What is CVE-2020-15661?

CVE-2020-15661 is a medium-severity vulnerability in Mozilla Firefox, classified under Insufficiently Protected Credentials. CVSS score: 6.5/10. Published 2020-08-10.

How severe is CVE-2020-15661?

Medium severity. CVSS v3 base score is 6.5 out of 10.

Vulnerability in Mozilla Firefox

CVE-2020-15661

A rogue webpage could override the injected WKUserScript used by the logins autofill, this exploit could result in leaking a password for the current domain. This vulnerability affects Firefox for iOS < 28.

Published 2020-08-10 · last modified 2026-08-19.

CVSS v3 metric

CVSS v3 base score 6.5 (Medium). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:N/A:N.

EPSS exploit prediction

EPSS: 0.008 (56.4th percentile), scored 2026-10-01.

Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 56th percentile — 56.4% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.

EPSS over last 30 days · oldest: 0.008 · newest: 0.008 · change: 0.000

Affected products

Mozilla Firefox
Mozilla Firefox For iOS — versions unspecified
Mozilla Firefox Mobile

Weakness classification (CWE)

CWE-522 — Insufficiently Protected Credentials

References

www.mozilla.org/security/advisories/mfsa2020-34/ (Vendor Advisory)
bugzilla.mozilla.org/show_bug.cgi (Permissions Required, Issue Tracking, Vendor Advisory)

Frequently asked questions

What is CVE-2020-15661?: CVE-2020-15661 is a medium-severity vulnerability in Mozilla Firefox, classified under Insufficiently Protected Credentials. CVSS score: 6.5/10. Published 2020-08-10.
How severe is CVE-2020-15661?: Medium severity. CVSS v3 base score is 6.5 out of 10.