CVE-2021-43540Public exploitNetwork attack vector

Persistent site-origin code via filtered ServiceWorker responses

Published Dec 8, 2021 · Updated Aug 4, 2024

Improper permission enforcement in Mozilla Firefox before 95 allows remote attackers to install persistent third-party ServiceWorkers. A permitted WebExtension can use blocking webRequest listeners and filterResponseData to replace a third-party ServiceWorker response body with extension-supplied JavaScript. A user must install or load the extension and grant the required permissions; removing it leaves the injected ServiceWorker associated with the targeted site's origin.

CVSS severity6.5
Medium
EPSS probability0.86%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Improper permission enforcement in Mozilla Firefox before 95 allows remote attackers to install persistent third-party ServiceWorkers. A permitted WebExtension can use blocking webRequest listeners and filterResponseData to replace a third-party ServiceWorker response body with extension-supplied JavaScript. A user must install or load the extension and grant the required permissions; removing it leaves the injected ServiceWorker associated with the targeted site's origin.

The record

CVE
CVE-2021-43540
Published
Dec 8, 2021
Updated
Aug 4, 2024
Vendor
Mozilla
Product
Firefox
Classifications
CWE-732, T1176.001
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Dec 8, 2021CVE publishedPublication date reported by the CVE source.
  2. Aug 4, 2024Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=unspecified <95

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Mozilla · Firefoxversion=unspecified <95

Published CVSS scores

4.3NIST National Vulnerability DatabaseAV:N/AC:M/Au:N/C:N/I:P/A:N
6.5NIST National Vulnerability DatabaseCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:H/A:N

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 9, 2026Sep 15, 2026
Latest reporting daySep 15, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitPublished

Weakness, pattern, technique

CWE-732Incorrect Permission Assignment for Critical Resource
T1176.001Browser Extensions

Public exploit references

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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