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ExploitedIn CISA KEVFix available

CVE-2020-6820

Exploitation confirmed. First attested on 2021-11-03 by CISA-ADP, CISA KEV, GCVE · kevintel.

  • Published2020-04-24
  • Fixed in74.0.1 · 68.6.1
  • WeaknessCWE-362

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

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.

Timeline

dates in UTC
  1. 2020-04-2416:15CVE publishedNVD
  2. 2021-11-03Added to the CISA KEV catalogCISA KEV
  3. 2021-11-03Listed as exploited by GCVEGCVE
  4. 2021-11-03Exploitation first reportedCISA KEV
  5. 2025-02-0712:49SSVC exploitation set to activeCISA-ADP

Observed activity

isMalicious corpus · updated 2026-11-03 21:37 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-2020-6820?format=txt
  • CSV export: https://api.ismalicious.com/edl/cve/CVE-2020-6820?format=csv
  • TAXII collection of exploited CVEs
  • The EDL and CSV take an API key on the Pro plan.

Versions per branch

NVD
Mozilla firefox
BranchAffectedFixed in
74.0< 74.0.174.0.1
68.6< 68.6.168.6.1
Mozilla thunderbird
BranchAffectedFixed in
68.7< 68.7.068.7.0

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

Products declared by the CNA3 products

As mozilla, the CNA that assigned this CVE, declared them in the CVE record; versions as written.

  • Mozilla Thunderbird
    Affected
    • < 68.7.0fixed in 68.7.0custom
  • Mozilla Firefox
    Affected
    • < 74.0.1fixed in 74.0.1custom
  • Mozilla Firefox ESR
    Affected
    • < 68.6.1fixed in 68.6.1custom

Source: the CNA record (CVE Program), © The MITRE Corporation,CVE Terms of UseCVE recordupdated 2020-04-29

Distribution builds and vendor advisories

Packages from OSV.dev and products from the vendors' CSAF advisories.

version not verifiedVersions not verified: read as OSV.dev and the vendors publish them, not checked against each vendor's release list.

Packages

  • firefoxUbuntu:20.04:LTSall versions · fixed in 75.0+build3-0ubuntu1
  • firefoxUbuntu:18.04:LTSall versions · fixed in 74.0.1+build1-0ubuntu0.18.04.1
  • firefoxUbuntu:16.04:LTSall versions · fixed in 74.0.1+build1-0ubuntu0.16.04.1
  • firefoxRed Hat:rhel_e4s:8.0::appstreamall versions · fixed in 0:68.6.1-1.el8_0and 2 other packages
  • firefoxRed Hat:enterprise_linux:8::appstreamall versions · fixed in 0:68.6.1-1.el8_1and 2 other packages
  • firefoxRed Hat:enterprise_linux:7::workstationall versions · fixed in 0:68.6.1-1.el7_8and 1 other package
  • firefoxRed Hat:enterprise_linux:7::serverall versions · fixed in 0:68.6.1-1.el7_8and 1 other package
  • firefoxRed Hat:enterprise_linux:7::clientall versions · fixed in 0:68.6.1-1.el7_8and 1 other package

95 more package ranges on OSV.dev

Vendor advisories

  • CVE-2020-6820 Red Hat Product Security205 products fixed · 2 known affected

    Mozilla: Use-after-free when handling a ReadableStream

    firefox as a component of Red Hat Enterprise Linux 5 · thunderbird as a component of Red Hat Enterprise Linux 5

  • RHSA-2020:1495 Red Hat Product Security7 products fixed

    Red Hat Security Advisory: thunderbird security update

  • RHSA-2020:1496 Red Hat Product Security7 products fixed

    Red Hat Security Advisory: thunderbird security update

  • RHSA-2020:1488 Red Hat Product Security19 products fixed

    Red Hat Security Advisory: thunderbird security update

  • RHSA-2020:1489 Red Hat Product Security11 products fixed

    Red Hat Security Advisory: thunderbird security update

  • RHSA-2020:1338 Red Hat Product Security21 products fixed

    Red Hat Security Advisory: firefox security update

3 more advisories

Full record on NVD

Fix

  1. Update Mozilla firefox to 74.0.1 · 68.6.1
  2. CISA’s required action: Apply updates per vendor instructions.due 2022-05-03
  3. Vendor fix Red Hat Product Security RHSA-2020:1495 · 7 productsFor details on how to apply this update, which includes the changes described in this advisory, refer to: https://access.redhat.com/articles/11258 All running instances of Thunderbird must be restarted for the update to take effect.
  4. Vendor fix Red Hat Product Security RHSA-2020:1338 · 21 productsFor details on how to apply this update, which includes the changes described in this advisory, refer to: https://access.redhat.com/articles/11258 After installing the update, Firefox must be restarted for the changes to take effect.
  5. No fix planned, says the vendor Red Hat Product Security CVE-2020-6820 · 2 productsOut of support scope

Advisories and patches

Detection

Nuclei template
none so far
ProjectDiscovery
Exploit-DB
no entry
Exploit-DB
Proof-of-concept repositories
none listed
PoC-in-GitHub
Metasploit modules
none listed
Metasploit Framework

CISA Known Exploited Vulnerability

Date Added
2021-11-03
Patch Due Date
2022-05-03
Ransomware Use
Unknown
Required Action
Apply updates per vendor instructions.

Sources

Identifiers

Technical details

CVSS v3 Vector
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H
Published
2020-04-24
Last Modified
2026-06-17
NVD status
Analyzed
GitHub Advisory
GHSA-f77r-rqc9-53hh

Frequently asked questions

What is CVE-2020-6820?

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.

Is CVE-2020-6820 actively exploited?

Yes. CVE-2020-6820 is on the CISA Known Exploited Vulnerabilities (KEV) catalog, meaning it has been confirmed as actively exploited in the wild. CISA requires federal agencies to patch by 2022-05-03.

What is the CVSS score for CVE-2020-6820?

CVE-2020-6820 has a CVSS v3 base score of 8.1 (HIGH severity), with vector string CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H.

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