CVE-2025-3909

Name	CVE-2025-3909
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.
Source	CVE (at NVD; CERT, LWN, oss-sec, fulldisc, Red Hat, Ubuntu, Gentoo, SUSE bugzilla/CVE, GitHub advisories/code/issues, web search, more)
References	DLA-4167-1, DSA-5921-1

Vulnerable and fixed packages

The table below lists information on source packages.

Source Package	Release	Version	Status
thunderbird (PTS)	bullseye	1:115.12.0-1~deb11u1	vulnerable
	bullseye (security)	1:128.10.1esr-1~deb11u1	fixed
	bookworm	1:128.10.0esr-1~deb12u1	vulnerable
	bookworm (security)	1:128.10.1esr-1~deb12u1	fixed
	trixie	1:128.10.0esr-1	vulnerable
	sid	1:128.10.1esr-1	fixed

The information below is based on the following data on fixed versions.

Package	Type	Release	Fixed Version	Origin
thunderbird	source	bullseye	1:128.10.1esr-1~deb11u1	DLA-4167-1
thunderbird	source	bookworm	1:128.10.1esr-1~deb12u1	DSA-5921-1
thunderbird	source	(unstable)	1:128.10.1esr-1

https://www.mozilla.org/en-US/security/advisories/mfsa2025-34/#CVE-2025-3909