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FirefoxWeb browser · Mozilla

CVE-2025-4083

CRITICAL · 9.1 CVSS v3.1 Published 2025-04-29
Fix available
A fix is available. Upgrade to 115.23 / 128.10 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
A process isolation vulnerability in Thunderbird stemmed from improper handling of javascript: URIs, which could allow content to execute in the top-level document's process instead of the intended frame, potentially enabling a sandbox escape. This vulnerability was fixed in Firefox 138, Firefox ESR 128.10, Firefox ESR 115.23, Thunderbird 138, and Thunderbird 128.10.

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 process isolation vulnerability in Thunderbird allowed javascript: URIs to execute in the top-level document's process instead of the intended sandboxed frame, enabling a sandbox escape. This bypasses the security model that isolates web content into separate processes.

MitigationUpdate Thunderbird to version 138 or 128.10 (or Firefox to version 138, ESR 128.10, or ESR 115.23) to apply the fix for proper javascript: URI handling and process isolation.

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:< 115.23< 138.0>= 128.0, < 128.10
ThunderbirdApplication
Affected:< 128.10.0< 138.0

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
None

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/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 Mozilla product
    Check if Thunderbird or Firefox is installed on the system by looking in typical installation directories (C:\Program Files\Mozilla Thunderbird, C:\Program Files\Mozilla Firefox on Windows; /Applications/Thunderbird.app, /Applications/Firefox.app on macOS; /usr/lib/thunderbird, /usr/lib/firefox on Linux) or checking the system's program list.
    Affected if Either Thunderbird or Firefox is installed, as both are affected by this vulnerability.
  2. Get Thunderbird version number
    If Thunderbird is installed, open it and go to Help > About Thunderbird, or check the version.ini or application.ini file in the Thunderbird installation directory. Record the full version number (for example, 128.9.0).
    Affected if Thunderbird version is less than 128.10.0 or less than 138.0.
  3. Get Firefox version number
    If Firefox is installed, open it and go to Help > About Firefox, or check the version.ini or application.ini file in the Firefox installation directory. Record the full version number (for example, 128.9.1).
    Affected if Firefox version is less than 115.23, OR less than 138.0 while also being 128.0 or higher but less than 128.10.
  4. Verify if any affected version is in use
    Compare the identified version against the affected ranges: for Thunderbird, check if version < 128.10.0 or version < 138.0; for Firefox, check if version < 115.23 OR (version >= 128.0 AND version < 128.10).
    Affected if The installed version falls into any of the affected ranges, meaning the javascript: URI process isolation flaw is present.

A user is affected if they have Thunderbird below 128.10.0 or below 138.0, or Firefox below 115.23 or between 128.0 and 128.9.x.

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 115.23 / 128.10 / 128.10.0 or later
Fixed in 115.23128.10128.10.0
Interim mitigation

Update Thunderbird to version 138 or 128.10 (or Firefox to version 138, ESR 128.10, or ESR 115.23) to apply the fix for proper javascript: URI handling and process isolation.

Recommended fix High confidence

Firefox 138 (stable), Firefox ESR 128.10, Firefox ESR 115.23, Thunderbird 138, or Thunderbird 128.10 - choose the appropriate version based on your current release channel

  1. Identify the current installed version of Firefox or Thunderbird
  2. If running Firefox ESR 115.x, upgrade to Firefox ESR 115.23 or later
  3. If running Firefox ESR 128.x, upgrade to Firefox ESR 128.10 or later
  4. If running Firefox 127.x or earlier, upgrade to Firefox 138 or later
  5. If running Thunderbird 128.x, upgrade to Thunderbird 128.10 or later
  6. If running Thunderbird 137.x or earlier, upgrade to Thunderbird 138 or later
  7. Restart the application after upgrading
Caveat Mozilla regular releases typically have minimal breaking changes; however, some older extensions or add-ons may be incompatible with major version upgrades

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

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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-2025-4083 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