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Fencer Vulns
high 8.8

CVE-2020-26974 json

When flex-basis was used on a table wrapper, a StyleGenericFlexBasis object could have been incorrectly cast to the wrong type.

published 2021-01-07 5 y ago modified 2026-06-17 4 mo ago also EUVD-2020-19498

cvss
8.8
version 3.1, NVD
cisa kev
Not listed
not in CISA's catalog
epss
1.5%
EPSS 1.5% chance of exploitation in 30 days, percentile 74%
public exploits
0
none known

Summary

When flex-basis was used on a table wrapper, a StyleGenericFlexBasis object could have been incorrectly cast to the wrong type. This resulted in a heap user-after-free, memory corruption, and a potentially exploitable crash. This vulnerability affects Firefox < 84, Thunderbird < 78.6, and Firefox ESR < 78.6.

Patch / fix

  • Mozilla Firefox 84
  • Mozilla Thunderbird 78.6
  • Mozilla Firefox ESR 78.6
  • firefox-esr Debian 14, Debian 13, Debian 12 78.6.0esr-1
  • thunderbird Debian 14, Debian 13, Debian 12 1:78.6.0-1

debian advisories DEBIAN-CVE-2020-26974

  • firefox Ubuntu 16.04 LTS 84.0+build3-0ubuntu0.16.04.1
  • firefox Ubuntu 18.04 LTS 84.0+build3-0ubuntu0.18.04.1
  • firefox Ubuntu 20.04 LTS 84.0+build3-0ubuntu0.20.04.1
  • thunderbird Ubuntu 18.04 LTS 1:78.8.1+build1-0ubuntu0.18.04.1
  • thunderbird Ubuntu 20.04 LTS 1:78.7.1+build1-0ubuntu0.20.04.1

ubuntu advisories UBUNTU-CVE-2020-26974

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 78.6.0-1.el7_9
  • thunderbird RHEL 8 78.6.0-1.el8_3
  • firefox RHEL 8 78.6.0-1.el8_3
  • firefox RHEL 7 78.6.0-1.el7_9
  • thunderbird RHEL 8.0 E4S 78.6.0-1.el8_0
  • thunderbird RHEL 8.1 EUS 78.6.0-1.el8_1
  • thunderbird RHEL 8.2 EUS 78.6.0-1.el8_2
  • firefox RHEL 8.2 EUS 78.6.0-1.el8_2

red hat advisories RHSA-2020:5645RHSA-2020:5644RHSA-2020:5618RHSA-2020:5622RHSA-2020:5624RHSA-2020:5562

  • MozillaThunderbird SLE Workstation Extension 15 SP2 78.6.0-8.3.1
  • MozillaFirefox SLES for SAP 12 SP5, SLES for SAP 12 SP4, SLES for SAP 12 SP3 +15 more 78.6.0-112.39.1
  • MozillaFirefox SLE Module for Desktop Applications 15 SP1 78.6.0-3.122.1
  • MozillaFirefox SLE Module for Desktop Applications 15 SP2 78.6.0-8.20.2
  • MozillaThunderbird SLE Workstation Extension 15 SP1 78.6.0-3.113.1
  • MozillaFirefox SLES 11 SP4-LTSS 78.6.0-78.108.1

suse advisories SUSE-SU-2020:3935-1SUSE-SU-2020:3903-1SUSE-SU-2020:3902-1SUSE-SU-2020:3901-1SUSE-SU-2020:3900-1SUSE-SU-2020:14584-1

  • firefox-esr openSUSE Tumbleweed 128.5.1-1.1
  • MozillaFirefox openSUSE Tumbleweed 92.0-1.2
  • MozillaThunderbird openSUSE Tumbleweed 91.1.1-1.1
  • MozillaFirefox openSUSE Leap 15.1 78.6.0-lp151.2.82.1
  • MozillaThunderbird openSUSE Leap 15.1 78.6.0-lp151.2.63.1
  • MozillaFirefox openSUSE Leap 15.2 78.6.0-lp152.2.34.1
  • MozillaThunderbird openSUSE Leap 15.2 78.6.0-lp152.2.23.1

opensuse advisories openSUSE-SU-2024:14572-1openSUSE-SU-2024:10600-1openSUSE-SU-2024:10601-1openSUSE-SU-2020:2325-1openSUSE-SU-2020:2324-1openSUSE-SU-2020:2318-1

  • firefox Alpine 3.23, Alpine 3.22, Alpine 3.21 +9 more 84.0.1-r0
  • librewolf Alpine 3.24, Alpine 3.23, Alpine 3.22 +1 more 84.0.1-r0
  • firefox-esr Alpine 3.23, Alpine 3.22, Alpine 3.21 +9 more 78.6.0-r0
  • thunderbird Alpine 3.23, Alpine 3.22, Alpine 3.21 +5 more 78.6.1-r0

alpine advisories ALPINE-CVE-2020-26974

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

  • thunderbird Arch Linux 78.6.0-1
  • firefox Arch Linux 84.0-1

arch linux advisories AVG-1315AVG-1362

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-2020-26974 updated cvss 8.8

Mozilla: Incorrect cast of StyleGenericFlexBasis resulted in a heap use-after-free

  • Red Hat Enterprise Linux 5 thunderbird Red Hat Enterprise Linux 6 firefox, thunderbird
    affected: no fix planned “Out of support scope”
  • Red Hat Enterprise Linux AppStream (v. 8) firefox 78.6.0-1.el8_3, thunderbird 78.6.0-1.el8_3
    fixed: vendor fix RHSA-2020:5562
  • Red Hat Enterprise Linux AppStream E4S (v. 8.0) firefox 78.6.0-1.el8_0, thunderbird 78.6.0-1.el8_0
    fixed: vendor fix RHSA-2020:5565
  • Red Hat Enterprise Linux AppStream EUS (v. 8.1) firefox 78.6.0-1.el8_1, thunderbird 78.6.0-1.el8_1
    fixed: vendor fix RHSA-2020:5564
  • Red Hat Enterprise Linux AppStream EUS (v. 8.2) firefox 78.6.0-1.el8_2, thunderbird 78.6.0-1.el8_2
    fixed: vendor fix RHSA-2020:5563
  • Red Hat Enterprise Linux Client (v. 7) firefox 78.6.0-1.el7_9, thunderbird 78.6.0-1.el7_9 Red Hat Enterprise Linux Client Optional (v. 7) firefox 78.6.0-1.el7_9 Red Hat Enterprise Linux Server (v. 7) firefox 78.6.0-1.el7_9 Red Hat Enterprise Linux Server Optional (v. 7) firefox 78.6.0-1.el7_9, thunderbird 78.6.0-1.el7_9 Red Hat Enterprise Linux Workstation (v. 7) firefox 78.6.0-1.el7_9, thunderbird 78.6.0-1.el7_9 Red Hat Enterprise Linux Workstation Optional (v. 7) firefox 78.6.0-1.el7_9
    fixed: vendor fix RHSA-2020:5561

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

1.53% chance of exploitation in the next 30 days, higher than 74% of all scored CVEs (FIRST EPSS, 2026-11-08).

CVSS scores

assigned by version score vector
NVD 3.1 8.8 CVSS:3.1/AV:N/AC:L/PR:N/UI:R/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
  • Mozilla Thunderbird
  • Mozilla Firefox ESR

Classification

Weakness

References

Sources

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.