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FirefoxWeb browser · Mozilla

CVE-2026-4696

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
Use-after-free 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

A use-after-free vulnerability in the Layout: Text and Fonts component allows memory corruption potentially leading to arbitrary code execution. The flaw occurs when the browser attempts to use memory that has already been freed, likely during font rendering or text layout operations.

MitigationUpdate affected systems to Firefox 149 or later, Firefox ESR 115.34/140.9 or later, or Thunderbird 149/140.9 or later to remediate this 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:< 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
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. Identify installed Mozilla product
    Check if Firefox or Thunderbird is installed by looking for executable files in typical installation paths (e.g., C:\Program Files\Mozilla Firefox\firefox.exe or /Applications/Firefox.app on macOS, /usr/bin/firefox on Linux). On Windows, you can also check Add/Remove Programs or the registry.
    Affected if Either Firefox or Thunderbird is installed on the system
  2. Determine Firefox version
    For Firefox: Type 'about:support' in the address bar and press Enter. Look for the 'Version' field in the page that loads. Alternatively, click the menu button, go to Help, then About Firefox. The version number will be displayed.
    Affected if Version is less than 115.34.0, OR between 128.0 and 139.x inclusive, OR between 140.0 and 140.8.x inclusive, OR less than 149.0 (all affected ranges)
  3. Determine Firefox ESR version
    For Firefox ESR: Type 'about:support' in the address bar and press Enter. Look for the 'Version' field. ESR versions typically include 'ESR' in the product name shown on the about:support page. The version format for ESR is like 115.x.x or 140.x.x.
    Affected if Version is less than 115.34.0 OR less than 140.9.0 (both ESR branches)
  4. Determine Thunderbird version
    For Thunderbird: Click the menu button (three horizontal lines or 'Thunderbird' on macOS), go to Help, then About Thunderbird. Alternatively, type 'about:support' in the address bar. The version number will be displayed in either location.
    Affected if Version is less than 115.34.0, OR between 128.0 and 139.x inclusive, OR between 140.0 and 140.8.x inclusive, OR less than 149.0 (all affected ranges)
  5. Confirm component exposure
    This vulnerability affects the Layout: Text and Fonts component. The flaw can be triggered by loading malicious web content (Firefox) or opening crafted email messages (Thunderbird). No additional configuration needs to be checked; if the vulnerable version is running and processing untrusted content, the condition is met.
    Affected if A vulnerable version is running and the application processes web content or email (standard normal use)

If any installed version of Firefox, Firefox ESR, or Thunderbird is below 115.34.0, below 140.9.0 (for ESR 128+ branches), or below 149.0, the system is affected by this use-after-free vulnerability in the Layout: Text and Fonts component.

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 systems to Firefox 149 or later, Firefox ESR 115.34/140.9 or later, or Thunderbird 149/140.9 or later to remediate this vulnerability.

Recommended fix High confidence

Firefox 149 (standard), Firefox ESR 115.34, Firefox ESR 140.9, Thunderbird 149, or Thunderbird 140.9 - depending on current release track

  1. 1. Check the current Firefox version by navigating to Menu > Help > About Firefox (or typing 'about:support' in the address bar)
  2. 2. Determine which Firefox release track is in use: Standard Firefox, Firefox ESR, or Thunderbird
  3. 3. Download the appropriate fixed version from the official Mozilla website: Firefox 149 (standard release), Firefox ESR 115.34 (for ESR 115.x users), Firefox ESR 140.9 (for ESR 140.x users), or Thunderbird 149/140.9 for Thunderbird users
  4. 4. Close all Firefox instances completely
  5. 5. Install the updated version over the existing installation or use the installer to update

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

Fix this in Firefox Scoped from the published advisory
  • Consultation2.0 h
  • Implementation1.0 h
  • Testing2.0 h
  • Review / QA1.0 h
6.0 hours of engineering $1,060
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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-2026-4696 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