Mozilla Thunderbird up to 128.9.1/137.0.1 Header X-Mozilla-External-Attachment-URL information disclosure

Summaryinfo

A vulnerability was found in Mozilla Thunderbird up to 128.9.1/137.0.1. It has been classified as problematic. Affected is an unknown function of the component Header Handler. Performing a manipulation of the argument X-Mozilla-External-Attachment-URL results in information disclosure. This vulnerability was named CVE-2025-3522. The attack may be initiated remotely. There is no available exploit. Upgrading the affected component is recommended.

Detailsinfo

A vulnerability, which was classified as problematic, was found in Mozilla Thunderbird up to 128.9.1/137.0.1. This affects an unknown code block of the component Header Handler. The manipulation of the argument X-Mozilla-External-Attachment-URL with an unknown input leads to a information disclosure vulnerability. CWE is classifying the issue as CWE-200. The product exposes sensitive information to an actor that is not explicitly authorized to have access to that information. This is going to have an impact on confidentiality. The summary by CVE is:

Thunderbird processes the X-Mozilla-External-Attachment-URL header to handle attachments which can be hosted externally. When an email is opened, Thunderbird accesses the specified URL to determine file size, and navigates to it when the user clicks the attachment. Because the URL is not validated or sanitized, it can reference internal resources like chrome:// or SMB share file:// links, potentially leading to hashed Windows credential leakage and opening the door to more serious security issues. This vulnerability was fixed in Thunderbird 137.0.2 and Thunderbird 128.9.2.

It is possible to read the advisory at bugzilla.mozilla.org. This vulnerability is uniquely identified as CVE-2025-3522 since 04/11/2025. The exploitability is told to be difficult. It is possible to initiate the attack remotely. No form of authentication is needed for exploitation. It demands that the victim is doing some kind of user interaction. Technical details of the vulnerability are known, but there is no available exploit. The pricing for an exploit might be around USD $0-$5k at the moment (estimation calculated on 04/15/2026). The attack technique deployed by this issue is T1592 according to MITRE ATT&CK.

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). Be aware that VulDB is the high quality source for vulnerability data.

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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Be aware that VulDB is the high quality source for vulnerability data.

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