high 8.1

CVE-2020-6820 kev in the critical feed json

Under certain conditions, when handling a ReadableStream, a race condition can cause a use-after-free.

published 2020-04-24 6 y ago modified 2026-06-17 4 mo ago also EUVD-2020-27964

cvss: 8.1; version 3.1, CNA
cisa kev: Exploited; added 2021-11-03
epss: 7.1%; EPSS 7.1% chance of exploitation in 30 days, percentile 94%
public exploits: 0; none known

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.

Patch / fix

cisa required action Apply updates per vendor instructions.

fixed in 3 distros 7

Mozilla Thunderbird 68.7.0
Mozilla Firefox 74.0.1
Mozilla Firefox ESR 68.6.1

debian ubuntu red hat suse opensuse alpine arch linux

firefox-esr Debian 14, Debian 13, Debian 12 68.6.1esr-1
thunderbird Debian 14, Debian 13, Debian 12 1:68.7.0-1

debian advisories DEBIAN-CVE-2020-6820

firefox Ubuntu 16.04 LTS 74.0.1+build1-0ubuntu0.16.04.1
firefox Ubuntu 18.04 LTS 74.0.1+build1-0ubuntu0.18.04.1
firefox Ubuntu 20.04 LTS 75.0+build3-0ubuntu1
thunderbird Ubuntu 16.04 LTS 1:68.7.0+build1-0ubuntu0.16.04.2
thunderbird Ubuntu 18.04 LTS 1:68.7.0+build1-0ubuntu0.18.04.1
thunderbird Ubuntu 20.04 LTS 1:68.7.0+build1-0ubuntu1

ubuntu advisories UBUNTU-CVE-2020-6820

Ubuntu security data © Canonical Ltd., from https://github.com/canonical/ubuntu-security-notices via OSV; changed: normalised to fixed package versions per release. CC BY-SA 4.0

thunderbird RHEL 8 68.7.0-1.el8_1
thunderbird RHEL 7 68.7.0-1.el7_8
thunderbird RHEL 6 68.7.0-1.el6_10
firefox RHEL 8 68.6.1-1.el8_1
firefox RHEL 7 68.6.1-1.el7_8
firefox RHEL 6 68.6.1-1.el6_10
thunderbird RHEL 8.0 E4S 68.7.0-1.el8_0
firefox RHEL 8.0 E4S 68.6.1-1.el8_0

red hat advisories RHSA-2020:1495 RHSA-2020:1496 RHSA-2020:1489 RHSA-2020:1488 RHSA-2020:1341 RHSA-2020:1340

MozillaThunderbird SLE Workstation Extension 15 SP1 68.7.0-3.77.1
MozillaFirefox SLE Module for Desktop Applications 15 SP1 68.6.1-3.81.1
MozillaFirefox SLES for SAP 12 SP5, SLES for SAP 12 SP4, SLES for SAP 12 SP3 +16 more 68.6.1-109.113.1
MozillaFirefox SLES 11 SP4-LTSS 68.6.1-78.67.1

suse advisories SUSE-SU-2020:1027-1 SUSE-SU-2020:14337-1 SUSE-SU-2020:0929-1 SUSE-SU-2020:0928-1

firefox-esr openSUSE Tumbleweed 128.5.1-1.1
MozillaFirefox openSUSE Tumbleweed 92.0-1.2
MozillaThunderbird openSUSE Tumbleweed 91.1.1-1.1
MozillaThunderbird openSUSE Leap 15.1 68.7.0-lp151.2.35.1
MozillaThunderbird openSUSE Leap 15.1 68.7.0-lp151.2.31.1
MozillaFirefox openSUSE Leap 15.1 68.6.1-lp151.2.39.1

opensuse advisories openSUSE-SU-2024:14572-1 openSUSE-SU-2024:10600-1 openSUSE-SU-2024:10601-1 openSUSE-SU-2020:0544-1 openSUSE-SU-2020:0520-1 openSUSE-SU-2020:0461-1

firefox Alpine 3.23, Alpine 3.22, Alpine 3.21 +9 more 74.0.1-r0
mozjs68 Alpine 3.12 68.7.0-r0
librewolf Alpine 3.24, Alpine 3.23, Alpine 3.22 +1 more 74.0.1-r0
firefox-esr Alpine 3.23, Alpine 3.22, Alpine 3.21 +10 more 68.6.1-r0
thunderbird Alpine 3.23, Alpine 3.22, Alpine 3.21 +5 more 68.7.0-r0

alpine advisories ALPINE-CVE-2020-6820

Alpine secdb © Alpine Linux, from https://secdb.alpinelinux.org via OSV; changed: normalised to fixed package versions per release. CC BY-SA 4.0

firefox Arch Linux 74.0.1-1
thunderbird Arch Linux 68.7.0-1

arch linux advisories AVG-1125 AVG-1132

Vendor advisories

What the vendors themselves say about their products: affected, fixed or not affected, and what to do.

red hat product security vex CVE-2020-6820 updated 2026-11-04 cvss 8.8

Mozilla: Use-after-free when handling a ReadableStream

Red Hat Enterprise Linux 5 firefox, thunderbird
affected: no fix planned “Out of support scope”
Red Hat Enterprise Linux AppStream (v. 8) firefox 68.6.1-1.el8_1, thunderbird 68.7.0-1.el8_1
fixed: vendor fix RHSA-2020:1341
Red Hat Enterprise Linux AppStream E4S (v. 8.0) firefox 68.6.1-1.el8_0, thunderbird 68.7.0-1.el8_0
fixed: vendor fix RHSA-2020:1340
Red Hat Enterprise Linux Client (v. 7) firefox 68.6.1-1.el7_8, thunderbird 68.7.0-1.el7_8 Red Hat Enterprise Linux Client Optional (v. 7) firefox 68.6.1-1.el7_8
fixed: vendor fix RHSA-2020:1338
Red Hat Enterprise Linux Desktop (v. 6) firefox 68.6.1-1.el6_10, thunderbird 68.7.0-1.el6_10 Red Hat Enterprise Linux Desktop Optional (v. 6) firefox 68.6.1-1.el6_10 Red Hat Enterprise Linux HPC Node Optional (v. 6) firefox 68.6.1-1.el6_10 Red Hat Enterprise Linux Server (v. 6) firefox 68.6.1-1.el6_10
fixed: vendor fix RHSA-2020:1339
Red Hat Enterprise Linux Server (v. 7) firefox 68.6.1-1.el7_8
fixed: vendor fix RHSA-2020:1338
Red Hat Enterprise Linux Server Optional (v. 6) firefox 68.6.1-1.el6_10, thunderbird 68.7.0-1.el6_10
fixed: vendor fix RHSA-2020:1339
Red Hat Enterprise Linux Server Optional (v. 7) firefox 68.6.1-1.el7_8, thunderbird 68.7.0-1.el7_8
fixed: vendor fix RHSA-2020:1338

show 4 more

Red Hat Enterprise Linux Workstation (v. 6) firefox 68.6.1-1.el6_10, thunderbird 68.7.0-1.el6_10
fixed: vendor fix RHSA-2020:1339
Red Hat Enterprise Linux Workstation (v. 7) firefox 68.6.1-1.el7_8, thunderbird 68.7.0-1.el7_8
fixed: vendor fix RHSA-2020:1338
Red Hat Enterprise Linux Workstation Optional (v. 6) firefox 68.6.1-1.el6_10
fixed: vendor fix RHSA-2020:1339
Red Hat Enterprise Linux Workstation Optional (v. 7) firefox 68.6.1-1.el7_8
fixed: vendor fix RHSA-2020:1338

Red Hat CSAF VEX © Red Hat, Inc., from https://security.access.redhat.com/data/csaf/v2/vex/; changed: reduced to product statuses and remediations. CC BY 4.0 · csaf

Timeline

2020-04-01 Found exploited as a 0-day (Project Zero)
2020-04-03 0-day patched (Project Zero)
2020-04-24 Published
2021-11-03 Added to CISA KEV
2022-05-03 CISA patch deadline
2025-02-07 CISA SSVC: exploitation active

Known exploited

Added: 5 y ago (2021-11-03)
Patch by: 2022-05-03 (4 y ago)
Ransomware: Not known
Required action: Apply updates per vendor instructions.
Vendor, product: Mozilla, Firefox and Thunderbird
CISA name: Mozilla Firefox And Thunderbird Use-After-Free Vulnerability

Zero-day

Detected in real attacks while still unknown to the vendor and the public, per Google Project Zero's 0day In-the-Wild tracker.

Found: 2020-04-01
Patched: 2020-04-03
Details: project zero root cause analysis

CISA SSVC

Exploitation: Active, exploited in the wild
Automatable: no
Technical impact: total
Assessed: 21 mo ago (2025-02-07)

Exploit prediction

7.06% chance of exploitation in the next 30 days, higher than 94% of all scored CVEs (FIRST EPSS, 2026-11-08).

CVSS scores

assigned by	version	score	vector
CISA ADP	3.1	8.1	CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H
NVD	3.1	8.1	CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H
CNA	3.1	8.1	CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H
NVD	2.0	6.8	AV:N/AC:M/Au:N/C:P/I:P/A:P

Affected products

Mozilla Thunderbird
Mozilla Firefox
Mozilla Firefox ESR

Classification

Weakness

CWE-362 Concurrent Execution using Shared Resource with Improper Synchronization

References

usn.ubuntu.com /4335-1/
vendor-advisory, third-party-advisory
mozilla.org /security/advisories/mfsa2020-14/
vendor-advisory
mozilla.org /security/advisories/mfsa2020-11/
vendor-advisory
bugzilla.mozilla.org /show_bug.cgi?id=1626728
issue-tracking, permissions-required
cisa.gov /known-exploited-vulnerabilities-catalog?field_cve=CVE-2020-6820
government-resource, us-government-resource

Sources

CVE List CVE-2020-6820
NVD CVE-2020-6820

Fencer Vulns keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.

Summary