dbcve.org
ThunderbirdApplication · Mozilla

CVE-2025-3523

MEDIUM · 6.4 CVSS v3.1 Published 2025-04-15
Fix available
A fix is available. Upgrade to 128.9.2 / 137.0.2 or later.
See remediation →
71/100
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
When an email contains multiple attachments with external links via the X-Mozilla-External-Attachment-URL header, only the last link is shown when hovering over any attachment. Although the correct link is used on click, the misleading hover text could trick users into downloading content from untrusted sources. This vulnerability was fixed in Thunderbird 137.0.2 and Thunderbird 128.9.2.

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

In Thunderbird, when emails contain multiple attachments with external links specified via the X-Mozilla-External-Attachment-URL header, the hover tooltip incorrectly displays only the last defined URL regardless of which attachment the user hovers over. While the actual click action uses the correct per-attachment URL, the misleading hover text could cause users to trust and download malicious content from untrusted sources.

MitigationUpgrade Thunderbird to version 137.0.2 or 128.9.2 or later. Users should verify attachment sources through alternative means rather than relying on hover text until patched.

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
ThunderbirdApplication
Affected:< 128.9.2>= 129.0, < 137.0.2

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
Required
Scope
Unchanged
Confidentiality
Low
Integrity
High
Availability
Low

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

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. Check if Thunderbird is installed and get its version
    In Thunderbird, go to Help > About Thunderbird. The version number will be displayed in the dialog that opens. On Linux, you can also run 'thunderbird --version' in terminal. On Windows, check Add/Remove Programs or the application properties.
    Affected if Version is less than 128.9.2, or greater than or equal to 129.0 but less than 137.0.2
  2. Identify emails with external attachment URLs
    Use Thunderbird's search or message filters to find emails containing the header 'X-Mozilla-External-Attachment-URL'. You can also view message source (View > Message Source) for specific emails to inspect headers.
    Affected if Emails exist that contain multiple attachments with X-Mozilla-External-Attachment-URL headers defined
  3. Verify the tooltip behavior with multiple external attachments
    Receive or open an email with multiple attachments where each has an X-Mozilla-External-Attachment-URL header defined. Hover the mouse cursor over each attachment individually and observe the URL shown in the tooltip.
    Affected if The tooltip shows the same URL for all attachments instead of the correct per-attachment URL

You are affected if your Thunderbird version is within the vulnerable range AND you handle emails containing multiple attachments with X-Mozilla-External-Attachment-URL headers, where hovering shows the same URL for every attachment.

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 128.9.2 / 137.0.2 or later
Fixed in 128.9.2137.0.2
Interim mitigation

Upgrade Thunderbird to version 137.0.2 or 128.9.2 or later. Users should verify attachment sources through alternative means rather than relying on hover text until patched.

Recommended fix High confidence

Thunderbird 128.9.2 (ESR) or Thunderbird 137.0.2 (standard release)

  1. Check current Thunderbird version by navigating to Help > About Thunderbird
  2. Download Thunderbird 128.9.2 (for users on the Extended Support Release channel) or Thunderbird 137.0.2 (for users on the standard release channel) from the official Mozilla Thunderbird website
  3. Close Thunderbird completely before installing the update
  4. Install the downloaded Thunderbird update by running the installer and following the prompts
  5. After installation, verify the version by checking Help > About Thunderbird to confirm the version number matches the expected fixed release
Caveat Standard Thunderbird upgrade; ensure any custom settings or add-ons are compatible with the target version

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

Fix this in Thunderbird Scoped from the published advisory
  • Consultation4.0 h
  • Implementation8.0 h
  • Testing6.0 h
  • Review / QA4.0 h
22.0 hours of engineering $3,860
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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-2025-3523 in production — separate from our analysis above.

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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