CVE-2021-43540

MEDIUMNVD 6.56.5—

6.5

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.

CVSS v3: 6.5
EG Score: 6.5(medium)
EPSS: 58.6%
KEV: Not listed

Published

December 8, 2021

Last Modified

November 21, 2024

References (6)

security@mozillahttps://bugzilla.mozilla.org/show_bug.cgi?id=1636629
security@mozillahttps://security.gentoo.org/glsa/202202-03
security@mozillahttps://www.mozilla.org/security/advisories/mfsa2021-52/
af854a3a-2127-422b-91ae-364da2661108https://bugzilla.mozilla.org/show_bug.cgi?id=1636629
af854a3a-2127-422b-91ae-364da2661108https://security.gentoo.org/glsa/202202-03
af854a3a-2127-422b-91ae-364da2661108https://www.mozilla.org/security/advisories/mfsa2021-52/

Patch Availability(1)

Vendor / Ecosystem	Fixed in / Patch	Released	Source
ubuntu	firefox-locale-ast (95.0+build1-0ubuntu0.21.10.1) @ impish	2026-05-26	ubuntu

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.

CWE-732Incorrect Permission Assignment for Critical Resource

All Vendor Advisories

(1)

Every vendor that published an advisory referencing this CVE — pulled from our cve_vendor_advisories aggregation. Click any row for the vendor's original advisory page.

UbuntuUSN-5186-1MEDIUM
Firefox vulnerabilities

Data Freshness Timeline

(refreshed 11× in last 7d / 20× in last 30d)

Every time one of our enrichment pipelines (NVD, MITRE cvelistV5, EPSS, CISA KEV, GHSA, OSV, vendor advisories) ran against this CVE. Most recent first.

2026-06-04 13:10 UTCepss_batch
2026-06-04 13:10 UTCepss_batch
2026-06-02 20:11 UTCepss_batch
2026-06-02 20:11 UTCepss_batch
2026-06-01 13:50 UTCepss_batch
2026-06-01 13:50 UTCepss_batch
2026-05-31 22:29 UTCepss_batch
2026-05-31 22:29 UTCepss_batch
2026-05-31 00:15 UTCepss_batch
2026-05-31 00:15 UTCepss_batch
2026-05-29 13:42 UTCepss_batch
2026-05-28 13:43 UTCepss_batch
2026-05-28 13:43 UTCepss_batch
2026-05-27 13:39 UTCepss_batch
2026-05-27 13:39 UTCepss_batch
2026-05-26 13:42 UTCepss_batch
2026-05-26 13:42 UTCepss_batch
2026-05-26 07:17 UTCepss_batch
2026-05-26 07:17 UTCepss_batch
2026-05-26 07:17 UTCepss_batch

Frequently asked(5)

What is CVE-2021-43540?

CVE-2021-43540 is a medium vulnerability published on December 8, 2021. 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.

When was CVE-2021-43540 disclosed?

CVE-2021-43540 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-2021-43540 actively exploited?

CVE-2021-43540 is not currently on CISA's Known Exploited Vulnerabilities catalog. FIRST EPSS estimates a 58.6% percentile likelihood of exploitation in the next 30 days — higher percentiles indicate greater predicted risk.

What is the CVSS score of CVE-2021-43540?

CVE-2021-43540 has a CVSS v3 base score of 6.5 (NVD).

How do I remediate CVE-2021-43540?

Patch to the fixed version published by the affected vendor. Where vendor advisories exist for CVE-2021-43540, 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.

Dependency Blast Radius

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CVE-2021-43540

Published

Last Modified

References (6)

Patch Availability(1)

Weakness Classification(1)

All Vendor Advisories

Data Freshness Timeline

Frequently asked(5)

Dependency Blast Radius

Is Your Infrastructure Affected by CVE-2021-43540?

Same vendor

Same CWE

CVE-2021-43540

📅 Published

🔄 Last Modified

🔗 References (6)

Patch Availability(1)

Weakness Classification(1)

All Vendor Advisories

Data Freshness Timeline

❓ Frequently asked(5)

Dependency Blast Radius

Is Your Infrastructure Affected by CVE-2021-43540?

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References (6)

Frequently asked(5)

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