FirefoxWeb browser · Mozilla
CVE-2021-43542
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 · uneditedUsing XMLHttpRequest, an attacker could have identified installed applications by probing error messages for loading external protocols. This vulnerability affects Thunderbird < 91.4.0, Firefox ESR < 91.4.0, and 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 confidenceXMLHttpRequest can be abused to probe for installed applications by triggering error messages when attempting to load external protocols. Attackers can enumerate software on a victim's system by analyzing these protocol-handling error responses.
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
< 95.0
Firefox EsrWeb browser
< 91.4.0
Debian LinuxOperating system
= 9.0= 10.0= 11.0< 91.4.0CVSS 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
- High
- Integrity
- None
- Availability
- None
CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:N/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.
-
Check Mozilla Firefox versionIn Firefox, type 'about:support' in the address bar or go to Help > About Firefox to view the installed version numberAffected if Version is less than 95.0 (for example, 94.x or earlier)
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Check Mozilla Firefox ESR versionIn Firefox ESR, type 'about:support' in the address bar or go to Help > About Firefox to view the installed version numberAffected if Version is less than 91.4.0 (for example, 91.3.x or earlier)
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Check Mozilla Thunderbird versionIn Thunderbird, go to Help > About Thunderbird to view the installed version numberAffected if Version is less than 91.4.0 (for example, 91.3.x or earlier)
You are affected if you have Firefox below 95.0, Firefox ESR below 91.4.0, or Thunderbird below 91.4.0 installed, as the XMLHttpRequest protocol-handling vulnerability exists in those unpatched versions.
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 · scoped91.4.095.0
Update affected browsers (Firefox < 95, Firefox ESR < 91.4.0, Thunderbird < 91.4.0) to patched versions. This is a client-side vulnerability requiring browser updates rather than network-level remediation.
Firefox 95.0+, Firefox ESR 91.4.0+, Thunderbird 91.4.0+
- Upgrade Firefox to version 95.0 or later
- Upgrade Firefox ESR to version 91.4.0 or later
- Upgrade Thunderbird to version 91.4.0 or later
- On Debian systems, ensure security updates are applied: apt update && apt upgrade (or apt dist-upgrade) to receive the Mozilla security patches
Generated from the published advisory — verify against the referenced sources before acting.
Fix this in Firefox
Scoped from the published advisory
- Consultation2.0 h
- Implementation1.0 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 $1,456.
Scan for this in your stack
Free · runs locallyCheck whether your project pulls in CVE-2021-43542 — 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-2021-43542 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