FirefoxWeb browser · Mozilla
CVE-2021-43541
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 · uneditedWhen invoking protocol handlers for external protocols, a supplied parameter URL containing spaces was not properly escaped. 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 confidenceWhen invoking protocol handlers for external protocols (e.g., mailto:, tel:), the browser fails to properly escape URLs containing spaces before passing them to external applications. This could allow injection of arbitrary parameters into external handler commands.
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
- 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.
-
Check Firefox versionNavigate to about:Firefox or run 'firefox --version' from command lineAffected if Installed version is earlier than 95.0
-
Check Firefox ESR versionNavigate to about:Firefox or run 'firefox --version' from command line. ESR versions include 'ESR' in the version string.Affected if Installed version is earlier than 91.4.0 and is an ESR release
-
Check Thunderbird versionNavigate to Help > About Thunderbird or run 'thunderbird --version' from command lineAffected if Installed version is earlier than 91.4.0
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Check Debian-packaged Firefox versionRun 'dpkg -l firefox-esr' or 'apt-cache policy firefox-esr' on Debian-based systemsAffected if Installed version is earlier than 91.4.0+deb10u1 (for Debian 10) or 91.4.0+deb11u1 (for Debian 11)
A user is affected if they run Firefox (any channel) below 95.0, Firefox ESR below 91.4.0, or Thunderbird below 91.4.0 and actively use external protocol handlers such as mailto: or tel: links.
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
Upgrade to Firefox 95, Firefox ESR 91.4.0, or Thunderbird 91.4.0 or later to obtain the patched version.
Firefox 95.0+, Firefox ESR 91.4.0+, or Thunderbird 91.4.0+
- Check the current installed version of Firefox, Firefox ESR, or Thunderbird
- For Debian systems: run 'apt update && apt upgrade' to apply security updates
- Alternatively, manually download and install the fixed version: Firefox 95.0 or later, Firefox ESR 91.4.0 or later, Thunderbird 91.4.0 or later
- Restart the application after upgrade
Generated from the published advisory — verify against the referenced sources before acting.
Fix this in Firefox
Scoped from the published advisory
- Consultation1.0 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 $1,200.
Scan for this in your stack
Free · runs locallyCheck whether your project pulls in CVE-2021-43541 — 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-2021-43541 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