Vulnerability Description
Thunderbird'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.
CVSS Score
HIGH
Affected Products
| Vendor | Product | Versions |
|---|---|---|
| Mozilla | Thunderbird | < 128.10.1 |
Related Weaknesses (CWE)
References
- https://bugzilla.mozilla.org/show_bug.cgi?id=1958376Permissions Required
- https://www.mozilla.org/security/advisories/mfsa2025-34/Vendor Advisory
- https://www.mozilla.org/security/advisories/mfsa2025-35/Vendor Advisory
- https://lists.debian.org/debian-lts-announce/2025/05/msg00022.html
FAQ
What is CVE-2025-3909?
CVE-2025-3909 is a vulnerability with a CVSS score of 8.1 (HIGH). Thunderbird'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 settin...
How severe is CVE-2025-3909?
CVE-2025-3909 has been rated HIGH with a CVSS base score of 8.1/10. Review the CVSS metrics above for detailed severity breakdown.
Is there a patch for CVE-2025-3909?
Check the references section above for vendor advisories and patch information. Affected products include: Mozilla Thunderbird.