CVE-2025-3909: thunderbird — Fix

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.

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CVSS v3.1 Score

8.1

HIGH

CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:N

Weakness Type (CWE)

CWE-356 CWE-356

Affected Products

Vendor	Product	Versions
mozilla	thunderbird	< 128.10.1
mozilla	thunderbird	129.0 — 138.0.1

References

Advisories & Patches

https://www.mozilla.org/security/advisories/mfsa2025-34/ https://www.mozilla.org/security/advisories/mfsa2025-35/

Other References

https://bugzilla.mozilla.org/show_bug.cgi?id=1958376 https://lists.debian.org/debian-lts-announce/2025/05/msg00022.html

Frequently Asked Questions

What is 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 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. It has a CVSS v3.1 base score of 8.1 (HIGH).

How severe is CVE-2025-3909? +

CVE-2025-3909 has a CVSS v3.1 score of 8.1 out of 10, rated HIGH. This is a high-severity vulnerability that should be prioritized for patching.

What products are affected by CVE-2025-3909? +

CVE-2025-3909 affects products from mozilla, specifically: thunderbird. Check the affected products table above for specific version ranges.

How do I check if I'm vulnerable to CVE-2025-3909? +

You can use Secably's free Website Scanner to check your website for known vulnerabilities. For infrastructure scanning, use the Port Scanner to identify exposed services that may be affected. Check the vendor advisories linked above for specific patch and version information.

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