Open Redirect in Mozilla Thunderbird
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.
Vulnerability class: Open Redirect
Published · last modified .
CVSS v3 metric
CVSS v3 base score 6.3 (Medium). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:L/I:L/A:L.
EPSS exploit prediction
EPSS: 0.003 (17.2th percentile), scored .
Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 17th percentile — 17.2% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.
EPSS over last 30 days · oldest: 0.003 · newest: 0.003 · change: +0.000
Affected products
- Mozilla Thunderbird — versions 128.9.2, 137.0.2
Weakness classification (CWE)
Public proof-of-concept exploits
References
- bugzilla.mozilla.org/show_bug.cgi (Permissions Required)
- www.mozilla.org/security/advisories/mfsa2025-26/ (Vendor Advisory)
- www.mozilla.org/security/advisories/mfsa2025-27/ (Vendor Advisory)
Frequently asked questions
- What is CVE-2025-3522?
- CVE-2025-3522 is a medium-severity vulnerability in Mozilla Thunderbird, classified under URL Redirection to Untrusted Site (Open Redirect). CVSS score: 6.3/10. Published 2025-04-15.
- How severe is CVE-2025-3522?
- Medium severity. CVSS v3 base score is 6.3 out of 10.
- Is CVE-2025-3522 known to be exploited?
- 1 public proof-of-concept repositories are indexed. Not currently listed in the CISA KEV catalog.