8.1
High
CVE-2025-3909 Thunderbird's handling of the X-Mozilla-External-Attachment-URL header can be exploited to execute JavaScript in the…
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 and versions
| Product | Affected versions | Fixed in |
|---|---|---|
| Thunderbird 138.0 | earliest → < 138.0.1 | 138.0.1 |
| Thunderbird 128.10 | earliest → < 128.10.1 | 128.10.1 |
Other CVEs of Thunderbird
The 1713 CVEs of Thunderbird →
CVE-2026-118180
Matrix homeserver could redirect the chat session to a local file or network share
7.5
High
CVE-2026-118179
Request forgery via news: links
8.8
High
CVE-2026-118178
RSS feed could redirect its subscription to a local file or network share
5.7
Medium
CVE-2026-118177
Calendar discovery accepted local file and remote network share locations
9.1
Critical