#VU109192 Input validation error in Mozilla Thunderbird - CVE-2025-3909

Vulnerability identifier: #VU109192

Vulnerability risk: High

CVSSv4.0: 6 [CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:A/VC:H/VI:H/VA:N/SC:N/SI:N/SA:N/E:U/U:Amber]

CVE-ID: CVE-2025-3909

CWE-ID: CWE-20

Exploitation vector: Network

Exploit availability: No

Vulnerable software:
Mozilla Thunderbird
Client/Desktop applications / Messaging software

Vendor: Mozilla

Description

The vulnerability allows a remote attacker to execute arbitrary JavaScript code.

The vulnerability exists due to incorrect handling of the X-Mozilla-External-Attachment-URL header. A remote attacker can create a nested email attachment, set its content type to application/pdf and force the application to execute arbitrary JavaScript code in the file:/// context.

Mitigation
Install updates from vendor's website.

Vulnerable software versions

Mozilla Thunderbird: 128.0, 128.0.1, 128.1.0, 128.1.1 - 135.0.1, 128.2.0, 128.2.1, 128.2.2, 128.2.3, 128.3.0, 128.3.1, 128.3.2, 128.3.3, 128.4.0, 128.4.1, 128.4.2, 128.4.3, 128.4.4, 128.5.0, 128.5.1, 128.5.2, 128.6.0, 128.6.1, 128.7.0, 128.7.1, 128.8.0, 128.8.1, 128.9.0, 128.9.1, 128.9.2, 128.10, 130.0, 132.0, 132.0.1, 133.0, 134.0, 135.0, 136.0, 136.0.1, 137.0, 137.0.1, 137.0.2, 138.0

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External links
https://www.mozilla.org/en-US/security/advisories/mfsa2025-34/
https://bugzilla.mozilla.org/show_bug.cgi?id=1958376
https://www.mozilla.org/en-US/security/advisories/mfsa2025-35/

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Q & A

Can this vulnerability be exploited remotely?

Yes. This vulnerability can be exploited by a remote non-authenticated attacker via the Internet.

Is there known malware, which exploits this vulnerability?

No. We are not aware of malware exploiting this vulnerability.