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FirefoxWeb browser · Mozilla

CVE-2021-43536

MEDIUM · 6.5 CVSS v3.1 Published 2021-12-08
Fix available
A fix is available. Upgrade to 91.4.0 / 95.0 or later.
See remediation →
72/100
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
Under certain circumstances, asynchronous functions could have caused a navigation to fail but expose the target URL. 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 · moderate confidence

In Firefox and Thunderbird, certain asynchronous functions could cause a navigation attempt to fail but inadvertently expose the target URL. This represents an information disclosure vulnerability where URLs that should remain private (e.g., behind authentication or in restricted contexts) could be leaked through failed navigation handling.

MitigationDeploy the patched versions: Firefox 95+, Firefox ESR 91.4.0+, or Thunderbird 91.4.0+. In enterprise environments, use patch management to identify and update all affected browser installations.

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
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 checks

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

  1. Check if Mozilla Firefox is installed
    On Windows, look for Firefox in Program Files or check Start menu. On Linux, run 'which firefox' or check /usr/bin/firefox. On macOS, check /Applications/Firefox.app
    Affected if Firefox is present on the system
  2. Determine Firefox version
    In Firefox, go to Help > About Firefox, or run 'firefox --version' from command line. On Windows, right-click firefox.exe and view Properties > Details for version number
    Affected if Installed version is below 95.0 (for standard Firefox) or below 91.4.0 (for Firefox ESR)
  3. Identify if Firefox ESR is in use
    Check if the installed version includes 'ESR' designation by running 'firefox --version' or viewing the About dialog - ESR versions display 'Firefox Extended Support Release'
    Affected if Firefox ESR version is below 91.4.0
  4. Check if Mozilla Thunderbird is installed
    On Windows, look in Program Files. On Linux, run 'which thunderbird' or check /usr/bin/thunderbird. On macOS, check /Applications/Thunderbird.app
    Affected if Thunderbird is present on the system
  5. Determine Thunderbird version
    In Thunderbird, go to Help > About Thunderbird, or run 'thunderbird --version' from command line
    Affected if Installed version is below 91.4.0

A user is affected if they have any installed version of Mozilla Firefox below 95.0 (or below 91.4.0 for ESR) or Thunderbird below 91.4.0, as the information disclosure flaw exists in the browser's navigation handling code for all users of those versions.

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

Deploy the patched versions: Firefox 95+, Firefox ESR 91.4.0+, or Thunderbird 91.4.0+. In enterprise environments, use patch management to identify and update all affected browser installations.

Recommended fix High confidence

Firefox 95.0+, Firefox ESR 91.4.0+, Thunderbird 91.4.0+

  1. Check current installed version of Firefox, Firefox ESR, or Thunderbird
  2. For Firefox: Download and install version 95.0 or later from mozilla.org
  3. For Firefox ESR: Download and install version 91.4.0 or later from mozilla.org
  4. For Thunderbird: Download and install version 91.4.0 or later from mozilla.org
  5. Alternatively, use your system's package manager to update (e.g., apt-get update && apt-get upgrade for Debian-based systems)
  6. Restart the application after upgrade
  7. Verify the installed version matches or exceeds the fixed release
Caveat Standard upgrade considerations apply - ensure add-on compatibility with newer 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
  • 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-2021-43536 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