CVE-2026-4371

Name	CVE-2026-4371
Description	A malicious mail server could send malformed strings with negative lengths, causing the parser to read memory outside the buffer. If a mail server or connection to a mail server were compromised, an attacker could cause the parser to malfunction, potentially crashing Thunderbird or leaking sensitive data. This vulnerability affects Thunderbird < 149 and Thunderbird < 140.9.
Source	CVE (at NVD; CERT, ENISA, LWN, oss-sec, fulldisc, Debian ELTS, Red Hat, Ubuntu, Gentoo, SUSE bugzilla/CVE, GitHub advisories/code/issues, web search, more)

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)	1:140.9.0esr-1~deb11u1	vulnerable
	bookworm	1:140.6.0esr-1~deb12u1	vulnerable
	bookworm (security)	1:140.8.0esr-1~deb12u1	vulnerable
	trixie (security), trixie	1:140.8.0esr-1~deb13u1	vulnerable
	forky, sid	1:140.8.0esr-1	vulnerable

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

Package	Type	Release	Fixed Version	Urgency	Origin	Debian Bugs
thunderbird	source	(unstable)	(unfixed)

https://www.mozilla.org/en-US/security/advisories/mfsa2026-24/#CVE-2026-4371