CVE-2025-3522: Mozilla Thunderbird

Medium severity, CVSS 6.3. EPSS: 0.3% chance of exploitation in the next 30 days.

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.

Affected products

• Mozilla Thunderbird: before 128.9.2 (fixed in 128.9.2); from 129.0, before 137.0.2 (fixed in 137.0.2)

Published 2025-04-15. Last modified 2026-06-17.