CVE-2021-38507

Name: CVE-2021-38507
Creator: NVD / NIST
Published: 2021-12-08T22:15:08.947+00:00
License: https://creativecommons.org/publicdomain/zero/1.0/

MEDIUMCVSS 6.5/10EPSS 0.80%

Last modified Jun 17, 2026

CVE-2021-38507 is a medium-severity vulnerability rated 6.5/10 on the CVSS scale. 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. EPSS estimates a 0.80% chance of exploitation in the next 30 days.

Description

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.

Metrics

CVSS 3.1

6.5/10

CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:H/A:N

EPSS Probability

0.80%

52.1th percentile

Probability of exploitation in the next 30 days. Learn more

Weakness Enumeration

CWE-346

Affected Software

Vendor	Product	Versions
Mozilla	Firefox	< 94.0
Mozilla	Firefox Esr	< 91.3.0
Mozilla	Thunderbird	< 91.3.0
Debian	Debian Linux	9.0
Debian	Debian Linux	10.0
Debian	Debian Linux	11.0

References

https://bugzilla.mozilla.org/show_bug.cgi?id=1730935Issue Tracking, Permissions Required, Vendor Advisory
https://lists.debian.org/debian-lts-announce/2021/12/msg00030.htmlMailing List, Third Party Advisory
https://lists.debian.org/debian-lts-announce/2022/01/msg00001.htmlMailing List, Third Party Advisory
https://security.gentoo.org/glsa/202202-03Third Party Advisory
https://security.gentoo.org/glsa/202208-14Third Party Advisory
https://www.debian.org/security/2021/dsa-5026Third Party Advisory
https://www.debian.org/security/2022/dsa-5034Third Party Advisory
https://www.mozilla.org/security/advisories/mfsa2021-48/Vendor Advisory
https://www.mozilla.org/security/advisories/mfsa2021-49/Vendor Advisory
https://www.mozilla.org/security/advisories/mfsa2021-50/Vendor Advisory
https://bugzilla.mozilla.org/show_bug.cgi?id=1730935Issue Tracking, Permissions Required, Vendor Advisory
https://lists.debian.org/debian-lts-announce/2021/12/msg00030.htmlMailing List, Third Party Advisory
https://lists.debian.org/debian-lts-announce/2022/01/msg00001.htmlMailing List, Third Party Advisory
https://security.gentoo.org/glsa/202202-03Third Party Advisory
https://security.gentoo.org/glsa/202208-14Third Party Advisory
https://www.debian.org/security/2021/dsa-5026Third Party Advisory
https://www.debian.org/security/2022/dsa-5034Third Party Advisory
https://www.mozilla.org/security/advisories/mfsa2021-48/Vendor Advisory
https://www.mozilla.org/security/advisories/mfsa2021-49/Vendor Advisory
https://www.mozilla.org/security/advisories/mfsa2021-50/Vendor Advisory

Timeline

Published: Dec 8, 2021
Last Modified: Jun 17, 2026
Status: Modified

Frequently Asked Questions

What is CVE-2021-38507?

How severe is CVE-2021-38507?

CVE-2021-38507 has a CVSS score of 6.5/10 (MEDIUM severity). The EPSS model estimates a 0.80% probability of exploitation in the next 30 days.

How do I fix CVE-2021-38507?

Check the vendor references and advisories linked above for patched versions and mitigation guidance. You can also run a Strix scan to test if your systems are affected.

Are you affected by CVE-2021-38507?

Run a free Strix scan to check your systems for this vulnerability.

Scan your code now

Source: NVD / NIST