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FirefoxWeb browser · Mozilla

CVE-2021-43543

MEDIUM · 6.1 CVSS v3.1 Published 2021-12-08
Fix available
A fix is available. Upgrade to 91.4.0 / 95.0 or later.
See remediation →
68/100
Remediation priority · Elevated
Remotely reachable No privileges

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 · unedited
Documents loaded with the CSP sandbox directive could have escaped the sandbox's script restriction by embedding additional content. 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 confidence

The Content Security Policy (CSP) sandbox directive in Firefox, Firefox ESR, and Thunderbird had a vulnerability where documents loaded with the sandbox directive could escape the script restriction by embedding additional content, allowing potential script execution that should have been blocked.

MitigationUpdate affected products to Thunderbird 91.4.0+, Firefox ESR 91.4.0+, or Firefox 95+ to patch the CSP sandbox bypass.

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
Affected:< 95.0
Firefox EsrWeb browser
Affected:< 91.4.0
Debian LinuxOperating system
Affected:= 9.0= 10.0= 11.0
ThunderbirdApplication
Affected:< 91.4.0

CVSS 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
Changed
Confidentiality
Low
Integrity
Low
Availability
None

CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/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 checks

Work through these to decide whether this CVE applies to you.

  1. Identify installed Mozilla product
    Run 'which firefox' or 'which thunderbird' to determine which product is installed. On Linux, check package manager: 'dpkg -l | grep -i firefox' or 'dpkg -l | grep -i thunderbird'. On Windows, check Program Files for Mozilla Firefox or Thunderbird folders.
    Affected if Any of Firefox, Firefox ESR, or Thunderbird is installed
  2. Get Firefox version
    Run 'firefox --version' or go to Help > About Firefox to display the version number.
    Affected if Version is below 95.0 (e.g., 94.x or earlier)
  3. Get Firefox ESR version
    Run 'firefox --version' or go to Help > About Firefox. ESR versions typically show 'Mozilla Firefox ESR 91.x' in the about dialog.
    Affected if Version is below 91.4.0 (e.g., 91.3.x or earlier)
  4. Get Thunderbird version
    Run 'thunderbird --version' or go to Help > About Thunderbird to display the version number.
    Affected if Version is below 91.4.0 (e.g., 91.3.x or earlier)
  5. Check Debian system package version
    On Debian-based systems, run 'dpkg -l | grep -i firefox-esr' or 'apt-cache policy firefox-esr' to see installed and available versions. Check the release version with 'cat /etc/debian_version'.
    Affected if Running Debian 9, 10, or 11 with Firefox ESR package version below 91.4.0

The environment is affected if Firefox (any variant) is installed with version below 95.0, or Firefox ESR/Thunderbird is installed with version below 91.4.0.

Generated from the published advisory. Verify against your own configuration.

Check your environment

Paste your version and any relevant configuration and it will be compared against the affected criteria above. Do not include secrets or credentials.

AI-assisted, checked against the advisory. Informational, not a guarantee.

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 · scoped
Upgrade available Upgrade to 91.4.0 / 95.0 or later
Fixed in 91.4.095.0
Interim mitigation

Update affected products to Thunderbird 91.4.0+, Firefox ESR 91.4.0+, or Firefox 95+ to patch the CSP sandbox bypass.

Recommended fix High confidence

Firefox 95.0, Firefox ESR 91.4.0, or Thunderbird 91.4.0 (depending on product); Debian security updates

  1. For Firefox users: Upgrade to Firefox 95.0 or later
  2. For Firefox ESR users: Upgrade to Firefox ESR 91.4.0 or later
  3. For Thunderbird users: Upgrade to Thunderbird 91.4.0 or later
  4. For Debian systems: Apply the Debian security updates for this CVE (DSA-5000-1 and subsequent updates)
  5. Restart the affected application after applying updates
  6. Verify the version after update matches or exceeds the fixed release

Generated from the published advisory — verify against the referenced sources before acting.

Fix this in Firefox Scoped from the published advisory
  • Consultation2.0 h
  • Implementation4.0 h
  • Testing2.0 h
  • Review / QA1.0 h
9.0 hours of engineering $1,600
Get the upgrade done

An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $2,560.

Scan for this in your stack

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References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.

Primary sources

Practitioner notes

Contributed

Peer-ranked notes from engineers who’ve handled CVE-2021-43543 in production — separate from our analysis above.

No notes yet

Be the first to add a field note for this CVE — a mitigation you’ve verified, a version caveat, or a link to a working fix. Sign in above to contribute.

What this is

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.

What belongs here
  • 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