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CVE-2025-3909

8.1 · HIGH

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

Published May 14, 2025CWE-356EPSS 0.47%

Overview

CVE-2025-3909 is a high-severity vulnerability. It was published on May 14, 2025 and has a CVSS 3.1 base score of 8.1 (HIGH).

This vulnerability has a CVSS 3.1 base score of 8.1, rated HIGH. It can be exploited remotely over the network. No authentication or special privileges are required for exploitation.

Technical 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.

Remediation

Check the references section for vendor advisories, patches, and mitigation guidance. If immediate patching is not possible, review the CVSS vector to understand the attack surface and apply compensating controls such as network segmentation or access restrictions.

References

Refbugzilla.mozilla.org/show_bug.cgi?id=1958376Refwww.mozilla.org/security/advisories/mfsa2025-34/Refwww.mozilla.org/security/advisories/mfsa2025-35/Reflists.debian.org/debian-lts-announce/2025/05/msg00022.html

How CyberStrike helps

Frequently asked questions

What is CVE-2025-3909?

CVE-2025-3909 is a high-severity vulnerability. It was published on May 14, 2025 and has a CVSS 3.1 base score of 8.1 (HIGH).

How severe is CVE-2025-3909?

This vulnerability has a CVSS 3.1 base score of 8.1, rated HIGH. It can be exploited remotely over the network. No authentication or special privileges are required for exploitation.

How do I fix or remediate CVE-2025-3909?

Check the references section for vendor advisories, patches, and mitigation guidance. If immediate patching is not possible, review the CVSS vector to understand the attack surface and apply compensating controls such as network segmentation or access restrictions.

How can CyberStrike help with CVE-2025-3909?

CyberStrike is an open-source, AI-augmented pentesting engine. Rather than flag a version number, it tests whether an issue like CVE-2025-3909 is actually reachable in your own applications and APIs, then verifies each finding before reporting it. That tells you if the weakness is a real, exploitable risk in your environment or a false positive you can safely deprioritize.

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