Vulnerability in Mozilla Thunderbird
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.
Published · last modified .
CVSS v3 metric
CVSS v3 base score 8.1 (High). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:N.
EPSS exploit prediction
EPSS: 0.004 (35.6th percentile), scored .
Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 36th percentile — 35.6% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.
EPSS over last 30 days · oldest: 0.004 · newest: 0.004 · change: 0.000
Affected products
- Mozilla Thunderbird — versions 128.10.1, 138.0.1
Weakness classification (CWE)
Public proof-of-concept exploits
References
- bugzilla.mozilla.org/show_bug.cgi (Permissions Required)
- www.mozilla.org/security/advisories/mfsa2025-34/ (Vendor Advisory)
- www.mozilla.org/security/advisories/mfsa2025-35/ (Vendor Advisory)
- af854a3a-2127-422b-91ae-364da2661108
Frequently asked questions
- What is CVE-2025-3909?
- CVE-2025-3909 is a high-severity vulnerability in Mozilla Thunderbird, classified under CWE-356. CVSS score: 8.1/10. Published 2025-05-14.
- How severe is CVE-2025-3909?
- High severity. CVSS v3 base score is 8.1 out of 10.
- Is CVE-2025-3909 known to be exploited?
- 1 public proof-of-concept repositories are indexed. Not currently listed in the CISA KEV catalog.