dbcve.org
FirefoxWeb browser · Mozilla

CVE-2021-23960

HIGH · 8.8 CVSS v3.1 Published 2021-02-26
Fix available
A fix is available. Upgrade to 78.7 / 85.0 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
Performing garbage collection on re-declared JavaScript variables resulted in a user-after-poison, and a potentially exploitable crash. This vulnerability affects Firefox < 85, Thunderbird < 78.7, and Firefox ESR < 78.7.

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

Memory safety vulnerability in Firefox, Thunderbird, and Firefox ESR where garbage collection performed on re-declared JavaScript variables results in a user-after-poison condition, leading to a potentially exploitable crash. The vulnerability stems from incorrect handling of variable redeclaration during JavaScript garbage collection cycles.

MitigationUpgrade affected products to patched versions: Firefox 85 or later, Thunderbird 78.7 or later, or Firefox ESR 78.7 or later.

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:< 85.0
Firefox EsrWeb browser
Affected:< 78.7
ThunderbirdApplication
Affected:< 78.7

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, Firefox ESR, or Thunderbird is installed on the system. On Windows, look in Program Files or Program Files (x86). On macOS, check /Applications. On Linux, check /usr/lib/firefox or similar paths, or run 'which firefox' / 'which thunderbird'.
    Affected if Any of Firefox, Firefox ESR, or Thunderbird is installed
  2. Determine Firefox version
    For Firefox: Open the browser, go to Help > About Firefox, or navigate to 'about:support' in the address bar. Alternatively, run 'firefox --version' from command line. Record the full version number (e.g., 84.0, 84.0.1, 78.5).
    Affected if Firefox version is less than 85.0 (any 84.x or earlier version)
  3. Determine Firefox ESR version
    For Firefox ESR: Open the browser, go to Help > About Firefox, or navigate to 'about:support' in the address bar. The ESR version is typically displayed as something like '78.6esr' or '78.5esr'. Run 'firefox --version' from command line if available.
    Affected if Firefox ESR version is less than 78.7 (any 78.x version before 78.7, or 68.x)
  4. Determine Thunderbird version
    For Thunderbird: Open the application, go to Help > About Thunderbird, or navigate to 'about:support' in the address bar. Alternatively, run 'thunderbird --version' from command line. Record the full version number.
    Affected if Thunderbird version is less than 78.7 (any 78.x version before 78.7, or earlier)

You are affected if any installed Mozilla product (Firefox, Firefox ESR, or Thunderbird) matches an affected version: Firefox < 85.0, Firefox ESR < 78.7, or Thunderbird < 78.7.

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.

From vendor data
Upgrade available Upgrade to 78.7 / 85.0 or later
Fixed in 78.785.0
Interim mitigation

Upgrade affected products to patched versions: Firefox 85 or later, Thunderbird 78.7 or later, or Firefox ESR 78.7 or later.

Fix this in Firefox Scoped from the published advisory
  • Consultation1.0 h
  • Implementation1.0 h
  • Testing2.0 h
  • Review / QA1.0 h
5.0 hours of engineering $860
Get the upgrade done

An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $1,376.

Scan for this in your stack

Free · runs locally
dbcve dependency scanner

Check whether your project pulls in CVE-2021-23960 — or any other known-vulnerable package — straight from your lock files. Free and open source; it runs locally and uploads nothing.

Get it on GitHub Scan in the browser

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-2021-23960 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