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FirefoxWeb browser · Mozilla

CVE-2026-12299

MEDIUM · 5.4 CVSS v3.1 Published 2026-06-16
Fix available
A fix is available. Upgrade to 115.37.0 / 140.12.0 or later.
See remediation →
61/100
Remediation priority · Elevated
Remotely reachable No privileges

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
JIT miscompilation in the DOM: Core & HTML component. This vulnerability was fixed in Firefox 152, Firefox ESR 140.12, Firefox ESR 115.37, Thunderbird 152, and Thunderbird 140.12.

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

JIT miscompilation vulnerability in the Firefox/Thunderbird JavaScript engine's DOM Core & HTML component. The Just-In-Time compiler generates incorrect machine code when processing DOM operations, which could lead to memory corruption and potentially allow arbitrary code execution.

MitigationUpdate affected installations to Firefox 152, Firefox ESR 115.37/140.12, or Thunderbird 140.12/152 or later versions to receive the JIT compiler fix.

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:< 115.37.0< 152.0>= 140.0, < 140.12.0
ThunderbirdApplication
Affected:< 152.0.0>= 140.0, < 140.12.0

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
Required
Scope
Unchanged
Confidentiality
Low
Integrity
Low
Availability
None

CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:L/I:L/A:N

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
    Run 'firefox --version' from command line, or in Firefox navigate to Help > About Firefox to see the exact version number
    Affected if Version is less than 115.37.0, or 115.x.x but less than 152.0, or 140.0.x but less than 140.12.0
  2. Identify installed Thunderbird version
    Run 'thunderbird --version' from command line, or in Thunderbird navigate to Help > About Thunderbird to see the exact version number
    Affected if Version is less than 152.0.0, or 140.0.x but less than 140.12.0
  3. Verify JIT compilation is enabled
    In Firefox address bar, go to 'about:config' and search for 'javascript.options.jit' - check if the value is set to true (enabled). Alternatively, check 'javascript.options.jit.content' preference.
    Affected if JIT is enabled (value is true) - the vulnerability exists in the JIT compiler, so disabling JIT would mitigate the issue even on a vulnerable version

A user is affected if they are running a vulnerable Firefox or Thunderbird version (as listed above) with JIT compilation 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 115.37.0 / 140.12.0 / 152.0 or later
Fixed in 115.37.0140.12.0152.0
Interim mitigation

Update affected installations to Firefox 152, Firefox ESR 115.37/140.12, or Thunderbird 140.12/152 or later versions to receive the JIT compiler fix.

Recommended fix High confidence

Firefox ESR 115.37, Firefox 152, Firefox ESR 140.12.0, Thunderbird 152, or Thunderbird 140.12.0 (depending on current branch)

  1. Check current Firefox or Thunderbird version via Help > About (or running --version from command line)
  2. If running Firefox ESR 115.x: upgrade to Firefox ESR 115.37
  3. If running Firefox 140.x (or ESR 140.x): upgrade to Firefox ESR 140.12.0
  4. If running Firefox 141-151: upgrade to Firefox 152
  5. If running Thunderbird 140.x (or ESR 140.x): upgrade to Thunderbird 140.12.0
  6. If running Thunderbird before 152: upgrade to Thunderbird 152
  7. Restart the application after upgrade
  8. Verify version shows the upgraded release in Help > About
Caveat Standard minor version upgrades typically have no breaking changes; ensure compatibility with any locally-installed add-ons or extensions

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
Get the upgrade done

An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $1,424.

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-2026-12299 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