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FirefoxWeb browser · Mozilla

CVE-2026-4721

CRITICAL · 9.8 CVSS v3.1 Published 2026-03-24
Fix available
A fix is available. Upgrade to 115.34.0 / 140.9.0 or later.
See remediation →
100/100
Remediation priority · Urgent
Remotely reachable No privileges Zero-click

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
Memory safety bugs present in Firefox ESR 115.33, Firefox ESR 140.8, Thunderbird ESR 140.8, Firefox 148 and Thunderbird 148. 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 was fixed in Firefox 149, Firefox ESR 115.34, Firefox ESR 140.9, Thunderbird 149, and Thunderbird 140.9.

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

Multiple memory safety vulnerabilities in Firefox ESR and Thunderbird allowing memory corruption with potential for arbitrary code execution. The bugs affected versions 115.33, 140.8, and 148, and were addressed in versions 115.34, 140.9, and 149.

MitigationUpdate affected Mozilla products (Firefox ESR, Firefox, Thunderbird) to the patched versions (115.34, 140.9, or 149) as listed in the official advisory.

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:< 115.34.0< 149.0>= 128.0, < 140.9.0
ThunderbirdApplication
Affected:< 140.9.0< 149.0

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
None
Scope
Unchanged
Confidentiality
High
Integrity
High
Availability
High

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/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. Check Firefox version
    Navigate to Firefox menu > Help > About Firefox, or enter 'about:support' in the address bar. The version is displayed at the top.
    Affected if The version is earlier than 115.34.0, OR is 128.x through 140.8.x (i.e., 128.0 to 140.8.x), OR is earlier than 149.0.
  2. Check Thunderbird version
    Navigate to Thunderbird menu > Help > About Thunderbird. The version is displayed at the top.
    Affected if The version is earlier than 140.9.0, OR is earlier than 149.0.
  3. Confirm product type
    Verify whether the installed application is Mozilla Firefox or Mozilla Thunderbird by checking the application name in the About dialog or the installed program name.
    Affected if The application is Mozilla Firefox or Mozilla Thunderbird and meets the version conditions above.

A user is affected if they have Firefox version < 115.34.0, or version 128.x through 140.8.x, or any version < 149.0 (depending on branch), or Thunderbird version < 140.9.0 or < 149.0.

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 115.34.0 / 140.9.0 / 149.0 or later
Fixed in 115.34.0140.9.0149.0
Interim mitigation

Update affected Mozilla products (Firefox ESR, Firefox, Thunderbird) to the patched versions (115.34, 140.9, or 149) as listed in the official advisory.

Recommended fix High confidence

Firefox 149 or Firefox ESR 140.9 or Firefox ESR 115.34; Thunderbird 149 or Thunderbird 140.9

  1. 1. Identify the currently installed Firefox or Thunderbird version by navigating to the application menu and selecting 'Help' > 'About Firefox' or 'About Thunderbird'
  2. 2. Close all instances of the affected application before proceeding with the update
  3. 3. For Firefox users: Navigate to the official Mozilla download page at www.mozilla.org and download Firefox 149 (stable release) or Firefox ESR 140.9 (for enterprise/ESR users)
  4. 4. For Thunderbird users: Navigate to www.mozilla.org and download Thunderbird 149 or Thunderbird 140.9
  5. 5. Run the installer and follow the prompts to complete the installation
  6. 6. Alternatively, open the application and select 'Help' > 'About' which will automatically check for and prompt installation of the latest version
  7. 7. After installation, verify the version by returning to 'Help' > 'About' to confirm the installed version matches the fixed release
Caveat Standard Mozilla release upgrade - minimal risk; minor add-on compatibility changes possible but usually handled by add-on developers

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-2026-4721 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