FirefoxWeb browser · Mozilla
CVE-2021-43540
Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.
NVD · uneditedWebExtensions 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.
Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.
dbcve analysis · high confidenceWebExtensions with the correct permissions could create and install ServiceWorkers for third-party websites that would persist after the extension was uninstalled, creating an unintended persistence mechanism for code on affected websites.
Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.
Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.
NVD · CPE data
FirefoxWeb browser
< 95.0CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.
From the vector- Attack vector
- Network
- Complexity
- Low
- Privileges
- None
- User interaction
- Required
- Scope
- Unchanged
- Confidentiality
- None
- Integrity
- High
- Availability
- None
CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:H/A:N
Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.
dbcve checksWork through these to decide whether this CVE applies to you.
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Check Firefox versionOpen Firefox, click the menu button (three lines), then Help > About Firefox. The version number will be displayed.Affected if The displayed version is below 95.0 (for example, 94.0, 93.0, etc.)
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Identify WebExtensions with broad permissionsNavigate to about:debugging in Firefox, click on 'This Firefox' under Extensions, and review the permissions listed for each installed extension.Affected if Any installed extension lists permissions that include host permissions (like <all_urls> or specific domains) and the ability to register ServiceWorkers.
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Inspect extension manifest filesLocate extension installation folders in the Firefox profile directory (typically in profiles/[profile_name]/extensions/), extract the .xpi file, and examine the manifest.json for 'permissions' and 'host_permissions' fields.Affected if The manifest.json contains permissions that allow the extension to register ServiceWorkers for arbitrary third-party websites.
The environment is affected if Firefox version is below 95.0 AND any installed WebExtension has permissions capable of registering ServiceWorkers for third-party sites, as this combination allows persisted code execution after extension removal.
Generated from the published advisory. Verify against your own configuration.
Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.
dbcve · scoped95.0
Upgrade Firefox to version 95 or later to remediate this vulnerability.
Firefox 95.0 or later
- 1. Back up any important browser data (bookmarks, passwords, etc.) if desired
- 2. Open Firefox browser
- 3. Navigate to Help > About Firefox (or Firefox > About Firefox on macOS)
- 4. The browser will automatically check for and download the latest version
- 5. If not automatically updated, download Firefox 95.0 or later from the official Mozilla website at www.mozilla.org
- 6. Restart the browser after updating
Generated from the published advisory — verify against the referenced sources before acting.
Fix this in Firefox
Scoped from the published advisory
- Consultation2.0 h
- Implementation2.0 h
- Testing1.0 h
- Review / QA1.0 h
An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $1,744.
Scan for this in your stack
Free · runs locallyCheck whether your project pulls in CVE-2021-43540 — or any other known-vulnerable package — straight from your lock files. Free and open source; it runs locally and uploads nothing.
References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.
Primary sourcesPractitioner notes
ContributedPeer-ranked notes from engineers who’ve handled CVE-2021-43540 in production — separate from our analysis above.
The advisory tells you what broke. It rarely tells you what actually worked. If you’ve dealt with this one, that detail is what the next engineer is searching for.
- The version that genuinely resolved it — not the one the vendor claimed
- A config change or rule that shut the vector down
- A gotcha in the upgrade path that cost you an afternoon
No notes yet
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A place for practitioners to share what actually worked: a mitigation you’ve tested, a configuration change, a version- or environment-specific caveat, or a link to a verified patch. The most useful notes rise to the top as peers upvote them, so the signal stays high.
- Verified mitigations, workarounds, and config changes
- Version or environment caveats, and links to real fixes
- No weaponised exploit code, or anything meant to cause harm
- No spam, self-promotion, credentials, or personal data