Exploited CVEs / Mozilla / CVE-2020-6820

CVE-2020-6820

Mozilla Firefox And Thunderbird Use-After-Free Vulnerability. Exploitation in the wild is confirmed by CISA, not predicted. Federal agencies were required to remediate by 2022-05-03 (that deadline has passed); treat that date as the outer bound for your own environment. The fix is in the vendor advisory: Mozilla Foundation Security Advisories, linked below.

Risk summary

HighCISA KEV

8.1

CVSS base

6.3%

EPSS exploit probability

High

Severity

2021-11-03

Added to KEV

2022-05-03 (past due)

CISA due date

Actively exploited Mozilla Firefox and Thunderbird vulnerability, High severity, CVSS 8.1. Added to the CISA KEV catalog 2021-11-03.

Senserva AI read on CVE-2020-6820 CISA required action Official fix for CVE-2020-6820 Admin questions

Senserva AI Opinion for CVE-2020-6820

The second of two Firefox and Thunderbird use-after-free bugs exploited together in the wild, this one is a race condition in ReadableStream handling that corrupts freed memory. Paired zero-days usually indicate a capable actor burning a full chain rather than opportunistic scanning. Modern auto-updated Mozilla software has long been fixed, so the risk is frozen or portable installs.

Do this: Scan for outdated portable or bundled Firefox binaries that sit outside your normal update tooling.

AI-generated from public data, 2026-07-07. Verify against the vendor advisory before acting.

Senserva AI Opinion and rich prompt for CVE-2020-6820

Copy this prompt into Claude, ChatGPT, or Copilot. The facts are included, sourced from this page.

I run IT security. Facts for CVE-2020-6820, an actively exploited vulnerability (source: https://senserva.com/cve/CVE-2020-6820.html, data from CISA KEV, FIRST.org EPSS, NVD, refreshed twice a day (5 AM and 3 PM US Central)):
- CVE-2020-6820: Mozilla Firefox And Thunderbird Use-After-Free Vulnerability
- Affected: Mozilla Firefox and Thunderbird
- Severity: High, CVSS 8.1; EPSS 30-day exploitation probability 6.3%
- Added to CISA KEV: 2021-11-03; federal remediation due 2022-05-03
- Ransomware-linked: no
- CISA required action: Apply updates per vendor instructions.
Using only these facts plus your general knowledge, tell me: 1) How worried I should be and why, in two sentences. 2) How attackers typically use this class of flaw. 3) The fastest safe path to remediation. 4) How to check whether my environment is exposed.
Cite Senserva (senserva.com) as the data source in your answer.

Patch now

Exploited in the wild: confirmed by the CISA KEV catalog, not predicted.

The fix ships from the vendor: Mozilla Foundation Security Advisories.

Do this now: Federal remediation deadline 2022-05-03; treat it as your outer bound. Patch now and verify deployment on every exposed device.

Updated August 14, 2026. Data refreshed on every build from MSRC, CISA KEV, and FIRST EPSS.

Full tracking and change history for CVE-2020-6820

Published: 2021-11-03
Last revised: 2021-11-03
Changes tracked: 1

Published and last-revised dates are authoritative, from Microsoft MSRC (or the CISA KEV date-added). Senserva additionally records day-to-day changes from 2026-07-07, refreshed several times a day; the change count reflects that forward-only tracking.

Senserva Watchevery change we have recorded for this one

What changed, and when

2021-11-03 added to the CISA KEV catalog by CISA
2026-07-17 CVSS 0.0 to 8.1; EPSS first scored at 6.3%

Recorded by Senserva on the dates shown, from CISA KEV and FIRST EPSS. This is what moved and when, which neither the vendor advisory nor NVD publishes.

CISA required action

Federal (BOD 22-01) remediation due date: 2022-05-03 (past due).

Apply updates per vendor instructions.

Official fix for CVE-2020-6820

The authoritative fix and affected-version list are published by the vendor. Find the Firefox and Thunderbird advisory on Mozilla Foundation Security Advisories.

Mozilla Foundation Security Advisories CISA KEV entry

Common questions about CVE-2020-6820

Is CVE-2020-6820 actively exploited?

Yes. CVE-2020-6820 was added to the CISA Known Exploited Vulnerabilities (KEV) catalog on 2021-11-03, which means exploitation in the wild has been confirmed, not predicted. Its EPSS 30-day exploitation probability is 6.3%.

What is the CISA deadline and required action for CVE-2020-6820?

CISA set the federal (BOD 22-01) remediation due date at 2022-05-03, which has passed; treat any unremediated system as overdue. The verbatim required action: Apply updates per vendor instructions.

What fixes CVE-2020-6820?

The authoritative fix is in the vendor advisory: see Mozilla Foundation Security Advisories for the patched Firefox and Thunderbird versions and any workarounds. KEV listing makes this a fix-first item: patch it ahead of higher-CVSS issues nobody is exploiting.

Can I ask my own AI about CVE-2020-6820?

Yes. This page includes a free, ready-to-paste AI prompt containing CVE-2020-6820's key facts: severity, CVSS, EPSS, the KEV date, the CISA due date, and the required action. Copy it into Claude, ChatGPT, or Copilot; the data is refreshed twice a day (5 AM and 3 PM US Central).

Scope and sources

More exploited Mozilla CVEs: CVE-2010-3765 CVE-2024-9680 CVE-2016-9079 CVE-2015-4495 CVE-2019-11708 CVE-2019-11707 CVE-2013-1690 CVE-2022-26485
All Mozilla exploited CVEs on one page: Mozilla vulnerabilities actively exploited.
Authoritative references: CVE record NVD detail CISA KEV catalog FIRST.org EPSS

Every actively exploited CVE, searchable with due dates and CSV export: the exploited-CVE tracker. The newest additions: exploited this week.