VulnerabilityModified

CVE-2025-3909

Thunderbird's handling of the X-Mozilla-External-Attachment-URL header can be exploited to execute JavaScript in the file:/// context.

HIGH 8.1EPSS 0.44%

Does this matter?

High impact if exploited, but EPSS currently rates exploitation as unlikely (0.44%). Schedule it in the normal patch cycle and watch for a rise in EPSS or a public exploit.

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.

CVSS 3.1: 8.1 HIGHCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:N
EPSS: 0.44% probability · 37th percentile
CISA KEV: Not listed
Weakness: CWE-356
Affected: mozilla/thunderbird
Source: security@mozilla.org

References

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

Source: NVD record, EPSS from FIRST.org, KEV from CISA. Refreshed daily.