The Opportunistic Encryption feature of HTTP2 (RFC 8164) allows a connection to be transparently upgraded to TLS while retaining the visual properties of an HTTP connection, including being same-origin with unencrypted connections on port 80. However, if a second encrypted port on the same IP address (e.g. port 8443) did not opt-in to opportunistic encryption; a network attacker could forward a connection from the browser to port 443 to port 8443, causing the browser to treat the content of port 8443 as same-origin with HTTP. This was resolved by disabling the Opportunistic Encryption feature, which had low usage. This vulnerability affects Firefox < 94, Thunderbird < 91.3, and Firefox ESR < 91.3.
| Product | Vendor Advisory | Solution File | Added | Published |
|---|---|---|---|---|
| Alma_linux | — | Upgrade firefoxUpgrade thunderbird | May 4, 2022 | Dec 8, 2021 |
| Alpine Linux | — | Upgrade librewolfUpgrade firefoxUpgrade firefox-esrUpgrade thunderbird | Aug 22, 2024 | Dec 8, 2021 |
| Arch Linux | — | Upgrade to the latest version of Arch Linux | Jul 11, 2025 | Dec 8, 2021 |
| Centos_linux | — | Upgrade thunderbirdUpgrade firefox-debuginfoUpgrade firefoxUpgrade thunderbird-debugsourceUpgrade thunderbird-debuginfoUpgrade firefox-debugsource | Nov 9, 2021 | Nov 3, 2021 |
| Debian | — | Upgrade firefox-esrUpgrade thunderbird | Dec 30, 2021 | Dec 8, 2021 |
| Gentoo Linux | — | Upgrade mail-client/thunderbird-bin.Upgrade www-client/firefox-bin.Upgrade www-client/firefox.Upgrade mail-client/thunderbird. | Feb 22, 2022 | Dec 8, 2021 |
| Mfsa2021 48 | — | Upgrade to Mozilla Firefox version 94.0Upgrade to the latest version of Mozilla Firefox | Nov 3, 2021 | Nov 2, 2021 |
| Mfsa2021 49 | — | Upgrade to the latest version of Mozilla FirefoxUpgrade to Mozilla Firefox ESR version 91.3 | Nov 3, 2021 | Nov 2, 2021 |
| Mozilla Thunderbird | — | Upgrade to the latest version of Mozilla ThunderbirdUpgrade to Mozilla Thunderbird version 91.3 | Nov 4, 2021 | Nov 3, 2021 |
| Oracle Solaris | — | Upgrade web/data/firefox-bookmarks to version 91.3.0-11.4.40.0.1.107.1 on Solaris 11.4Upgrade web/browser/firefox to version 91.3.0-11.4.40.0.1.107.1 on Solaris 11.4Upgrade mail/thunderbird to version 91.3.0-11.4.40.0.1.107.1 on Solaris 11.4 | Dec 13, 2021 | Dec 8, 2021 |
| Oracle_linux | — | Upgrade firefoxUpgrade thunderbird | Nov 4, 2021 | Nov 2, 2021 |
| Redhat_linux | — | Upgrade firefoxUpgrade thunderbirdUpgrade thunderbird-debugsourceUpgrade firefox-debugsourceUpgrade firefox-debuginfoUpgrade thunderbird-debuginfoNo solution exists | Nov 9, 2021 | Nov 3, 2021 |
| Rocky_linux | — | Upgrade firefoxUpgrade thunderbird-debugsourceUpgrade thunderbirdUpgrade firefox-debuginfoUpgrade firefox-debugsourceUpgrade thunderbird-debuginfo | May 5, 2022 | Dec 8, 2021 |
| Suse | — | Upgrade MozillaFirefoxUpgrade MozillaFirefox-translations-commonUpgrade MozillaThunderbirdUpgrade MozillaFirefox-develUpgrade MozillaThunderbird-translations-otherUpgrade MozillaThunderbird-translations-commonUpgrade MozillaFirefox-translations-otherUpgrade mozillafirefox-branding-upstream | Nov 11, 2021 | Nov 3, 2021 |
| Ubuntu | — | Upgrade thunderbirdUpgrade firefox | Nov 4, 2021 | Nov 3, 2021 |
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