CVE-2021-23969

Name: CVE-2021-23969
Creator: NVD / NIST
Published: 2021-02-26T02:15:12.947+00:00
License: https://creativecommons.org/publicdomain/zero/1.0/

MEDIUMCVSS 4.3/10EPSS 1.22%

Last modified Jun 17, 2026

CVE-2021-23969 is a medium-severity vulnerability rated 4.3/10 on the CVSS scale. As specified in the W3C Content Security Policy draft, when creating a violation report, "User agents need to ensure that the source file is the URL requested by the page, pre-redirects. If that’s not possible, user agents need to strip the URL down to an origin to avoid unintentional leakage." Under certain types of redirects, Firefox incorrectly set the source file to be the destination of the redirects. EPSS estimates a 1.22% chance of exploitation in the next 30 days.

Description

As specified in the W3C Content Security Policy draft, when creating a violation report, "User agents need to ensure that the source file is the URL requested by the page, pre-redirects. If that’s not possible, user agents need to strip the URL down to an origin to avoid unintentional leakage." Under certain types of redirects, Firefox incorrectly set the source file to be the destination of the redirects. This was fixed to be the redirect destination's origin. This vulnerability affects Firefox < 86, Thunderbird < 78.8, and Firefox ESR < 78.8.

Metrics

CVSS 3.1

4.3/10

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

EPSS Probability

1.22%

64.9th percentile

Probability of exploitation in the next 30 days. Learn more

Affected Software

Vendor	Product	Versions
Mozilla	Firefox	< 86.0
Mozilla	Firefox Esr	< 78.8
Mozilla	Thunderbird	< 78.8
Debian	Debian Linux	9.0
Debian	Debian Linux	10.0

References

https://bugzilla.mozilla.org/show_bug.cgi?id=1542194Issue Tracking, Permissions Required, Vendor Advisory
https://lists.debian.org/debian-lts-announce/2021/03/msg00000.htmlMailing List, Third Party Advisory
https://security.gentoo.org/glsa/202104-09Third Party Advisory
https://security.gentoo.org/glsa/202104-10Third Party Advisory
https://www.debian.org/security/2021/dsa-4866Third Party Advisory
https://www.mozilla.org/security/advisories/mfsa2021-07/Release Notes, Vendor Advisory
https://www.mozilla.org/security/advisories/mfsa2021-08/Release Notes, Vendor Advisory
https://www.mozilla.org/security/advisories/mfsa2021-09/Release Notes, Vendor Advisory
https://bugzilla.mozilla.org/show_bug.cgi?id=1542194Issue Tracking, Permissions Required, Vendor Advisory
https://lists.debian.org/debian-lts-announce/2021/03/msg00000.htmlMailing List, Third Party Advisory
https://security.gentoo.org/glsa/202104-09Third Party Advisory
https://security.gentoo.org/glsa/202104-10Third Party Advisory
https://www.debian.org/security/2021/dsa-4866Third Party Advisory
https://www.mozilla.org/security/advisories/mfsa2021-07/Release Notes, Vendor Advisory
https://www.mozilla.org/security/advisories/mfsa2021-08/Release Notes, Vendor Advisory
https://www.mozilla.org/security/advisories/mfsa2021-09/Release Notes, Vendor Advisory

Timeline

Published: Feb 26, 2021
Last Modified: Jun 17, 2026
Status: Modified

Frequently Asked Questions

What is CVE-2021-23969?

How severe is CVE-2021-23969?

CVE-2021-23969 has a CVSS score of 4.3/10 (MEDIUM severity). The EPSS model estimates a 1.22% probability of exploitation in the next 30 days.

How do I fix CVE-2021-23969?

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-2021-23969?

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

Scan your code now

Source: NVD / NIST