CVE-2025-3909: Thunderbird's handling of the X-Mozilla-External-Attachment-URL header can be exploited to execute JavaScript in the file:/// context. By crafting...
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.
Classification
CVE ID: CVE-2025-3909
CVSS Base Severity: MEDIUM
CVSS Base Score: 6.5
CVSS Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N
Problem Types
JavaScript Execution via Spoofed PDF Attachment and file:/// LinkAffected Products
Vendor: Mozilla
Product: Thunderbird
Exploit Prediction Scoring System (EPSS)
EPSS Score: 0.06% (probability of being exploited)
EPSS Percentile: 17.47% (scored less or equal to compared to others)
EPSS Date: 2025-06-12 (when was this score calculated)
References
https://nvd.nist.gov/vuln/detail/CVE-2025-3909https://bugzilla.mozilla.org/show_bug.cgi?id=1958376
https://www.mozilla.org/security/advisories/mfsa2025-34/
https://www.mozilla.org/security/advisories/mfsa2025-35/