CVE-2025-3909
First published: Tue May 13 2025(Updated: )
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.
Credit: security@mozilla.org
| Affected Software | Affected Version | How to fix |
|---|---|---|
| Thunderbird | <128.10.1<138.0.1 | |
| Thunderbird | <128.10.1 | 128.10.1 |
| Thunderbird | <138.0.1 | 138.0.1 |
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Reference Links
Peer vulnerabilities
(Found alongside the following vulnerabilities)
Frequently Asked Questions
What is the severity of CVE-2025-3909?
CVE-2025-3909 is classified as a high severity vulnerability due to the potential for executing JavaScript in insecure contexts.
How do I fix CVE-2025-3909?
To fix CVE-2025-3909, update your Thunderbird application to versions 128.10.1 or 138.0.1 or later.
What versions of Thunderbird are affected by CVE-2025-3909?
CVE-2025-3909 affects Mozilla Thunderbird versions prior to 128.10.1 and 138.0.1.
What type of attack is possible with CVE-2025-3909?
CVE-2025-3909 allows for the execution of JavaScript code through crafted email attachments.
Is there a workaround for CVE-2025-3909?
Currently, the recommended approach for CVE-2025-3909 is to ensure you are using the latest version of Thunderbird, since no specific workaround is noted.
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