Vulnerability in Mozilla Firefox
CVE-2025-6434
The exception page for the HTTPS-Only feature, displayed when a website is opened via HTTP, lacked an anti-clickjacking delay, potentially allowing an attacker to trick a user into granting an exception and loading a webpage over HTTP. This vulnerability was fixed in Firefox 140 and Thunderbird 140.
Published · last modified .
CVSS v3 metric
CVSS v3 base score 4.3 (Medium). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:L/A:N.
EPSS exploit prediction
EPSS: 0.003 (18.7th percentile), scored .
Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 19th percentile — 18.7% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.
EPSS over last 30 days · oldest: 0.002 · newest: 0.003 · change: +0.000
Affected products
- Mozilla Firefox — versions 140
- Mozilla Thunderbird — versions 140
Weakness classification (CWE)
References
- bugzilla.mozilla.org/show_bug.cgi (Permissions Required)
- www.mozilla.org/security/advisories/mfsa2025-51/ (Vendor Advisory)
- www.mozilla.org/security/advisories/mfsa2025-54/
Frequently asked questions
- What is CVE-2025-6434?
- CVE-2025-6434 is a medium-severity vulnerability in Mozilla Firefox, classified under Improper Restriction of Rendered UI Layers or Frames (Clickjacking). CVSS score: 4.3/10. Published 2025-06-24.
- How severe is CVE-2025-6434?
- Medium severity. CVSS v3 base score is 4.3 out of 10.