CVE-2025-3909: Mozilla Thunderbird

High severity, CVSS 8.1. EPSS: 0.5% chance of exploitation in the next 30 days.

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.

Affected products

• Mozilla Thunderbird: before 128.10.1 (fixed in 128.10.1); from 129.0, before 138.0.1 (fixed in 138.0.1)

Published 2025-05-14. Last modified 2026-06-17.