CVE-2025-3522: thunderbird Open Redirect — Fix

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.

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CVSS v3.1 Score

6.3

MEDIUM

CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:L/I:L/A:L

Weakness Type (CWE)

CWE-601 CWE-601

Affected Products

Vendor	Product	Versions
mozilla	thunderbird	< 128.9.2
mozilla	thunderbird	129.0 — 137.0.2

References

Advisories & Patches

https://www.mozilla.org/security/advisories/mfsa2025-26/ https://www.mozilla.org/security/advisories/mfsa2025-27/

Other References

https://bugzilla.mozilla.org/show_bug.cgi?id=1955372

Frequently Asked Questions

What is CVE-2025-3522? +

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 has a CVSS v3.1 base score of 6.3 (MEDIUM).

How severe is CVE-2025-3522? +

CVE-2025-3522 has a CVSS v3.1 score of 6.3 out of 10, rated MEDIUM. This is a medium-severity vulnerability that should be remediated as part of regular maintenance.

What products are affected by CVE-2025-3522? +

CVE-2025-3522 affects products from mozilla, specifically: thunderbird. Check the affected products table above for specific version ranges.

How do I check if I'm vulnerable to CVE-2025-3522? +

You can use Secably's free Website Scanner to check your website for known vulnerabilities. For infrastructure scanning, use the Port Scanner to identify exposed services that may be affected. Check the vendor advisories linked above for specific patch and version information.

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