What is CVE-2020-15656?

CVE-2020-15656 is a high-severity vulnerability in Mozilla Firefox, classified under Access of Resource Using Incompatible Type (Type Confusion). CVSS score: 8.8/10. Published 2020-08-10.

How severe is CVE-2020-15656?

High severity. CVSS v3 base score is 8.8 out of 10.

Is CVE-2020-15656 known to be exploited?

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

Vulnerability in Mozilla Firefox

CVE-2020-15656

JIT optimizations involving the Javascript arguments object could confuse later optimizations. This risk was already mitigated by various precautions in the code, resulting in this bug rated at only moderate severity. 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 8.8 (High). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H.

EPSS exploit prediction

EPSS: 0.015 (73.4th percentile), scored 2026-09-29.

Low probability of exploitation in the next 30 days, but still tracked. 73rd percentile — 73.4% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.

EPSS over last 30 days · oldest: 0.015 · newest: 0.015 · change: 0.000

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
openSUSE Leap — versions 15.2

Weakness classification (CWE)

CWE-843 — Access of Resource Using Incompatible Type (Type Confusion)

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 (Permissions Required, 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-2020-15656?: CVE-2020-15656 is a high-severity vulnerability in Mozilla Firefox, classified under Access of Resource Using Incompatible Type (Type Confusion). CVSS score: 8.8/10. Published 2020-08-10.
How severe is CVE-2020-15656?: High severity. CVSS v3 base score is 8.8 out of 10.
Is CVE-2020-15656 known to be exploited?: 8 public proof-of-concept repositories are indexed. Not currently listed in the CISA KEV catalog.