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FirefoxWeb browser · Mozilla

CVE-2021-43539

HIGH · 8.8 CVSS v3.1 Published 2021-12-08
Fix available
A fix is available. Upgrade to 91.4.0 / 95.0 or later.
See remediation →
95/100
Remediation priority · Urgent
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
Failure to correctly record the location of live pointers across wasm instance calls resulted in a GC occurring within the call not tracing those live pointers. This could have led to a use-after-free causing a potentially exploitable crash. 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

Use-after-free vulnerability in SpiderMonkey's WebAssembly implementation where live pointers were not properly recorded across wasm instance calls. When garbage collection occurred during a wasm call, it failed to trace live pointers, leading to freed memory being accessed.

MitigationUpdate affected products to Firefox >= 95, Firefox ESR >= 91.4.0, or Thunderbird >= 91.4.0. No application-level code changes needed - this is a browser engine vulnerability.

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
Unchanged
Confidentiality
High
Integrity
High
Availability
High

CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H

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
    Check if Firefox or Thunderbird is installed. On Linux, use 'dpkg -l' or 'rpm -qa | grep -i firefox' or 'thunderbird'. On Windows, check Program Files for Mozilla folders. On macOS, check /Applications for Firefox or Thunderbird.
    Affected if Either Firefox or Thunderbird is installed
  2. Determine Firefox version
    For Firefox, type 'about:support' in the address bar or run 'firefox --version' from command line. Look for the 'Version' field.
    Affected if Version is below 95.0 for standard Firefox, or below 91.4.0 for Firefox ESR
  3. Determine Thunderbird version
    For Thunderbird, type 'about:support' in the address bar or run 'thunderbird --version' from command line. Look for the 'Version' field.
    Affected if Version is below 91.4.0 for Thunderbird
  4. Verify WebAssembly is enabled (if applicable)
    In Firefox, go to about:config and search for 'javascript.options.wasm'. In Thunderbird, the same config applies. Verify the option exists and is not explicitly disabled.
    Affected if WebAssembly is explicitly disabled - however, this is not a typical configuration as Wasm is enabled by default

The environment is affected if Mozilla Firefox is installed at a version below 95.0 (or below 91.4.0 for ESR), or Thunderbird is below 91.4.0, with WebAssembly enabled (default).

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 Firefox >= 95, Firefox ESR >= 91.4.0, or Thunderbird >= 91.4.0. No application-level code changes needed - this is a browser engine vulnerability.

Recommended fix High confidence

Firefox 95.0, Firefox ESR 91.4.0, or Thunderbird 91.4.0 (depending on product)

  1. Check current installed version of Firefox, Firefox ESR, or Thunderbird
  2. Back up bookmarks, passwords, and important browser data
  3. Download the latest stable release from Mozilla (Firefox 95+, Firefox ESR 91.4.0+, or Thunderbird 91.4.0+)
  4. Close all instances of the affected application
  5. Install the updated version using your operating system's package manager or the installer from Mozilla
  6. Restart the application and verify the update was successful
Caveat Upgrades are generally safe; some older extensions may need updates for compatibility with newer Firefox versions

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

Fix this in Firefox Scoped from the published advisory
  • Consultation2.0 h
  • Implementation40.0 h
  • Testing24.0 h
  • Review / QA8.0 h
74.0 hours of engineering $12,640
Get the upgrade done

An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $20,224.

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-2021-43539 in production — separate from our analysis above.

No notes yet

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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