CVE-2025-3909
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 · uneditedThunderbird's handling of the X-Mozilla-External-Attachment-URL header can be exploited to execute JavaScript in the file:/// context. By crafting a nested email attachment (message/rfc822) and setting its content type to application/pdf, Thunderbird may incorrectly render it as HTML when opened, allowing the embedded JavaScript to run without requiring a file download. This behavior relies on Thunderbird auto-saving the attachment to /tmp and linking to it via the file:/// protocol, potentially enabling JavaScript execution as part of the HTML. This vulnerability was fixed in Thunderbird 128.10.1 and Thunderbird 138.0.1.
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 confidenceThunderbird incorrectly renders a nested email attachment (message/rfc822) with a forged content-type of application/pdf as HTML, allowing embedded JavaScript to execute in the file:/// context. This occurs because Thunderbird auto-saves the attachment to /tmp and links to it, enabling JavaScript execution without requiring a file download.
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< 128.10.1>= 129.0, < 138.0.1CVSS 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
- High
- Integrity
- High
- Availability
- None
CVSS:3.1/AV:N/AC:L/PR:N/UI:R/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 checksWork through these to decide whether this CVE applies to you.
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Identify Thunderbird versionRun: thunderbird --version or check About Thunderbird in the application menuAffected if The version is < 128.10.1 OR >= 129.0 and < 138.0.1
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Check for nested email attachmentsInspect any received emails that contain nested attachments (message/rfc822 parts). This typically occurs when an email contains another email as an attachment.Affected if An email with a nested message/rfc822 attachment using a forged content-type of application/pdf is opened
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Verify auto-save behaviorExamine the /tmp directory for files created by Thunderbird when previewing attachments. Look for files with .html or .pdf extensions that Thunderbird may have auto-saved.Affected if Thunderbird auto-saves attachments to /tmp and renders them, allowing JavaScript execution in the file:/// context
If Thunderbird version is < 128.10.1 or between 129.0 and 138.0.1, and the user opens a specially crafted email with a forged nested attachment, they are affected by this vulnerability.
Generated from the published advisory. Verify against your own configuration.
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 · scoped128.10.1138.0.1
Update Thunderbird to version 128.10.1 or 138.0.1 or later to remediate this vulnerability.
Thunderbird 128.10.1 or Thunderbird 138.0.1
- Check current Thunderbird version by going to Help > About Thunderbird
- Close Thunderbird completely before upgrading
- Download Thunderbird 128.10.1 or Thunderbird 138.0.1 from the official Mozilla Thunderbird website (www.thunderbird.net) or your system's package manager
- Install the downloaded version using your system's package manager or by running the installer
- After installation, verify the version by checking Help > About Thunderbird to confirm it shows 128.10.1 or 138.0.1 or higher
- Ensure your Thunderbird auto-update is enabled to receive future security updates
Generated from the published advisory — verify against the referenced sources before acting.
- Consultation2.0 h
- Implementation4.0 h
- Testing3.0 h
- Review / QA1.0 h
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Scan for this in your stack
Free · runs locallyCheck whether your project pulls in CVE-2025-3909 — or any other known-vulnerable package — straight from your lock files. Free and open source; it runs locally and uploads nothing.
References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.
Primary sourcesPractitioner notes
ContributedPeer-ranked notes from engineers who’ve handled CVE-2025-3909 in production — separate from our analysis above.
The advisory tells you what broke. It rarely tells you what actually worked. If you’ve dealt with this one, that detail is what the next engineer is searching for.
- The version that genuinely resolved it — not the one the vendor claimed
- A config change or rule that shut the vector down
- A gotcha in the upgrade path that cost you an afternoon
No notes yet
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- 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