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.
CVSS Details
- CVSS 3.1 Base Score: 6.5
- CVSS 3.1 Vector: (CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:H/A:N)
Covered by Rapid7
| Product | Vendor Advisory | Solution File | Added | Published |
|---|---|---|---|---|
| Alma_linux | — | Upgrade firefoxUpgrade thunderbird | May 4, 2022 | Dec 8, 2021 |
| Alpine Linux | — | Upgrade thunderbirdUpgrade firefoxUpgrade firefox-esrUpgrade librewolf | 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 firefoxUpgrade firefox-debuginfoUpgrade thunderbird-debugsourceUpgrade thunderbird-debuginfoUpgrade firefox-debugsource | Nov 9, 2021 | Nov 3, 2021 |
| Debian | — | Upgrade thunderbirdUpgrade firefox-esr | 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 94.0 | Nov 3, 2021 | Nov 2, 2021 |
| Mfsa2021 49 | — | Upgrade to Mozilla Firefox ESR version 91.3 | Nov 3, 2021 | Nov 2, 2021 |
| Mozilla Thunderbird | — | Upgrade to Mozilla Thunderbird version 91.3 | Nov 4, 2021 | Nov 3, 2021 |
| Oracle Solaris | — | Upgrade mail/thunderbird 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 web/data/firefox-bookmarks 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 thunderbird-debugsourceUpgrade thunderbirdUpgrade firefoxNo solution existsUpgrade firefox-debugsourceUpgrade thunderbird-debuginfoUpgrade firefox-debuginfo | Nov 9, 2021 | Nov 3, 2021 |
| Rocky_linux | — | Upgrade firefox-debuginfoUpgrade firefox-debugsourceUpgrade thunderbird-debuginfoUpgrade thunderbird-debugsourceUpgrade firefoxUpgrade thunderbird | May 5, 2022 | Dec 8, 2021 |
| Suse | — | Upgrade mozillathunderbird-translations-otherUpgrade mozillafirefox-translations-commonUpgrade mozillathunderbirdUpgrade mozillafirefox-translations-otherUpgrade mozillathunderbird-translations-commonUpgrade mozillafirefox-develUpgrade mozillafirefoxUpgrade mozillafirefox-branding-upstream | Nov 11, 2021 | Nov 3, 2021 |
| Ubuntu | — | Upgrade thunderbirdUpgrade firefox | Nov 4, 2021 | Nov 3, 2021 |
Prioritise with Active Threat Intelligence
With curated Threat Intelligence, you can see which vulnerabilities truly put you at risk, prioritize what matters most, and act before attackers do.
Explore Intelligence Hub