CVE-2021-43540

Name: CVE-2021-43540
Creator: NVD / NIST
Published: 2021-12-08T22:15:09.787+00:00
License: https://creativecommons.org/publicdomain/zero/1.0/

MEDIUMCVSS 6.5/10EPSS 0.86%

Last modified Jun 17, 2026

CVE-2021-43540 is a medium-severity vulnerability rated 6.5/10 on the CVSS scale. WebExtensions with the correct permissions were able to create and install ServiceWorkers for third-party websites that would not have been uninstalled with the extension. This vulnerability affects Firefox < 95.. EPSS estimates a 0.86% chance of exploitation in the next 30 days.

Description

WebExtensions with the correct permissions were able to create and install ServiceWorkers for third-party websites that would not have been uninstalled with the extension. This vulnerability affects Firefox < 95.

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

0.86%

53.9th percentile

Probability of exploitation in the next 30 days. Learn more

Affected Software

Vendor	Product	Versions
Mozilla	Firefox	< 95.0

References

https://bugzilla.mozilla.org/show_bug.cgi?id=1636629Issue Tracking, Permissions Required, Vendor Advisory
https://security.gentoo.org/glsa/202202-03Third Party Advisory
https://www.mozilla.org/security/advisories/mfsa2021-52/Vendor Advisory
https://bugzilla.mozilla.org/show_bug.cgi?id=1636629Issue Tracking, Permissions Required, Vendor Advisory
https://security.gentoo.org/glsa/202202-03Third Party Advisory
https://www.mozilla.org/security/advisories/mfsa2021-52/Vendor Advisory

Timeline

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

Frequently Asked Questions

What is CVE-2021-43540?

How severe is CVE-2021-43540?

CVE-2021-43540 has a CVSS score of 6.5/10 (MEDIUM severity). The EPSS model estimates a 0.86% probability of exploitation in the next 30 days.

How do I fix CVE-2021-43540?

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-2021-43540?

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

Scan your code now

Source: NVD / NIST