CVE-2025-3522

What it 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.

At a glance

Exploitability

Can be exploited over the network without logging in, but needs a user to act. Limited direct impact.

Exploit odds

0.3% chance of exploitation in the next 30 days (EPSS).

Fix

A patch, advisory or fixed version is referenced. Upgrade or apply it.

Timeline

1. 2025-04-15 15:16 UTC PublishedDisclosed publicly on CVE.org and NVD
2. 2026-06-17 09:20 UTC Last updatedMost recent change to the record

Details

Published

2025-04-15 15:16 UTC

Last modified

2026-06-17 09:20 UTC

Exploit probability

0.3% · EPSS, next 30 days

Weakness

CWE-601

Record status

Modified

Affected

• Mozilla Thunderbird < 128.9.2
• Mozilla Thunderbird < 137.0.2

Attack profile CVSS 3.1 · 6.3

Attack vector

Network

Complexity

Low

Privileges needed

None

User interaction

Required

Scope

Unchanged

Confidentiality

Low

Integrity

Low

Availability

Low