Arbitrary file upload in Mozilla Firefox
CVE-2025-6435
If a user saved a response from the Network tab in Devtools using the Save As context menu option, that file may not have been saved with the `.download` file extension. This could have led to the user inadvertently running a malicious executable. This vulnerability was fixed in Firefox 140 and Thunderbird 140.
Vulnerability class: Unrestricted File Upload
Published · last modified .
CVSS v3 metric
CVSS v3 base score 8.1 (High). Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H.
EPSS exploit prediction
EPSS: 0.005 (37.5th percentile), scored .
Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 37th percentile — 37.5% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.
EPSS over last 30 days · oldest: 0.004 · newest: 0.005 · change: +0.001
Affected products
- Mozilla Firefox — versions 140
- Mozilla Thunderbird — versions 140
Weakness classification (CWE)
References
- bugzilla.mozilla.org/show_bug.cgi (Permissions Required)
- bugzilla.mozilla.org/show_bug.cgi (Permissions Required)
- www.mozilla.org/security/advisories/mfsa2025-51/ (Vendor Advisory)
- www.mozilla.org/security/advisories/mfsa2025-54/ (Vendor Advisory)
Frequently asked questions
- What is CVE-2025-6435?
- CVE-2025-6435 is a high-severity vulnerability in Mozilla Firefox, classified under Unrestricted Upload of File with Dangerous Type. CVSS score: 8.1/10. Published 2025-06-24.
- How severe is CVE-2025-6435?
- High severity. CVSS v3 base score is 8.1 out of 10.