CVE-2025-3909 JavaScript Execution via Spoofed PDF Attachment and file:/// Link
Attackers can execute JavaScript in the file:/// context by spoofing PDF attachments, potentially compromising user systems.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- A user has to click or open something
- Fix
- Available
What is CVE-2025-3909?
CVE-2025-3909 is a high-severity cross-site scripting vulnerability in Mozilla Thunderbird, published on 14 May 2025. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.
Official 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 was fixed in Thunderbird 128.10.1 and Thunderbird 138.0.1.
Weakness: CWE-356
Which versions are affected by CVE-2025-3909?
It affects 1 product.
Products include Mozilla Thunderbird.
How do I fix CVE-2025-3909?
Upgrade to a fixed version.
Escape output for the context it lands in and set a strict Content Security Policy. Upgrading the affected component is the real fix. More about Cross-site scripting.
Is CVE-2025-3909 being exploited?
There is no known exploitation of CVE-2025-3909 in the wild and no public exploit code.
Other vulnerabilities in Mozilla Thunderbird
Sources
Also known as DEBIAN-CVE-2025-3909, GHSA-h6cg-6m9j-xj9g, UBUNTU-CVE-2025-3909.