Vulnerability · CVE-2025-3522
Thunderbird processes the X-Mozilla-External-Attachment-URL header to handle attachments which can be hosted externally.
Published Apr 15, 2025 · catalog updated Apr 13, 2026 · Thunderbird processes the X-Mozilla-External-Attachment-URL
Check if you run it
No confirmed exploitation is on record for CVE-2025-3522. EPSS is 0.27%. Act only if a listed version of Thunderbird processes the X-Mozilla-External-Attachment-URL is installed.
Gaps on this record
- The stored product name is a package slug. The name on this page is taken from the title (Thunderbird processes the X-Mozilla-External-Attachment-URL).
- No version range is stored. A customer cannot check an installed version against this record.
- No vendor fix or required action is stored. Compare the linked advisory before you change production.
- Severity
- Medium6.3
- Known exploitation
- Not in CISA/VulnCheck KEV
- EPSS
- 0.27%
- Remediation
- Advisory link
- Affected products
- vendor mozilla
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.
Are you affected?
Enter the version you run. The result uses only the ranges stored on this record.
The check returns affected, not affected, or incomplete. It does not guess a version that is missing from the record.
- vendor mozillaversion unspecified
How to fix CVE-2025-3522
Recommended remediation
A patch or advisory reference is linked. Open the original publisher for availability by product version.
Official advisories
What to do
Three questions, each answered from this record only. Confirm the vendor advisory before you patch.
Should you patch?
No confirmed exploitation is on record for CVE-2025-3522. EPSS is 0.27%. Act only if a listed version of Thunderbird processes the X-Mozilla-External-Attachment-URL is installed.
Which versions are affected?
Affected version ranges are not on this record. Compare the installed version with the linked vendor advisory before you change production.
What should an analyst verify?
No weakness class is stored. EPSS 0.27%. There is no CISA KEV listing. No Exploit-DB or Metasploit reference is linked. Use the source links for the original advisory.
Code
The project this record applies to. Exploit listings stay in References.
No public source repository is identified for this product.
Timeline
Detection, updates, and resolution for this record.
Apr 15, 2025 · EUVD
Detected
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, potentia
Apr 13, 2026 · EUVD
Updated
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, potentia
Sources
- CVE.orgVulnerability recordCVE-2025-3522
Official CVE identifier
cve.org - FIRST EPSSExploitation probabilityCVE-2025-3522
30-day exploitation probability 0.3%
first.org - EUVDVulnerability recordEUVD-2025-10965
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 UR
euvd.enisa.europa.eu
References
Freshness
Last verified in this catalog Oct 18, 2026. Significant claims (KEV, EPSS, CVSS, affected ranges) come from the source rows above — open the original publisher for the authoritative advisory.