{"id":"UBUNTU-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":"2026-01-30T02:33:04.270569Z","published":"2021-11-03T00:00:00Z","related":["USN-5131-1","USN-5152-1","USN-5248-1"],"upstream":["CVE-2021-38507"],"references":[{"type":"REPORT","url":"https://ubuntu.com/security/CVE-2021-38507"},{"type":"REPORT","url":"https://www.mozilla.org/en-US/security/advisories/mfsa2021-48/#CVE-2021-38507"},{"type":"REPORT","url":"https://www.mozilla.org/en-US/security/advisories/mfsa2021-49/#CVE-2021-38507"},{"type":"ADVISORY","url":"https://ubuntu.com/security/notices/USN-5131-1"},{"type":"ADVISORY","url":"https://ubuntu.com/security/notices/USN-5152-1"},{"type":"ADVISORY","url":"https://ubuntu.com/security/notices/USN-5248-1"},{"type":"REPORT","url":"https://www.cve.org/CVERecord?id=CVE-2021-38507"}],"affected":[{"package":{"name":"firefox","ecosystem":"Ubuntu:18.04:LTS","purl":"pkg:deb/ubuntu/firefox@94.0+build3-0ubuntu0.18.04.1?arch=source&distro=bionic"},"ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"fixed":"94.0+build3-0ubuntu0.18.04.1"}]}],"versions":["56.0+build6-0ubuntu1","57.0.1+build2-0ubuntu1","59.0.1+build1-0ubuntu1","59.0.2+build1-0ubuntu1","60.0+build2-0ubuntu1","60.0.1+build2-0ubuntu0.18.04.1","60.0.2+build1-0ubuntu0.18.04.1","61.0+build3-0ubuntu0.18.04.1","61.0.1+build1-0ubuntu0.18.04.1","62.0+build2-0ubuntu0.18.04.3","62.0+build2-0ubuntu0.18.04.4","62.0+build2-0ubuntu0.18.04.5","62.0.3+build1-0ubuntu0.18.04.1","63.0+build2-0ubuntu0.18.04.2","63.0.3+build1-0ubuntu0.18.04.1","64.0+build3-0ubuntu0.18.04.1","65.0+build2-0ubuntu0.18.04.1","65.0.1+build2-0ubuntu0.18.04.1","66.0+build3-0ubuntu0.18.04.1","66.0.1+build1-0ubuntu0.18.04.1","66.0.2+build1-0ubuntu0.18.04.1","66.0.3+build1-0ubuntu0.18.04.1","66.0.4+build3-0ubuntu0.18.04.1","66.0.5+build1-0ubuntu0.18.04.1","67.0+build2-0ubuntu0.18.04.1","67.0.1+build1-0ubuntu0.18.04.1","67.0.2+build2-0ubuntu0.18.04.1","67.0.3+build1-0ubuntu0.18.04.1","67.0.4+build1-0ubuntu0.18.04.1","68.0+build3-0ubuntu0.18.04.1","68.0.1+build1-0ubuntu0.18.04.1","68.0.2+build1-0ubuntu0.18.04.1","69.0+build2-0ubuntu0.18.04.1","69.0.1+build1-0ubuntu0.18.04.1","69.0.2+build1-0ubuntu0.18.04.1","70.0+build2-0ubuntu0.18.04.1","70.0.1+build1-0ubuntu0.18.04.1","71.0+build5-0ubuntu0.18.04.1","72.0.1+build1-0ubuntu0.18.04.1","72.0.2+build1-0ubuntu0.18.04.1","73.0+build3-0ubuntu0.18.04.1","73.0.1+build1-0ubuntu0.18.04.1","74.0+build3-0ubuntu0.18.04.1","74.0.1+build1-0ubuntu0.18.04.1","75.0+build3-0ubuntu0.18.04.1","76.0+build2-0ubuntu0.18.04.1","76.0.1+build1-0ubuntu0.18.04.1","77.0.1+build1-0ubuntu0.18.04.1","78.0.1+build1-0ubuntu0.18.04.1","78.0.2+build2-0ubuntu0.18.04.1","79.0+build1-0ubuntu0.18.04.1","80.0+build2-0ubuntu0.18.04.1","80.0.1+build1-0ubuntu0.18.04.1","81.0+build2-0ubuntu0.18.04.1","81.0.2+build1-0ubuntu0.18.04.1","82.0+build2-0ubuntu0.18.04.1","82.0.2+build1-0ubuntu0.18.04.1","82.0.3+build1-0ubuntu0.18.04.1","83.0+build2-0ubuntu0.18.04.2","84.0+build3-0ubuntu0.18.04.1","84.0.1+build1-0ubuntu0.18.04.1","84.0.2+build1-0ubuntu0.18.04.1","85.0+build1-0ubuntu0.18.04.1","85.0.1+build1-0ubuntu0.18.04.1","86.0+build3-0ubuntu0.18.04.1","86.0.1+build1-0ubuntu0.18.04.1","87.0+build3-0ubuntu0.18.04.2","88.0+build2-0ubuntu0.18.04.2","88.0.1+build1-0ubuntu0.18.04.2","89.0+build2-0ubuntu0.18.04.2","89.0.1+build1-0ubuntu0.18.04.1","89.0.2+build1-0ubuntu0.18.04.1","90.0+build1-0ubuntu0.18.04.1","90.0.2+build1-0ubuntu0.18.04.1","91.0+build2-0ubuntu0.18.04.1","91.0.1+build1-0ubuntu0.18.04.1","91.0.2+build1-0ubuntu0.18.04.1","92.0+build3-0ubuntu0.18.04.1","93.0+build1-0ubuntu0.18.04.1"],"ecosystem_specific":{"binaries":[{"binary_name":"firefox","binary_version":"94.0+build3-0ubuntu0.18.04.1"},{"binary_name":"firefox-dev","binary_version":"94.0+build3-0ubuntu0.18.04.1"},{"binary_name":"firefox-geckodriver","binary_version":"94.0+build3-0ubuntu0.18.04.1"},{"binary_name":"firefox-mozsymbols","binary_version":"94.0+build3-0ubuntu0.18.04.1"}],"availability":"No 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