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FirefoxWeb browser · Mozilla

CVE-2026-2758

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: GC 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 · moderate confidence

A use-after-free vulnerability in the JavaScript garbage collection component allows remote attackers to execute arbitrary code via crafted JavaScript. This memory corruption flaw can be triggered by visiting malicious web pages in affected Firefox or Thunderbird versions.

MitigationUpdate affected systems to Firefox 148, Firefox ESR 115.33/140.8, Thunderbird 148/140.8 or later versions to remediate the 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:< 115.33.0< 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 installed Firefox version
    In Firefox, navigate to Help > About Firefox. The version number is displayed in the window title or main content area. Alternatively, type 'about:support' in the address bar and locate the 'Version' field under 'Application Basics'.
    Affected if The displayed version is lower than 115.33.0, OR is 128.0 through 140.7.x, OR is between 141.0 and 147.x inclusive.
  2. Identify installed Thunderbird version
    In Thunderbird, go to Help > About Thunderbird. The version number appears in the dialog window. Alternatively, check the Windows installed programs list, or run 'thunderbird --version' from command line on Linux systems.
    Affected if The displayed version is lower than 140.8.0, OR is between 141.0 and 147.x inclusive.
  3. Determine if JavaScript is enabled
    The vulnerability exists in the JavaScript garbage collector. In Firefox, verify JS is enabled by typing 'about:config' in the address bar and searching for 'javascript.enabled'. In Thunderbird, JavaScript can be enabled for message viewing in Settings > Privacy & Security, or via config editor.
    Affected if JavaScript is enabled in the browser or mail client settings.
  4. Verify application binary version via command line
    On Windows, locate the Firefox or Thunderbird executable (firefox.exe or thunderbird.exe), right-click and select Properties > Details > File Version. On Linux, run 'firefox --version' or 'thunderbird --version' in terminal.
    Affected if The command output or file version matches the affected version ranges in steps 1 or 2.

A user is affected if they are running any version of Firefox lower than 115.33.0, any version from 128.0 up to 140.7.x, any version from 141.0 up to 147.x, or any version of Thunderbird lower than 140.8.0 or between 141.0 and 147.x, with JavaScript enabled.

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 systems to Firefox 148, Firefox ESR 115.33/140.8, Thunderbird 148/140.8 or later versions to remediate the vulnerability.

Recommended fix High confidence

Firefox 148 (Stable), Firefox ESR 115.33, Firefox ESR 140.8, Thunderbird 148, or Thunderbird 140.8

  1. Check current Firefox or Thunderbird version by navigating to: Menu > Help > About (or visiting about:support)
  2. If using Firefox Stable: Upgrade to Firefox 148 or later by downloading from https://www.mozilla.org/firefox/
  3. If using Firefox ESR 115.x: Upgrade to Firefox ESR 115.33 or later
  4. If using Firefox ESR 128.x or 140.x: Upgrade to Firefox ESR 140.8 or later
  5. If using Thunderbird Stable: Upgrade to Thunderbird 148 or later
  6. If using Thunderbird ESR 128.x or 140.x: Upgrade to Thunderbird 140.8 or later
  7. After upgrade, verify version in About to confirm the fix is applied
  8. Ensure automatic updates are enabled in settings to receive future security patches

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

Fix this in Firefox Scoped from the published advisory
  • Consultation1.0 h
  • Testing1.0 h
  • Review / QA1.0 h
3.0 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.

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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-2026-2758 in production — separate from our analysis above.

No notes yet

Be the first to add a field note for this CVE — a mitigation you’ve verified, a version caveat, or a link to a working fix. Sign in above to contribute.

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