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ThunderbirdApplication · Mozilla

CVE-2025-3522

MEDIUM · 6.3 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 →
70/100
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
Thunderbird processes the X-Mozilla-External-Attachment-URL header to handle attachments which can be hosted externally. When an email is opened, Thunderbird accesses the specified URL to determine file size, and navigates to it when the user clicks the attachment. Because the URL is not validated or sanitized, it can reference internal resources like chrome:// or SMB share file:// links, potentially leading to hashed Windows credential leakage and opening the door to more serious security issues. 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

Thunderbird processes the X-Mozilla-External-Attachment-URL header to handle externally hosted attachments. When an email is opened, Thunderbird accesses the specified URL to determine file size and navigates to it when the user clicks the attachment. Because the URL is not validated or sanitized, it can reference internal resources like chrome:// or file:// (SMB) links, enabling Windows credential hash leakage and potential access to internal services.

MitigationUpgrade Thunderbird to version 137.0.2 or 128.9.2 or later. Until then, warn users not to open attachments from untrusted senders and consider blocking external content loading in the mail client.

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
Low
Privileges
None
User interaction
Required
Scope
Unchanged
Confidentiality
Low
Integrity
Low
Availability
Low

CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:L/I:L/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 Thunderbird installed version
    Open Thunderbird, go to Help > About Thunderbird (or Thunderbird > About Thunderbird on macOS) to see the version number. Alternatively, check the program files directory or use system package manager.
    Affected if The version is < 128.9.2 or >= 129.0 but < 137.0.2
  2. Verify external content loading is enabled
    Go to Thunderbird Settings > Privacy & Security > and look for 'Allow remote content in messages' or 'Load remote content in messages' checkbox. Check if it is checked/enabled.
    Affected if Remote content loading is enabled, allowing Thunderbird to process the X-Mozilla-External-Attachment-URL header and access external URLs when opening emails.
  3. Check for untrusted email accounts
    Review the email accounts configured in Thunderbird. Identify accounts that receive email from untrusted or unknown senders, particularly those that could deliver emails with the malicious X-Mozilla-External-Attachment-URL header.
    Affected if The account processes emails from untrusted sources, increasing the likelihood of receiving a specially crafted email that exploits this vulnerability.

You are affected if your Thunderbird version is below 128.9.2 or between 129.0 and 137.0.2, and you have remote content loading enabled while receiving emails from untrusted senders.

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. Until then, warn users not to open attachments from untrusted senders and consider blocking external content loading in the mail client.

Recommended fix High confidence

Thunderbird 137.0.2 or Thunderbird 128.9.2 (ESR)

  1. 1. Verify current Thunderbird version by navigating to Help > About Thunderbird
  2. 2. Download Thunderbird 137.0.2 (current stable) or Thunderbird 128.9.2 (ESR) from the official Mozilla website at www.mozilla.org
  3. 3. Close Thunderbird completely before installing the update
  4. 4. Run the installer and follow the prompts to complete the upgrade
  5. 5. After installation, verify the version shows 137.0.2 or 128.9.2 in Help > About Thunderbird

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

Fix this in Thunderbird Scoped from the published advisory
  • Consultation2.0 h
  • Implementation1.0 h
  • Testing2.0 h
  • Review / QA1.0 h
6.0 hours of engineering $1,060
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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-2025-3522 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