What is CVE-2025-3859?

CVE-2025-3859 is a medium-severity vulnerability in Mozilla Firefox Focus, classified under URL Redirection to Untrusted Site (Open Redirect). CVSS score: 6.1/10. Published 2025-04-30.

How severe is CVE-2025-3859?

Medium severity. CVSS v3 base score is 6.1 out of 10.

Open Redirect in Mozilla Firefox Focus

CVE-2025-3859

Websites directing users to long URLs that caused eliding to occur in the location view could leverage the truncating behavior to potentially trick users into thinking they were on a different webpage. This vulnerability was fixed in Focus 138.

Vulnerability class: Open Redirect

Published 2025-04-30 · last modified 2026-06-17.

CVSS v3 metric

CVSS v3 base score 6.1 (Medium). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N.

EPSS exploit prediction

EPSS: 0.002 (9.3th percentile), scored 2026-09-30.

Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 9th percentile — 9.3% 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: 0.002 · change: +0.000

Affected products

Mozilla Firefox Focus
Mozilla Focus — versions 138

Weakness classification (CWE)

References

bugzilla.mozilla.org/show_bug.cgi (Issue Tracking)
www.mozilla.org/security/advisories/mfsa2025-33/ (Vendor Advisory)

Frequently asked questions

What is CVE-2025-3859?: CVE-2025-3859 is a medium-severity vulnerability in Mozilla Firefox Focus, classified under URL Redirection to Untrusted Site (Open Redirect). CVSS score: 6.1/10. Published 2025-04-30.
How severe is CVE-2025-3859?: Medium severity. CVSS v3 base score is 6.1 out of 10.