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Fencer Vulns
medium 6.5

CVE-2021-43540 json

WebExtensions with the correct permissions were able to create and install ServiceWorkers for third-party websites that would not have been…

published 2021-12-08 4 y ago modified 2026-06-17 4 mo ago also EUVD-2021-30469

cvss
6.5
version 3.1, NVD
cisa kev
Not listed
not in CISA's catalog
epss
0.9%
EPSS 0.9% chance of exploitation in 30 days, percentile 59%
public exploits
0
none known

Summary

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.

Patch / fix

  • Mozilla Firefox 95
  • firefox Ubuntu 18.04 LTS 95.0+build1-0ubuntu0.18.04.1
  • firefox Ubuntu 20.04 LTS 95.0+build1-0ubuntu0.20.04.1
  • firefox Ubuntu 22.04 LTS 95.0+build1-0ubuntu1

ubuntu advisories UBUNTU-CVE-2021-43540

Ubuntu security data © Canonical Ltd., from https://github.com/canonical/ubuntu-security-notices via OSV; changed: normalised to fixed package versions per release. CC BY-SA 4.0

  • firefox-esr openSUSE Tumbleweed 128.5.1-1.1
  • MozillaFirefox openSUSE Tumbleweed 95.0-1.1

opensuse advisories openSUSE-SU-2024:14572-1openSUSE-SU-2024:11669-1

  • firefox Alpine 3.23, Alpine 3.22, Alpine 3.21 +5 more 95.0-r0
  • librewolf Alpine 3.24, Alpine 3.23, Alpine 3.22 +1 more 95.0-r0

alpine advisories ALPINE-CVE-2021-43540

Alpine secdb © Alpine Linux, from https://secdb.alpinelinux.org via OSV; changed: normalised to fixed package versions per release. CC BY-SA 4.0

  • firefox Arch Linux 95.0-1

arch linux advisories AVG-2606

Exploit prediction

0.89% chance of exploitation in the next 30 days, higher than 59% of all scored CVEs (FIRST EPSS, 2026-11-08).

CVSS scores

assigned by version score vector
NVD 3.1 6.5 CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:H/A:N
NVD 2.0 4.3 AV:N/AC:M/Au:N/C:N/I:P/A:N

Affected products

  • Mozilla Firefox

References

Sources

Fencer Vulns keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.