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FirefoxWeb browser · Mozilla

CVE-2021-23964

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
Mozilla developers reported memory safety bugs present in Firefox 84 and Firefox ESR 78.6. Some of these bugs showed evidence of memory corruption and we presume that with enough effort some of these could have been exploited to run arbitrary code. 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 bugs in Firefox 84 and Firefox ESR 78.6 allowed memory corruption, which could potentially be exploited to achieve arbitrary code execution. This is a client-side vulnerability affecting Firefox, Thunderbird, and Firefox ESR.

MitigationUpgrade to Firefox 85+, Thunderbird 78.7+, or Firefox ESR 78.7+ to patch the memory corruption 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:< 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 by looking in Program Files (Windows), /Applications (macOS), or system package managers (Linux)
    Affected if Any of these three products are installed
  2. Check Firefox version
    Launch Firefox, go to Help > About Firefox, or navigate to about:support, to view the exact version number
    Affected if The displayed version is below 85.0 (for example, 84.0.x)
  3. Check Firefox ESR version
    Launch Firefox ESR, go to Help > About Firefox ESR, to view the exact version number
    Affected if The displayed version is below 78.7 (for example, 78.6.x)
  4. Check Thunderbird version
    Launch Thunderbird, go to Help > About Thunderbird, to view the exact version number
    Affected if The displayed version is below 78.7 (for example, 78.6.x)

If Firefox is below 85.0, or Firefox ESR/Thunderbird is below 78.7, the memory corruption vulnerability is present in the installed version.

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 to Firefox 85+, Thunderbird 78.7+, or Firefox ESR 78.7+ to patch the memory corruption vulnerability.

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-2021-23964 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