{"id":"CVE-2021-38507","details":"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 \u003c 94, Thunderbird \u003c 91.3, and Firefox ESR \u003c 91.3.","modified":"2026-03-13T05:06:45.474180Z","published":"2021-12-08T22:15:08.947Z","related":["ALSA-2021:4123","ALSA-2021:4130","MGASA-2021-0505","MGASA-2021-0506","SUSE-SU-2021:3651-1","SUSE-SU-2021:3721-1","SUSE-SU-2021:3745-1","SUSE-SU-2021:4150-1","openSUSE-SU-2021:1635-1","openSUSE-SU-2021:3745-1","openSUSE-SU-2021:4150-1","openSUSE-SU-2024:11607-1","openSUSE-SU-2024:11614-1","openSUSE-SU-2024:14572-1"],"references":[{"type":"ADVISORY","url":"https://lists.debian.org/debian-lts-announce/2021/12/msg00030.html"},{"type":"ADVISORY","url":"https://lists.debian.org/debian-lts-announce/2022/01/msg00001.html"},{"type":"ADVISORY","url":"https://security.gentoo.org/glsa/202202-03"},{"type":"ADVISORY","url":"https://security.gentoo.org/glsa/202208-14"},{"type":"ADVISORY","url":"https://www.debian.org/security/2021/dsa-5026"},{"type":"ADVISORY","url":"https://www.debian.org/security/2022/dsa-5034"},{"type":"ADVISORY","url":"https://www.mozilla.org/security/advisories/mfsa2021-48/"},{"type":"ADVISORY","url":"https://www.mozilla.org/security/advisories/mfsa2021-49/"},{"type":"ADVISORY","url":"https://www.mozilla.org/security/advisories/mfsa2021-50/"},{"type":"REPORT","url":"https://bugzilla.mozilla.org/show_bug.cgi?id=1730935"}],"affected":[{"database_specific":{"unresolved_ranges":[{"events":[{"introduced":"0"},{"fixed":"94.0"}]},{"events":[{"introduced":"0"},{"fixed":"91.3.0"}]},{"events":[{"introduced":"0"},{"fixed":"91.3.0"}]},{"events":[{"introduced":"0"},{"last_affected":"9.0"}]},{"events":[{"introduced":"0"},{"last_affected":"10.0"}]},{"events":[{"introduced":"0"},{"last_affected":"11.0"}]}],"source":"https://storage.googleapis.com/osv-test-cve-osv-conversion/osv-output/CVE-2021-38507.json"}}],"schema_version":"1.7.5","severity":[{"type":"CVSS_V3","score":"CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:H/A:N"}]}