No exploitation reportedFix available

CVE-2020-15656

Q: Is CVE-2020-15656 actively exploited?

Active exploitation of CVE-2020-15656 has not been confirmed. Its EPSS score was 1.5% on 2026-10-22, the estimated probability of exploitation in the next 30 days.

Q: What is the CVSS score for CVE-2020-15656?

CVE-2020-15656 has a CVSS v3 base score of 8.8 (HIGH severity), with vector string CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H.

No exploitation reported by the sources we read.

Published2020-08-10
WeaknessCWE-843

Am I affected?

With a domain, we read the version its public headers and pages expose. A distribution build can carry the fix under an older number.

Description

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.

Timeline

dates in UTC

2020-08-1018:15CVE publishedNVD

Observed activity

isMalicious corpus · updated 2026-10-23 18:48 UTC

Indicators tagged with this CVE, per day0 indicators over 7 days

Block these indicators

Firewall list (EDL, text): https://api.ismalicious.com/edl/cve/CVE-2020-15656?format=txt
CSV export: https://api.ismalicious.com/edl/cve/CVE-2020-15656?format=csv
TAXII collection of exploited CVEs
The EDL and CSV take an API key on the Pro plan.

Versions per branch

NVD

Canonical ubuntu linux

Branch	Affected	Fixed in	Source
16.4	= 16.04	no fix published	NVD
18.4	= 18.04	no fix published	NVD
20.4	= 20.04	no fix published	NVD

Mozilla firefox

Branch	Affected	Fixed in	Source
79.0	< 79.0	79.0	NVD

Mozilla firefox esr

Branch	Affected	Fixed in	Source
78.1	< 78.1	78.1	NVD

2 more products in the sources.

as publishedRanges as NVD and OSV publish them; a distribution’s build can be fixed under an older number.

Fix

Advisories and patches

bugzilla.mozilla.orgVendor Advisory
www.mozilla.orgVendor Advisory
www.mozilla.orgVendor Advisory
www.mozilla.orgVendor Advisory

Detection

Nuclei template: none so far
Exploit-DB: no entry
Proof-of-concept repositories: none listed

Sources

NVDCVSS 3.18.8read 2026-10-23
NVDCVSS 2.09.3read 2026-10-23
NVDweakness (CWE)CWE-843read 2026-10-23
FIRST EPSSEPSS score1.5 %2026-10-22

Technical details

CVSS v3 Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H
Published: 2020-08-10
Last Modified: 2026-06-17
NVD status: Modified

Frequently asked questions

What is CVE-2020-15656?

Is CVE-2020-15656 actively exploited?

Active exploitation of CVE-2020-15656 has not been confirmed. Its EPSS score was 1.5% on 2026-10-22, the estimated probability of exploitation in the next 30 days.

What is the CVSS score for CVE-2020-15656?

CVE-2020-15656 has a CVSS v3 base score of 8.8 (HIGH severity), with vector string CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H.

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