Vulnerability Monitor

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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 affects Thunderbird < 128.10.1 and Thunderbird < 138.0.1.

Published

2025-05-14T17:15:48.660

Last Modified

2025-11-03T20:18:48.220

Status

Modified

Source

[email protected]

Severity

CVSSv3.1: 6.5 (MEDIUM)

Weaknesses

Type: Secondary
CWE-290

Affected Vendors & Products

Type	Vendor	Product	Version/Range	Vulnerable?
Application	mozilla	thunderbird	< 128.10.1	Yes
Application	mozilla	thunderbird	< 138.0.1	Yes

References

https://bugzilla.mozilla.org/show_bug.cgi?id=1958376 Permissions Required ([email protected])
https://www.mozilla.org/security/advisories/mfsa2025-34/ Vendor Advisory ([email protected])
https://www.mozilla.org/security/advisories/mfsa2025-35/ Vendor Advisory ([email protected])
https://lists.debian.org/debian-lts-announce/2025/05/msg00022.html (af854a3a-2127-422b-91ae-364da2661108)