CVE-2021-38507

MEDIUMNVD 6.56.5—
EchelonGraph scoreMEDIUM confidence

Score 6.5 from GitHub Security Advisory published 2021-12-09. NVD baseline CVSS 6.5; sources differ by 0.0.

Triggered by: GitHub Security Advisory CVSS
Sources: epss, ghsa, nvd
Trending — 3 sources updated this week
6.5
EchelonGraph verdictMonitorLow exploitation likelihood right now — keep watching.
  • Lower severity and no public exploit yet
CISA-KEV: Not listedEPSS: 1%CVSS: 6.5Exploit: NoneExposed: 0

A fix is available — apply it.

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 v3
6.5
EG Score
6.5(medium)
EPSS
52.9%
KEV
Not listed

Published

December 8, 2021

Last Modified

November 21, 2024

References (20)

Vendor Advisories for CVE-2021-38507(2)

These vendors published their own advisory mentioning this CVE — often with vendor-specific remediation steps + affected product lists not in NVD.

Patch Availability(11)

Vendor / EcosystemFixed in / PatchReleasedSource
ubuntufirefox-locale-ast (94.0+build3-0ubuntu0.21.10.1) @ impish2026-05-26ubuntu
ubuntuthunderbird-locale-id (1:91.3.1+build1-0ubuntu0.21.10.1) @ impish2026-05-26ubuntu
ubuntuthunderbird-locale-id (1:91.5.0+build1-0ubuntu0.18.04.1) @ bionic2026-05-26ubuntu
redhatfirefox-0:91.3.0-1.el8_22021-11-10redhat
redhatfirefox-0:91.3.0-1.el8_12021-11-10redhat
redhatthunderbird-0:91.3.0-2.el8_42021-11-04redhat
redhatthunderbird-0:91.3.0-2.el8_12021-11-04redhat
redhatthunderbird-0:91.3.0-2.el8_22021-11-04redhat
redhatthunderbird-0:91.3.0-2.el7_92021-11-04redhat
redhatfirefox-0:91.3.0-1.el8_42021-11-03redhat
redhatfirefox-0:91.3.0-1.el7_92021-11-03redhat

Patches are aggregated from vendor advisories (Red Hat, Microsoft, Cisco, GitHub) and package ecosystems (OSV, GHSA). Multiple rows for the same upstream release have been deduplicated.

Weakness Classification(1)

MITRE Common Weakness Enumeration — the root-cause categories this CVE belongs to.

Additional Vendor Advisories

(9)

Vendors that published advisories for this CVE beyond the curated set above. Broader coverage but minimal per-row detail — click through for the original advisory.

Data Freshness Timeline

(refreshed 14× in last 7d / 46× in last 30d)

Each row is a source pipeline that fetched or updated this CVE on that date, with what changed. For example, "NVD update" means NVD published or revised its analysis for this CVE; "MITRE cvelistV5" means we ingested or refreshed it from the CNA feed. Most recent first.

Showing the most recent 100 of 102 total refreshes for this CVE.

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Frequently asked(5)

What is CVE-2021-38507?
CVE-2021-38507 is a medium vulnerability published on December 8, 2021. 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…
When was CVE-2021-38507 disclosed?
CVE-2021-38507 was first published in the National Vulnerability Database on December 8, 2021, with the most recent update on November 21, 2024. EchelonGraph re-ingests CVE updates from NVD on a 2-hour cycle, so this page reflects the latest published state.
Is CVE-2021-38507 actively exploited?
CVE-2021-38507 is not currently on CISA's Known Exploited Vulnerabilities catalog. FIRST EPSS estimates a 52.9% percentile likelihood of exploitation in the next 30 days — higher percentiles indicate greater predicted risk.
What is the CVSS score of CVE-2021-38507?
CVE-2021-38507 has a CVSS v3 base score of 6.5 (NVD).
How do I remediate CVE-2021-38507?
Patch to the fixed version published by the affected vendor. Where vendor advisories exist for CVE-2021-38507, EchelonGraph cross-links them in the Vendor Advisories panel below — those typically contain the canonical remediation steps, fixed version numbers, and any vendor-specific mitigations.

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