CVE-2025-3909 Information

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 affects Thunderbird < 128.10.1 and Thunderbird < 138.0.1.

Reference

https://bugzilla.mozilla.org/show_bug.cgi?id=1958376 https://www.mozilla.org/security/advisories/mfsa2025-34/ https://www.mozilla.org/security/advisories/mfsa2025-35/