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Fencer Intel
high 8.8

CVE-2020-26959 json

During browser shutdown, reference decrementing could have occured on a previously freed object, resulting in a use-after-free, memory corr…

published 5y ago modified 4mo ago also EUVD-2020-19483

cvss
8.8
version 3.1, NVD
cisa kev
Not listed
not in CISA's catalog
epss
1.3%
EPSS 1.3% chance of exploitation in 30 days, percentile 70%
public exploits
0
none known

Summary

During browser shutdown, reference decrementing could have occured on a previously freed object, resulting in a use-after-free, memory corruption, and a potentially exploitable crash. This vulnerability affects Firefox < 83, Firefox ESR < 78.5, and Thunderbird < 78.5.

Patch / fix

  • mozilla firefox 83.0
  • mozilla firefox_esr 78.5
  • mozilla thunderbird 78.5
  • firefox-esr Debian 14, Debian 13, Debian 12 78.5.0esr-1
  • thunderbird Debian 14, Debian 13, Debian 12 1:78.5.0-1

debian advisories DEBIAN-CVE-2020-26959

  • firefox Ubuntu 16.04 LTS 83.0+build2-0ubuntu0.16.04.3
  • firefox Ubuntu 18.04 LTS 83.0+build2-0ubuntu0.18.04.2
  • firefox Ubuntu 20.04 LTS 83.0+build2-0ubuntu0.20.04.1
  • 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-2020-26959

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

  • firefox RHEL 7 78.5.0-1.el7_9
  • thunderbird RHEL 7 78.5.0-1.el7_9
  • firefox RHEL 8 78.5.0-1.el8_3
  • thunderbird RHEL 6 78.5.0-1.el6_10
  • thunderbird RHEL 8 78.5.0-1.el8_3
  • firefox RHEL 6 78.5.0-1.el6_10
  • thunderbird RHEL 8.0 E4S 78.5.0-1.el8_0
  • thunderbird RHEL 8.1 EUS 78.5.0-1.el8_1

red hat advisories RHSA-2020:5239RHSA-2020:5235RHSA-2020:5237RHSA-2020:5238RHSA-2020:5240RHSA-2020:5231

  • MozillaFirefox SLES for SAP 12 SP5, SLES for SAP 12 SP4, SLES for SAP 12 SP3 +15 more 78.5.0-112.36.1
  • MozillaThunderbird SLE Workstation Extension 15 SP2, SLE Workstation Extension 15 SP1 78.5.0-3.107.1
  • MozillaFirefox SLE Module for Desktop Applications 15 SP2 78.5.0-8.17.1
  • MozillaFirefox SLE Module for Desktop Applications 15 SP1 78.5.0-3.119.1
  • MozillaFirefox SLES 11 SP4-LTSS 78.5.0-78.105.1

suse advisories SUSE-SU-2020:3548-1SUSE-SU-2020:3528-1SUSE-SU-2020:3458-1SUSE-SU-2020:14548-1SUSE-SU-2020:3383-1

  • 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 15.2 78.5.0-lp152.2.33.1
  • MozillaThunderbird openSUSE Leap 15.1 78.5.0-lp151.2.59.1
  • MozillaThunderbird openSUSE Leap 15.2 78.5.0-lp152.2.19.1
  • MozillaFirefox openSUSE Leap 15.1 78.5.0-lp151.2.79.1
  • MozillaFirefox openSUSE Leap 15.2 78.5.0-lp152.2.30.1

opensuse advisories openSUSE-SU-2024:14572-1openSUSE-SU-2024:10600-1openSUSE-SU-2024:10601-1openSUSE-SU-2020:2315-1openSUSE-SU-2020:2187-1openSUSE-SU-2020:2096-1

  • firefox Alpine 3.23, Alpine 3.22, Alpine 3.21 +9 more 83.0-r0
  • librewolf Alpine 3.24, Alpine 3.23, Alpine 3.22 +1 more 83.0-r0
  • firefox-esr Alpine 3.23, Alpine 3.22, Alpine 3.21 +9 more 78.5.0-r0
  • thunderbird Alpine 3.23, Alpine 3.22, Alpine 3.21 +5 more 78.5.1-r0

alpine advisories ALPINE-CVE-2020-26959

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 83.0-1

arch linux advisories AVG-1279

Is CVE-2020-26959 in your stack? Fencer shows which of your repositories are affected as soon as an advisory lands, and opens the fix as a pull request.

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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-2020-26959 updated cvss 8.8

Mozilla: Use-after-free in WebRequestService

  • Red Hat Enterprise Linux 5 firefox
    affected: no fix planned “Out of support scope”
  • Red Hat Enterprise Linux AppStream (v. 8) firefox 78.5.0-1.el8_3, thunderbird 78.5.0-1.el8_3
    fixed: vendor fix RHSA-2020:5237
  • Red Hat Enterprise Linux AppStream E4S (v. 8.0) firefox 78.5.0-1.el8_0, thunderbird 78.5.0-1.el8_0
    fixed: vendor fix RHSA-2020:5314
  • Red Hat Enterprise Linux AppStream EUS (v. 8.1) firefox 78.5.0-1.el8_1, thunderbird 78.5.0-1.el8_1
    fixed: vendor fix RHSA-2020:5233
  • Red Hat Enterprise Linux AppStream EUS (v. 8.2) firefox 78.5.0-1.el8_2, thunderbird 78.5.0-1.el8_2
    fixed: vendor fix RHSA-2020:5234
  • Red Hat Enterprise Linux Client (v. 7) firefox 78.5.0-1.el7_9, thunderbird 78.5.0-1.el7_9 Red Hat Enterprise Linux Client Optional (v. 7) firefox 78.5.0-1.el7_9
    fixed: vendor fix RHSA-2020:5239
  • Red Hat Enterprise Linux Desktop (v. 6) firefox 78.5.0-1.el6_10, thunderbird 78.5.0-1.el6_10 Red Hat Enterprise Linux HPC Node Optional (v. 6) firefox 78.5.0-1.el6_10 Red Hat Enterprise Linux Server (v. 6) firefox 78.5.0-1.el6_10
    fixed: vendor fix RHSA-2020:5257
  • Red Hat Enterprise Linux Server (v. 7) firefox 78.5.0-1.el7_9
    fixed: vendor fix RHSA-2020:5239
show 4 more
  • Red Hat Enterprise Linux Server Optional (v. 6) thunderbird 78.5.0-1.el6_10
    fixed: vendor fix RHSA-2020:5238
  • Red Hat Enterprise Linux Server Optional (v. 7) firefox 78.5.0-1.el7_9, thunderbird 78.5.0-1.el7_9
    fixed: vendor fix RHSA-2020:5239
  • Red Hat Enterprise Linux Workstation (v. 6) firefox 78.5.0-1.el6_10, thunderbird 78.5.0-1.el6_10
    fixed: vendor fix RHSA-2020:5257
  • Red Hat Enterprise Linux Workstation (v. 7) firefox 78.5.0-1.el7_9, thunderbird 78.5.0-1.el7_9 Red Hat Enterprise Linux Workstation Optional (v. 7) firefox 78.5.0-1.el7_9
    fixed: vendor fix RHSA-2020:5239

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

1.30% chance of exploitation in the next 30 days, higher than 70% of all scored CVEs (FIRST EPSS, 2026-11-09).

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 Firefox ESR
  • Mozilla Thunderbird

Classification

Weakness

References

Sources

Fencer Intel 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.