/fix/linux/ubuntu-bionic-xul-ext-lightning-1-91.5.0-build1-0ubuntu0.18.04.1.md) | Feb 16, 2023 |
| Linux | xul-ext-lightning Ubuntu 20.04 LTS (focal) | 1:91.5.0+build1-0ubuntu0.20.04.1 | [xul-ext-lightning 1:91.5.0+build1-0ubuntu0.20.04.1](https://patchblox.com/security/data/fix/linux/ubuntu-focal-xul-ext-lightning-1-91.5.0-build1-0ubuntu0.20.04.1.md) | Feb 16, 2023 |

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