What’s Vulnerable › Vulnerabilities

CVE-2025-3522: Leak of hashed Window credentials via crafted attachment URL

• Severity: medium (CVSS 6.3 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.3% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2025-04-15 · updated 2026-06-17
• Weakness: CWE-601

Description

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; from 129.0 before 137.0.2 — fixed in 128.9.2, 137.0.2

How to fix

Upgrade to a fixed version: Mozilla Thunderbird 128.9.2, 137.0.2. Check the vendor advisory for mitigations.

References

• https://bugzilla.mozilla.org/show_bug.cgi?id=1955372
• https://www.mozilla.org/security/advisories/mfsa2025-26/
• https://www.mozilla.org/security/advisories/mfsa2025-27/

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