CVE-2025-3909

CVE Details

Release Date:	2025-05-14
Impact:	Important	What is this?

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.

See more information about CVE-2025-3909 from MITRE CVE dictionary and NIST NVD

NOTE: The following CVSS metrics and score provided are preliminary and subject to review.

CVSS v3 metrics

Base Score:	6.5
Vector String:	CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N
Version:	3.1
Attack Vector:	Network
Attack Complexity:	Low
Privileges Required:	None
User Interaction:	None
Scope:	Unchanged
Confidentiality Impact:	Low
Integrity Impact:	Low
Availability Impact:	None

Errata information

Platform	Errata	Release Date
Oracle Linux version 8 (thunderbird)	ELSA-2025-8756	2025-06-10
Oracle Linux version 9 (thunderbird)	ELSA-2025-8203	2025-05-27