CVE-2025-3909
HIGHDescription
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.
How to fix
This vulnerability affects an unusually large number of packages. The highest-confidence fixes are shown above; the full list is longer. Check the referenced advisories for complete coverage.
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Remediation is compiled from vendor and distribution security advisories. Always confirm against the linked source for your exact version and platform.
CVSS v3 Vector
Exploitability
Impact
CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:N
Exploit Intelligence
Low risk: more likely to be exploited than 36% of all known CVEs.
References
Related Vulnerabilities
Other CWE-356 vulnerabilities, ordered by exploit likelihood. View all
| CVE | Severity | CVSS | EPSS | Exploited | Fix |
|---|---|---|---|---|---|
| CVE-2018-16858 | High | 7.8 | 67% | - | Fix |
| CVE-2019-6737 | High | 8.8 | 3.8% | - | - |
| CVE-2019-6738 | High | 8.8 | 3.7% | - | - |
| CVE-2019-6736 | High | 8.8 | 3.7% | - | - |
| CVE-2019-13322 | High | 8.8 | 2.6% | - | Fix |
| CVE-2019-17151 | Medium | 5.4 | 1.4% | - | Fix |
Common questions
How do I fix CVE-2025-3909?
Published advisories record a fix for 9 affected products. The "How to fix" section on this page lists the fixed version and source advisory for each one, so apply the entry matching what you actually run.
Is CVE-2025-3909 being actively exploited?
Not that we know of. CVE-2025-3909 is not in the CISA Known Exploited Vulnerabilities catalog. Its EPSS score of 0.44% is the estimated probability that it will be exploited in the next 30 days. That is higher than 36% of all scored CVEs.
How severe is CVE-2025-3909?
CVE-2025-3909 has a CVSS v3 base score of 8.1, rated high. CVSS rates the technical impact if the vulnerability is exploited, not how likely that is, so weigh it alongside the exploit-prediction score when you decide what to patch first.
What does CVE-2025-3909 affect?
Published advisories record a fix for thunderbird (Debian), thunderbird (Red Hat / RHEL), thunderbird (Rocky), thunderbird-debuginfo (Red Hat / RHEL), and 5 more. Only products with a sourced advisory are listed, so treat this as what we can cite rather than a complete inventory.
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This product uses NVD data but is not endorsed or certified by the NVD. EPSS scores courtesy of FIRST.org (https://www.first.org/epss). Source: CISA KEV Catalog. Data as of 2026-04-13.