What’s Vulnerable › Vulnerabilities

CVE-2025-3909: JavaScript Execution via Spoofed PDF Attachment and file:/// Link

• Severity: high (CVSS 8.1 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.5% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2025-05-14 · updated 2026-06-17
• Weakness: CWE-356

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.

Affected products

• Mozilla Thunderbird: before 128.10.1; from 129.0 before 138.0.1 — fixed in 128.10.1, 138.0.1

How to fix

Upgrade to a fixed version: Mozilla Thunderbird 128.10.1, 138.0.1. Check the vendor advisory for mitigations.

References

• https://bugzilla.mozilla.org/show_bug.cgi?id=1958376
• https://www.mozilla.org/security/advisories/mfsa2025-34/
• https://www.mozilla.org/security/advisories/mfsa2025-35/
• https://lists.debian.org/debian-lts-announce/2025/05/msg00022.html

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