Input validation error in Mozilla Thunderbird - CVE-2025-3909

Vulnerability identifier: #VU109192

CSH Severity: High

CVSS v4.0: CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:A/VC:H/VI:H/VA:N/SC:N/SI:N/SA:N/E:U/U:Amber

CVE-ID: CVE-2025-3909

CWE-ID: CWE-20

Exploitation vector: Remote access

Exploit availability: No public exploit available

Affected software:
Mozilla Thunderbird
Debian Linux
SUSE Linux Enterprise Desktop 15
SUSE Linux Enterprise Real Time 15
SUSE Linux Enterprise Server 15
SUSE Linux Enterprise Server for SAP Applications 15
SUSE Linux Enterprise Workstation Extension 15
Red Hat Enterprise Linux for ARM 64
Anolis OS
Red Hat Enterprise Linux for Power, little endian
Red Hat Enterprise Linux for IBM z Systems
Red Hat Enterprise Linux for x86_64
Red Hat Enterprise Linux Server - AUS
Red Hat Enterprise Linux Server - TUS
Red Hat Enterprise Linux for ARM 64 - Extended Update Support
Red Hat Enterprise Linux for Power, little endian - Extended Update Support
Red Hat Enterprise Linux for IBM z Systems - Extended Update Support
Red Hat Enterprise Linux for x86_64 - Extended Update Support
Slackware Linux
SUSE Package Hub 15
openSUSE Leap
Ubuntu
openEuler
Red Hat Enterprise Linux Server for Power LE - Update Services for SAP Solutions
mozilla-thunderbird
thunderbird (Debian package)
thunderbird (Red Hat package)
MozillaThunderbird-translations-other
MozillaThunderbird-translations-common
MozillaThunderbird-debuginfo
MozillaThunderbird
MozillaThunderbird-debugsource
thunderbird
thunderbird-debuginfo
thunderbird-debugsource
thunderbird-librnp-rnp
thunderbird-wayland
thunderbird (Ubuntu package)

The vulnerability allows a remote attacker to execute arbitrary JavaScript code.

The vulnerability exists due to incorrect handling of the X-Mozilla-External-Attachment-URL header. A remote attacker can create a nested email attachment, set its content type to application/pdf and force the application to execute arbitrary JavaScript code in the file:/// context.

Install updates from vendor's website.

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