CVE-2025-3909

The Mozilla Foundation's Security Advisory describes the following issue: 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.

Multiple security issues were discovered in Thunderbird, which could result in spoofing of From: mail headers, execution of JavaScript or information disclosure. For the stable distribution (bookworm), these problems have been fixed in version 1:128.10.1esr-1~deb12u1.