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CVE-2020-6820

8.1, high Known exploited

Mozilla Firefox And Thunderbird Use-After-Free Vulnerability

NVD (opens in a new tab) CVE.org (opens in a new tab) CISA KEV catalogue (opens in a new tab)
CVSS
8.1, high high CVSS v3.1
EPSS, next 30 days
7% higher than 94.1% of scored CVEs
Exploitation
Confirmed in the wild
Published
24 Apr 20206 years ago

Known exploited

CISA added this to the Known Exploited Vulnerabilities Catalog on 3 Nov 2021. US federal agencies had to remediate by 3 May 2022, a deadline that has passed.

Required action. Apply updates per vendor instructions.

Description

Under certain conditions, when handling a ReadableStream, a race condition can cause a use-after-free. We are aware of targeted attacks in the wild abusing this flaw. This vulnerability affects Thunderbird < 68.7.0, Firefox < 74.0.1, and Firefox ESR < 68.6.1.

Details

Vendor
Mozilla
Product
Thunderbird
Weakness
CWE-362 (opens in a new tab)
Status
Analyzed
Updated
17 Jun 2026
Assigner
security@mozilla.org

CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H

References 5

Advisories 3

Other references 2

The kind is read from NVD's tags where present and from the shape of the URL otherwise, so treat it as a hint.

Other Mozilla CVEs

Every Mozilla CVE we hold

Same vendor in what we hold, not a known link between the flaws.

  • CVE-2024-9680 Mozilla Firefox Use-After-Free Vulnerability KEV EPSS 23% 9.8, critical
  • CVE-2022-26485 Mozilla Firefox Use-After-Free Vulnerability KEV EPSS 14% 8.8, high
  • CVE-2022-26486 Mozilla Firefox Use-After-Free Vulnerability KEV EPSS 2% 9.6, critical
  • CVE-2020-6819 Mozilla Firefox And Thunderbird Use-After-Free Vulnerability KEV EPSS 3% 8.1, high
  • CVE-2019-17026 Mozilla Firefox And Thunderbird Type Confusion Vulnerability KEV EPSS 46% 8.8, high
  • CVE-2019-11707 Mozilla Firefox and Thunderbird Type Confusion Vulnerability KEV EPSS 38% 8.8, high
  • CVE-2019-11708 Mozilla Firefox and Thunderbird Sandbox Escape Vulnerability KEV EPSS 56% 10, critical
  • CVE-2016-9079 Mozilla Firefox, Firefox ESR, and Thunderbird Use-After-Free Vulnerability KEV EPSS 88% 7.5, high

Beyond this page

Is CVE-2020-6820 in your network?

This page tells you how dangerous CVE-2020-6820 is out there. Whether it sits on one of your devices, only a scan of your own network can tell. That is what our systems do, every day, for the companies we protect.

CVE-2020-6820 is in the CISA catalogue of exploited vulnerabilities: attackers are already exploiting it.

  • EXYS7600 OCELOT

    Find it before attackers do

    Ocelot scans your network on a schedule and reports every vulnerability it finds by its CVE identifier, with its CVSS score and how many of your hosts are affected. A finding like CVE-2020-6820 comes with the devices to patch.

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  • EXYS7950 KONDRA

    Detect it if it's exploited

    Until the patch is applied, Kondra monitors your network, events and endpoints in real time. It keeps an inventory of the software installed on every endpoint, and its anomaly detection rules flag what an attacker does once inside.

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Check your exposure

Assembled from the NVD 2.0 API, the CISA Known Exploited Vulnerabilities Catalog and FIRST EPSS. Scores are as of our last collection on 17 Oct 2026, not the live upstream state.