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FirefoxWeb browser · Mozilla

CVE-2025-4091

HIGH · 8.1 CVSS v3.1 Published 2025-04-29
Fix available
A fix is available. Upgrade to 128.10 / 128.10.0 or later.
See remediation →
90/100
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
Memory safety bugs present in Firefox 137, Thunderbird 137, Firefox ESR 128.9, and Thunderbird 128.9. Some of these bugs showed evidence of memory corruption and we presume that with enough effort some of these could have been exploited to run arbitrary code. This vulnerability was fixed in Firefox 138, Firefox ESR 128.10, 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 · moderate confidence

Memory safety bugs in Firefox 137, Thunderbird 137, Firefox ESR 128.9, and Thunderbird 128.9 allowed evidence of memory corruption, potentially exploitable for arbitrary code execution via the browser or email client.

MitigationUpdate to Firefox 138, Firefox ESR 128.10, Thunderbird 138, Thunderbird 128.10 or later versions to patch the memory safety vulnerabilities.

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:< 128.10< 138.0
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
High
Privileges
None
User interaction
None
Scope
Unchanged
Confidentiality
High
Integrity
High
Availability
High

CVSS:3.1/AV:N/AC:H/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 Mozilla product
    Check if Firefox or Thunderbird is installed. On Linux: which firefox thunderbird. On Windows: check Program Files folders or registry. On macOS: check /Applications for Firefox.app or Thunderbird.app
    Affected if Either Firefox or Thunderbird is installed on the system
  2. Get Firefox version number
    For Firefox Desktop: type 'about:firefox' in the address bar and press Enter. From command line: firefox --version (Linux/macOS) or check Help > About Firefox (Windows)
    Affected if Firefox version is below 128.10 OR between 128.10.x and 137.x (i.e., < 138.0)
  3. Get Thunderbird version number
    For Thunderbird: type 'about:thunderbird' in the address bar and press Enter. From command line: thunderbird --version (Linux/macOS) or check Help > About Thunderbird (Windows)
    Affected if Thunderbird version is below 128.10.0 OR between 128.10.x and 137.x (i.e., < 138.0)
  4. Confirm the specific vulnerable versions
    Review the version number obtained. Affected Firefox: 128.0 through 128.9.x, and 137.0 through 137.x. Affected Thunderbird: 128.0 through 128.9.x, and 137.0 through 137.x.
    Affected if The installed version exactly matches any version in the affected ranges

A system is affected if Firefox or Thunderbird is installed with a version below 128.10, or between 128.10.x and 137.x (i.e., any version < 138.0 for the 128.10+ branch or the 137.x branch).

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 128.10 / 128.10.0 / 138.0 or later
Fixed in 128.10128.10.0138.0
Interim mitigation

Update to Firefox 138, Firefox ESR 128.10, Thunderbird 138, Thunderbird 128.10 or later versions to patch the memory safety vulnerabilities.

Recommended fix High confidence

Firefox 138.0, Firefox ESR 128.10, Thunderbird 138.0, or Thunderbird 128.10 depending on product and release channel

  1. Check the current version of Firefox or Thunderbird by navigating to Menu > Help > About (or typing 'about:support' in the address bar)
  2. Download Firefox 138 (or Firefox ESR 128.10 for extended support) from the official Mozilla website: www.mozilla.org/firefox/
  3. Download Thunderbird 138 (or Thunderbird 128.10 for extended support) from the official Mozilla website: www.mozilla.org/thunderbird/
  4. Close all instances of Firefox or Thunderbird
  5. Run the downloaded installer and follow the prompts to complete the upgrade
  6. Restart the browser or email client after installation completes
  7. Verify the upgrade was successful by checking Menu > Help > About shows version 138.0 (or 128.10 for ESR)
Caveat Firefox 138 is a major version update - may include web compatibility changes, removed deprecated APIs, or UI modifications; minor version updates within the 128.x ESR channel typically have fewer breaking changes

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

Fix this in Firefox Scoped from the published advisory
  • Consultation2.0 h
  • Implementation3.0 h
  • Testing2.0 h
  • Review / QA1.0 h
8.0 hours of engineering $1,420
Get the upgrade done

An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $2,272.

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-2025-4091 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