What is CVE-2020-15654?

CVE-2020-15654 is a medium-severity vulnerability in Mozilla Firefox, classified under Loop with Unreachable Exit Condition (Infinite Loop). CVSS score: 6.5/10. Published 2020-08-10.

How severe is CVE-2020-15654?

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

Is CVE-2020-15654 known to be exploited?

2 public proof-of-concept repositories are indexed. Not currently listed in the CISA KEV catalog.

Vulnerability in Mozilla Firefox

CVE-2020-15654

When in an endless loop, a website specifying a custom cursor using CSS could make it look like the user is interacting with the user interface, when they are not. This could lead to a perceived broken state, especially when interactions with existing browser dialogs and warnings do not work. This vulnerability affects Firefox ESR < 78.1, Firefox < 79, and Thunderbird < 78.1.

Published 2020-08-10 · last modified 2026-06-17.

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:N/I:H/A:N.

EPSS exploit prediction

EPSS: 0.014 (71.8th percentile), scored 2026-09-29.

Low probability of exploitation in the next 30 days, but still tracked. 72nd percentile — 71.8% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.

EPSS over last 30 days · oldest: 0.012 · newest: 0.014 · change: +0.002

Affected products

Mozilla Firefox — versions unspecified
Mozilla Firefox Esr — versions unspecified
Mozilla Firefox Esr
Mozilla Thunderbird — versions unspecified
Canonical Ubuntu Linux — versions 16.04, 18.04, 20.04

Weakness classification (CWE)

CWE-835 — Loop with Unreachable Exit Condition (Infinite Loop)

Public proof-of-concept exploits

References

www.mozilla.org/security/advisories/mfsa2020-30/ (Vendor Advisory)
www.mozilla.org/security/advisories/mfsa2020-32/ (Vendor Advisory)
www.mozilla.org/security/advisories/mfsa2020-33/ (Vendor Advisory)
bugzilla.mozilla.org/show_bug.cgi (Issue Tracking, Vendor Advisory)
lists.opensuse.org/opensuse-security-announce/2020-08/msg00025.html (vendor-advisory, Third Party Advisory)
usn.ubuntu.com/4443-1/ (vendor-advisory, Third Party Advisory)

Frequently asked questions

What is CVE-2020-15654?: CVE-2020-15654 is a medium-severity vulnerability in Mozilla Firefox, classified under Loop with Unreachable Exit Condition (Infinite Loop). CVSS score: 6.5/10. Published 2020-08-10.
How severe is CVE-2020-15654?: Medium severity. CVSS v3 base score is 6.5 out of 10.
Is CVE-2020-15654 known to be exploited?: 2 public proof-of-concept repositories are indexed. Not currently listed in the CISA KEV catalog.