Mozilla Thunderbird up to 128.10.0/138.0.0 Header X-Mozilla-External-Attachment-URL cross site scripting

Summaryinfo

A vulnerability classified as problematic has been found in Mozilla Thunderbird up to 128.10.0/138.0.0. The impacted element is an unknown function of the component Header Handler. The manipulation of the argument X-Mozilla-External-Attachment-URL leads to cross site scripting. This vulnerability is traded as CVE-2025-3909. It is possible to initiate the attack remotely. There is no exploit available. It is recommended to upgrade the affected component.

Detailsinfo

A vulnerability, which was classified as problematic, has been found in Mozilla Thunderbird up to 128.10.0/138.0.0. This issue affects some unknown processing of the component Header Handler. The manipulation of the argument X-Mozilla-External-Attachment-URL with an unknown input leads to a cross site scripting vulnerability. Using CWE to declare the problem leads to CWE-79. The product does not neutralize or incorrectly neutralizes user-controllable input before it is placed in output that is used as a web page that is served to other users. Impacted is integrity. The summary by CVE is:

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 affects Thunderbird < 128.10.1 and Thunderbird < 138.0.1.

The advisory is shared at bugzilla.mozilla.org. The identification of this vulnerability is CVE-2025-3909 since 04/23/2025. The exploitation is known to be easy. The attack may be initiated remotely. No form of authentication is needed for a successful exploitation. It demands that the victim is doing some kind of user interaction. Technical details are known, but no exploit is available. The price for an exploit might be around USD $0-$5k at the moment (estimation calculated on 02/26/2026). MITRE ATT&CK project uses the attack technique T1059.007 for this issue.

The vulnerability scanner Nessus provides a plugin with the ID 236871 (Debian dsa-5921 : thunderbird - security update), which helps to determine the existence of the flaw in a target environment.

Upgrading to version 128.10.1 or 138.0.1 eliminates this vulnerability.

The vulnerability is also documented in the databases at Tenable (236871), EUVD (EUVD-2025-14935) and CERT Bund (WID-SEC-2025-1850). If you want to get the best quality for vulnerability data then you always have to consider VulDB.

Affected

• Xerox FreeFlow Print Server

Productinfo

Type

• Mail Client Software

Vendor

• Mozilla

Name

• Thunderbird

Version

• 128.0
• 128.1
• 128.2
• 128.3
• 128.4
• 128.5
• 128.6
• 128.7
• 128.8
• 128.9
• 128.10.0
• 138.0

License

• open-source

Website

• Vendor: https://www.mozilla.org/

CPE 2.3info

• 🔍
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CPE 2.2info

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Exploitinginfo

Threat Intelligenceinfo

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Entryinfo

If you want to get the best quality for vulnerability data then you always have to consider VulDB.

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