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FirefoxWeb browser · Mozilla

CVE-2026-4699

HIGH · 7.5 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 →
84/100
Remediation priority · High
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
Incorrect boundary conditions in the Layout: Text and Fonts component. 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 · moderate confidence

Memory safety vulnerability in Mozilla Firefox and Thunderbird's Layout: Text and Fonts component caused by incorrect boundary conditions during text and font processing. This could allow memory corruption leading to potential code execution or crash conditions.

MitigationUpdate affected Mozilla products (Firefox and Thunderbird) to the patched versions: Firefox 149/ESR 115.34/ESR 140.9 or later, Thunderbird 149/140.9 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:< 115.34.0< 149.0>= 128.0, < 140.9.0

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
None
Integrity
None
Availability
High

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/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. Determine installed Firefox version
    In Firefox, go to Menu > Help > About Firefox (or navigate to about:support). The version number is displayed on this page.
    Affected if The installed version is < 115.34.0 OR (>= 128.0 AND < 140.9.0) OR < 149.0 (version falls within any of the vulnerable ranges)
  2. Determine installed Thunderbird version
    In Thunderbird, go to Menu > Help > About Thunderbird (or navigate to about:support). The version number is displayed on this page.
    Affected if The installed version is < 115.34.0 OR (>= 128.0 AND < 140.9.0) OR < 149.0 (version falls within any of the vulnerable ranges)
  3. Verify Text and Fonts processing is active
    This vulnerability affects the Layout: Text and Fonts component. The flaw is triggered during text and font rendering operations in the browser or email client. Any page displaying text or any email being viewed would exercise this code path.
    Affected if The product is actively used to render web content (Firefox) or view emails with text/fonts (Thunderbird), meaning the affected code path is being exercised

You are affected if your Firefox or Thunderbird version is less than 115.34.0, or between 128.0 and 140.8.x, or less than 149.0 (checking against the specific version ranges provided), and you actively use the browser or email client to render content.

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 and Thunderbird) to the patched versions: Firefox 149/ESR 115.34/ESR 140.9 or later, Thunderbird 149/140.9 or later.

Recommended fix High confidence

Firefox 149 (or Firefox ESR 115.34 / Firefox ESR 140.9 for ESR users); Thunderbird 149 or Thunderbird 140.9

  1. Check the current Firefox version by navigating to Menu > Help > About Firefox (or entering 'about:support' in the address bar)
  2. If version is < 115.34.0, upgrade to Firefox 115.34.0 or later (or ESR 115.34)
  3. If version is >= 128.0 and < 140.9.0, upgrade to Firefox 140.9.0 or later (Firefox 149 is recommended)
  4. If version is >= 140.9.0 and < 149, upgrade to Firefox 149 or later
  5. For Thunderbird users: upgrade to Thunderbird 140.9 or Thunderbird 149 depending on current version
  6. After upgrade, verify the version by checking Menu > Help > About to confirm the fix is applied

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

Fix this in Firefox Scoped from the published advisory
  • Consultation1.0 h
  • Implementation1.0 h
  • Testing1.0 h
  • Review / QA1.0 h
4.0 hours of engineering $750
Get the upgrade done

An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $1,200.

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-2026-4699 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