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Actively exploited

CVE-2020-6820 Mozilla Firefox And Thunderbird Use-After-Free Vulnerability

Mozilla Firefox and Thunderbird contain a race condition vulnerability when handling a ReadableStream under certain conditions. The race condition creates a use-after-free vulnerability, causing unspecified impacts.

From the CISA KEV catalogue version 2026.11.06, with Cybergen analysis. Updated 6 Nov 2026.

Our Assessment

Should You Worry?

Patch urgentlyCybergen assessment
  • CISA confirmed exploitation in the wild and added it to the Known Exploited Vulnerabilities catalogue on 3 Nov 2021.
  • Rated High severity (CVSS 8.1, scored by CISA).
  • It has been in the catalogue for 1,829 days. Systems still unpatched now have almost certainly been scanned for it.
The Vulnerability

What It Is

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.

Affected Versions

ProductVersions
Mozilla Thunderbirdunspecified to before 68.7.0
Mozilla Firefoxunspecified to before 74.0.1
Mozilla Firefox ESRunspecified to before 68.6.1

From the public CVE record, as supplied by the vendor. Always confirm against the vendor advisory.

From Watch to Action

What To Do Now

  1. 1

    Find it

    Search your asset inventory and external attack surface for Mozilla Firefox and Thunderbird. Include cloud images, appliances and anything a supplier runs for you.

    Attack Surface Management
  2. 2

    Fix it

    Apply updates per vendor instructions.

    Vulnerability Management
  3. 3

    Hunt for compromise

    If an affected system was reachable before you patched, assume it may have been hit. Check logs, new accounts, web shells and outbound connections from the date of exposure.

    Incident Response
  4. 4

    Prove it is fixed

    Rescan or test the system after patching. Upgrades that fail silently and forgotten secondary instances are common.

    Penetration Testing

Vendor Advisories

Regulation

The Compliance Clock

Cyber EssentialsRated high, so if it affects devices in your Cyber Essentials scope, the update must be installed within 14 days of the vendor releasing it.NIS2If exploitation causes a significant incident, essential and important entities owe their CSIRT an early warning within 24 hours and a notification within 72 hours.DORAFinancial entities must classify ICT incidents and send an initial notification for a major incident within strict deadlines, with exploited flaws a common cause.US federal (BOD 22-01)Federal civilian agencies had until 3 May 2022 to remediate. A sensible benchmark for everyone else.

General guidance, not legal advice. Your obligations depend on your sector, scope and jurisdiction.

Stay Ahead

More Exploited Mozilla Flaws

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CVE-2010-3765Mozilla Multiple Products Remote Code Execution VulnerabilityAdded 6 Oct 2025CVE-2024-9680 · RansomwareMozilla Firefox Use-After-Free VulnerabilityAdded 15 Oct 2024CVE-2016-9079Mozilla Firefox, Firefox ESR, and Thunderbird Use-After-Free VulnerabilityAdded 22 Jun 2023CVE-2015-4495Mozilla Firefox Security Feature Bypass VulnerabilityAdded 25 May 2022CVE-2019-11707Mozilla Firefox and Thunderbird Type Confusion VulnerabilityAdded 23 May 2022CVE-2019-11708Mozilla Firefox and Thunderbird Sandbox Escape VulnerabilityAdded 23 May 2022

All 13 exploited Mozilla vulnerabilities

Not Sure If You Are Exposed?

Our attack surface and vulnerability teams can check your internet-facing estate for CVE-2020-6820 and every other exploited flaw on the list.

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Evidence

Sources

  1. CISA Known Exploited Vulnerabilities catalogue
  2. NVD: CVE-2020-6820
  3. CVE record: CVE-2020-6820
  4. FIRST EPSS
  5. mozilla.org/security/advisories/mfsa2020-14/
  6. mozilla.org/security/advisories/mfsa2020-11/
  7. bugzilla.mozilla.org/show_bug.cgi?id=1626728
  8. usn.ubuntu.com/4335-1/