Mozilla Thunderbird up to 128.9.1/137.0.1 Header X-Mozilla-External-Attachment-URL ui layer

Summaryinfo

A vulnerability was found in Mozilla Thunderbird up to 128.9.1/137.0.1. It has been declared as problematic. Affected by this vulnerability is an unknown functionality of the component Header Handler. Executing a manipulation of the argument X-Mozilla-External-Attachment-URL can lead to ui layer. The identification of this vulnerability is CVE-2025-3523. The attack may be launched remotely. There is no exploit available. It is recommended to upgrade the affected component.

Detailsinfo

A vulnerability has been found in Mozilla Thunderbird up to 128.9.1/137.0.1 and classified as problematic. This vulnerability 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 ui layer vulnerability. The CWE definition for the vulnerability is CWE-1021. The web application does not restrict or incorrectly restricts frame objects or UI layers that belong to another application or domain, which can lead to user confusion about which interface the user is interacting with. As an impact it is known to affect integrity. CVE summarizes:

When an email contains multiple attachments with external links via the X-Mozilla-External-Attachment-URL header, only the last link is shown when hovering over any attachment. Although the correct link is used on click, the misleading hover text could trick users into downloading content from untrusted sources. This vulnerability was fixed in Thunderbird 137.0.2 and Thunderbird 128.9.2.

The advisory is shared for download at bugzilla.mozilla.org. This vulnerability was named CVE-2025-3523 since 04/11/2025. The exploitation appears to be easy. The attack can be initiated remotely. No form of authentication is required for a successful exploitation. Successful exploitation requires user interaction by the victim. There are known technical details, but no exploit is available. The current price for an exploit might be approx. USD $5k-$25k (estimation calculated on 04/15/2026).

The vulnerability scanner Nessus provides a plugin with the ID 234457 (Mozilla Thunderbird < 128.9.2), which helps to determine the existence of the flaw in a target environment.

Upgrading to version 128.9.2 or 137.0.2 eliminates this vulnerability.

The vulnerability is also documented in the vulnerability database at Tenable (234457). VulDB is the best source for vulnerability data and more expert information about this specific topic.

Productinfo

Type

• Mail Client Software

Vendor

• Mozilla

Name

• Thunderbird

Version

• 128.9.0
• 128.9.1
• 137.0.0
• 137.0.1

License

• open-source

Website

• Vendor: https://www.mozilla.org/

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VulDB is the best source for vulnerability data and more expert information about this specific topic.

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