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FirefoxWeb browser · Mozilla

CVE-2026-2778

CRITICAL · 10.0 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
Sandbox escape due to incorrect boundary conditions in the DOM: Core & HTML 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 sandbox escape vulnerability exists in Firefox and Thunderbird due to incorrect boundary conditions in the DOM: Core & HTML component, allowing an attacker to break out of the browser's security sandbox and potentially execute arbitrary code with full user privileges.

MitigationUpdate affected installations to Firefox 148, Firefox ESR 115.33/140.8, or Thunderbird 148/140.8 as specified in the vendor advisory to remediate this critical 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
Changed
Confidentiality
High
Integrity
High
Availability
High

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/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
    Open Firefox, go to Menu > Help > About Firefox, or enter 'about:support' in the address bar. Alternatively, check via command line: 'firefox --version' or check the application file properties.
    Affected if The displayed version falls below 115.33.0, is between 128.0 and 140.7.x, or is below 148.0 (depending on release channel)
  2. Identify installed Thunderbird version
    Open Thunderbird, go to Menu > Help > About Thunderbird, or enter 'about:support' in the address bar. Alternatively, check via command line: 'thunderbird --version' or check the application file properties.
    Affected if The displayed version is below 140.8.0 or below 148.0 (depending on release channel)
  3. Determine Firefox release channel
    In Firefox, enter 'about:support' and look at the 'Application Basics > Release Channel' field. This indicates whether you are on Release, Beta, ESR, or Nightly.
    Affected if The channel is Release or ESR and the version is in any of the affected ranges from step 1. Note: This vulnerability affects the DOM: Core & HTML component which processes web content.
  4. Confirm DOM component is in use
    No separate check needed - the DOM: Core & HTML component is always active when the browser processes web pages or renders HTML content in emails (Thunderbird).
    Affected if Any regular use of Firefox to browse web pages or Thunderbird to view HTML emails means the vulnerable component is active.

You are affected if you have Firefox version < 115.33.0, 128.0 through 140.7.x, or < 148.0; or Thunderbird version < 140.8.0 or < 148.0, and you use the browser or email client to process web/HTML 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 115.33.0 / 140.8.0 / 148.0 or later
Fixed in 115.33.0140.8.0148.0
Interim mitigation

Update affected installations to Firefox 148, Firefox ESR 115.33/140.8, or Thunderbird 148/140.8 as specified in the vendor advisory to remediate this critical vulnerability.

Recommended fix High confidence

Firefox 148.0+ / Firefox ESR 115.33.0+ / Firefox ESR 140.8.0+ / Thunderbird 140.8.0+ / Thunderbird 148+

  1. 1. Determine which Firefox or Thunderbird branch you are currently using (standard release or ESR)
  2. 2. For Firefox standard release users: Upgrade to Firefox 148.0 or later
  3. 3. For Firefox ESR 115.x users: Upgrade to Firefox ESR 115.33.0 or later
  4. 4. For Firefox ESR 128.x users: Upgrade to Firefox ESR 140.8.0 or later
  5. 5. For Thunderbird users: Upgrade to Thunderbird 140.8.0 or later, or Thunderbird 148 or later
  6. 6. Restart the application after updating
  7. 7. Verify the installed version matches or exceeds the fixed versions listed above

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
  • Testing1.0 h
  • Review / QA1.0 h
5.0 hours of engineering $910
Get the upgrade done

An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $1,456.

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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-2026-2778 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