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FirefoxWeb browser · Mozilla

CVE-2026-2763

CRITICAL · 9.8 CVSS v3.1 Published 2026-02-24
Fix available
A fix is available. Upgrade to 115.33.0 / 140.8.0 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 JavaScript Engine component. This vulnerability was fixed in Firefox 148, Firefox ESR 115.33, Firefox ESR 140.8, Thunderbird 148, and Thunderbird 140.8.

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

A use-after-free memory corruption vulnerability in the Mozilla JavaScript Engine allows remote attackers to execute arbitrary code via crafted JavaScript. The vulnerability is exploitable network-side with no authentication required, achieving critical severity (CVSS 9.8).

MitigationUpdate affected Mozilla products (Firefox to 148/ESR 115.33/ESR 140.8, Thunderbird to 148/140.8) to remediate the use-after-free condition in the JavaScript engine.

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:< 115.33.0< 148.0>= 128.0, < 140.8.0
ThunderbirdApplication
Affected:< 140.8.0< 148.0

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 if Mozilla Firefox is installed
    On Windows, check for 'C:\Program Files\Mozilla Firefox\firefox.exe' or run 'Get-ItemProperty HKLM:\Software\Mozilla\Mozilla Firefox | Select-Object -ExpandProperty CurrentVersion'. On macOS, run 'ls /Applications/Firefox.app' or check via 'mdls -name kMDItemVersion /Applications/Firefox.app'. On Linux, run 'firefox --version' or check package manager.
    Affected if Firefox is installed and its version falls below 115.33.0, between 115.33.0 and 140.8.0 exclusive, or between 140.8.0 and 148.0 exclusive
  2. Identify if Mozilla Thunderbird is installed
    On Windows, check for 'C:\Program Files\Mozilla Thunderbird\thunderbird.exe' or run 'Get-ItemProperty HKLM:\Software\Mozilla\Thunderbird | Select-Object -ExpandProperty CurrentVersion'. On macOS, run 'ls /Applications/Thunderbird.app' or check via 'mdls -name kMDItemVersion /Applications/Thunderbird.app'. On Linux, run 'thunderbird --version' or check package manager.
    Affected if Thunderbird is installed and its version falls below 140.8.0 or below 148.0 (both ranges are affected)
  3. Verify exact version number of Firefox
    Run 'firefox --version' from command line, or in Firefox navigate to 'about:support' and read the 'Version' field under 'Application Basics', or check Help > About Firefox
    Affected if The displayed version is less than 115.33.0, or is 115.33.0 through 139.x, or is between 140.8.0 and 147.x (all these ranges are vulnerable)
  4. Verify exact version number of Thunderbird
    Run 'thunderbird --version' from command line, or in Thunderbird navigate to 'about:support' and read the 'Version' field under 'Application Basics', or check Help > About Thunderbird
    Affected if The displayed version is less than 140.8.0 or is between 140.8.0 and 147.x (both ranges are vulnerable)

A user is affected if they have Firefox version below 115.33.0, between 115.33.0 and 140.8.0, between 140.8.0 and 148.0, or Thunderbird version below 140.8.0 or between 140.8.0 and 148.0 - all of these versions contain the vulnerable JavaScript Engine code.

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 115.33.0 / 140.8.0 / 148.0 or later
Fixed in 115.33.0140.8.0148.0
Interim mitigation

Update affected Mozilla products (Firefox to 148/ESR 115.33/ESR 140.8, Thunderbird to 148/140.8) to remediate the use-after-free condition in the JavaScript engine.

Recommended fix High confidence

Firefox 148 (or ESR 115.33.0 / ESR 140.8.0); Thunderbird 140.8 (or 148)

  1. 1. Identify the installed Firefox or Thunderbird version via Help > About (or 'firefox --version' / 'thunderbird --version' from command line).
  2. 2. For Firefox: Upgrade to version 148 or later for the rapid release, OR upgrade to Firefox ESR 115.33.0 or later, OR upgrade to Firefox ESR 140.8.0 or later.
  3. 3. For Thunderbird: Upgrade to version 140.8.0 or later, OR upgrade to version 148 or later.
  4. 4. Restart the application after upgrade completes.
  5. 5. Verify the installed version matches or exceeds one of the fixed versions (Firefox 148, Firefox ESR 115.33.0, Firefox ESR 140.8.0, Thunderbird 140.8.0, or Thunderbird 148).

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

Fix this in Firefox Scoped from the published advisory
  • Consultation1.0 h
  • Implementation2.0 h
  • Testing1.0 h
  • Review / QA1.0 h
5.0 hours of engineering $890
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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-2026-2763 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