high 8.1

CVE-2020-12387 json

A race condition when running shutdown code for Web Worker led to a use-after-free vulnerability.

published 2020-05-26 6 y ago modified 2026-06-17 4 mo ago also EUVD-2020-4699

cvss: 8.1; version 3.1, NVD
cisa kev: Not listed; not in CISA's catalog
epss: 1.4%; EPSS 1.4% chance of exploitation in 30 days, percentile 73%
public exploits: 0; none known

A race condition when running shutdown code for Web Worker led to a use-after-free vulnerability. This resulted in a potentially exploitable crash. This vulnerability affects Firefox ESR < 68.8, Firefox < 76, and Thunderbird < 68.8.0.

Patch / fix

fixed in 3 distros 7

Mozilla Firefox ESR 68.8
Mozilla Firefox 76
Mozilla Thunderbird 68.8.0

debian ubuntu red hat suse opensuse alpine arch linux

firefox-esr Debian 14, Debian 13, Debian 12 68.8.0esr-1
thunderbird Debian 14, Debian 13, Debian 12 1:68.8.0-1

debian advisories DEBIAN-CVE-2020-12387

firefox Ubuntu 16.04 LTS 76.0+build2-0ubuntu0.16.04.1
firefox Ubuntu 18.04 LTS 76.0+build2-0ubuntu0.18.04.1
firefox Ubuntu 20.04 LTS 76.0+build2-0ubuntu0.20.04.1
thunderbird Ubuntu 16.04 LTS 1:68.8.0+build2-0ubuntu0.16.04.2
thunderbird Ubuntu 18.04 LTS 1:68.8.0+build2-0ubuntu0.18.04.2
thunderbird Ubuntu 20.04 LTS 1:68.8.0+build2-0ubuntu0.20.04.2

ubuntu advisories UBUNTU-CVE-2020-12387

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 7 68.8.0-1.el7_8
thunderbird RHEL 8 68.8.0-1.el8_2
thunderbird RHEL 6 68.8.0-1.el6_10
firefox RHEL 7 68.8.0-1.el7_8
firefox RHEL 6 68.8.0-1.el6_10
firefox RHEL 8 68.8.0-1.el8_2
thunderbird RHEL 8.1 EUS 68.8.0-1.el8_1
thunderbird RHEL 8.0 E4S 68.8.0-1.el8_0

red hat advisories RHSA-2020:2050 RHSA-2020:2046 RHSA-2020:2049 RHSA-2020:2047 RHSA-2020:2048 RHSA-2020:2037

MozillaThunderbird SLE Workstation Extension 15 SP1 68.8.0-3.80.2
MozillaFirefox SLES for SAP 12 SP5, SLES for SAP 12 SP4, SLES for SAP 12 SP3 +16 more 68.8.0-109.119.1
MozillaFirefox SLE Module for Desktop Applications 15 SP1 68.8.0-3.87.1
MozillaFirefox SLES 11 SP4-LTSS 68.8.0-78.73.1

suse advisories SUSE-SU-2020:14359-1 SUSE-SU-2020:1225-1 SUSE-SU-2020:1218-1 SUSE-SU-2020:1209-1

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 68.8.0-lp151.2.38.2
MozillaFirefox openSUSE Leap 15.1 68.8.0-lp151.2.45.1

opensuse advisories openSUSE-SU-2024:14572-1 openSUSE-SU-2024:10600-1 openSUSE-SU-2024:10601-1 openSUSE-SU-2020:0643-1 openSUSE-SU-2020:0621-1

firefox Alpine 3.23, Alpine 3.22, Alpine 3.21 +9 more 76.0-r0
mozjs68 Alpine 3.12 68.8.0-r0
librewolf Alpine 3.24, Alpine 3.23, Alpine 3.22 +1 more 76.0-r0
firefox-esr Alpine 3.23, Alpine 3.22, Alpine 3.21 +10 more 68.8.0-r0
thunderbird Alpine 3.23, Alpine 3.22, Alpine 3.21 +5 more 68.8.0-r0

alpine advisories ALPINE-CVE-2020-12387

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 76.0-1
thunderbird Arch Linux 68.8.0-1

arch linux advisories AVG-1148 AVG-1155

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-2020-12387 updated 2026-11-04 cvss 8.8

Mozilla: Use-after-free during worker shutdown

Red Hat Enterprise Linux 5 firefox, thunderbird
affected: no fix planned “Out of support scope”
Red Hat Enterprise Linux AppStream (v. 8) firefox 68.8.0-1.el8_2, thunderbird 68.8.0-1.el8_2
fixed: vendor fix RHSA-2020:2031
Red Hat Enterprise Linux AppStream E4S (v. 8.0) firefox 68.8.0-1.el8_0, thunderbird 68.8.0-1.el8_0
fixed: vendor fix RHSA-2020:2033
Red Hat Enterprise Linux AppStream EUS (v. 8.1) firefox 68.8.0-1.el8_1, thunderbird 68.8.0-1.el8_1
fixed: vendor fix RHSA-2020:2032
Red Hat Enterprise Linux Client (v. 7) firefox 68.8.0-1.el7_8, thunderbird 68.8.0-1.el7_8 Red Hat Enterprise Linux Client Optional (v. 7) firefox 68.8.0-1.el7_8
fixed: vendor fix RHSA-2020:2037
Red Hat Enterprise Linux Desktop (v. 6) firefox 68.8.0-1.el6_10, thunderbird 68.8.0-1.el6_10 Red Hat Enterprise Linux Desktop Optional (v. 6) firefox 68.8.0-1.el6_10 Red Hat Enterprise Linux HPC Node Optional (v. 6) firefox 68.8.0-1.el6_10 Red Hat Enterprise Linux Server (v. 6) firefox 68.8.0-1.el6_10
fixed: vendor fix RHSA-2020:2036
Red Hat Enterprise Linux Server (v. 7) firefox 68.8.0-1.el7_8
fixed: vendor fix RHSA-2020:2037
Red Hat Enterprise Linux Server Optional (v. 6) firefox 68.8.0-1.el6_10, thunderbird 68.8.0-1.el6_10
fixed: vendor fix RHSA-2020:2036

show 5 more

Red Hat Enterprise Linux Server Optional (v. 7) firefox 68.8.0-1.el7_8, thunderbird 68.8.0-1.el7_8
fixed: vendor fix RHSA-2020:2037
Red Hat Enterprise Linux Workstation (v. 6) firefox 68.8.0-1.el6_10, thunderbird 68.8.0-1.el6_10
fixed: vendor fix RHSA-2020:2036
Red Hat Enterprise Linux Workstation (v. 7) firefox 68.8.0-1.el7_8, thunderbird 68.8.0-1.el7_8
fixed: vendor fix RHSA-2020:2037
Red Hat Enterprise Linux Workstation Optional (v. 6) firefox 68.8.0-1.el6_10
fixed: vendor fix RHSA-2020:2036
Red Hat Enterprise Linux Workstation Optional (v. 7) firefox 68.8.0-1.el7_8
fixed: vendor fix RHSA-2020:2037

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

Exploit prediction

1.44% chance of exploitation in the next 30 days, higher than 73% of all scored CVEs (FIRST EPSS, 2026-11-08).

CVSS scores

assigned by	version	score	vector
NVD	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 Firefox ESR
Mozilla Firefox
Mozilla Thunderbird

Classification

Weakness

CWE-362 Concurrent Execution using Shared Resource with Improper Synchronization
CWE-416 Use After Free

References

usn.ubuntu.com /4373-1/
vendor-advisory, broken-link
mozilla.org /security/advisories/mfsa2020-18/
third-party-advisory
mozilla.org /security/advisories/mfsa2020-16/
third-party-advisory
mozilla.org /security/advisories/mfsa2020-17/
third-party-advisory
bugzilla.mozilla.org /show_bug.cgi?id=1545345
issue-tracking, permissions-required
security.gentoo.org /glsa/202005-03
third-party-advisory
security.gentoo.org /glsa/202005-04
third-party-advisory

Sources

CVE List CVE-2020-12387
NVD CVE-2020-12387

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