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Fencer Vulns
high 8.8

CVE-2020-26950 in the critical feed json

In certain circumstances, the MCallGetProperty opcode can be emitted with unmet assumptions resulting in an exploitable use-after-free cond…

published 2020-12-09 5 y ago modified 2026-06-17 4 mo ago also EUVD-2020-19474

cvss
8.8
version 3.1, NVD
cisa kev
Not listed
not in CISA's catalog
epss
42.3%
EPSS 42.3% chance of exploitation in 30 days, percentile 98%
public exploits
1
Metasploit

Summary

In certain circumstances, the MCallGetProperty opcode can be emitted with unmet assumptions resulting in an exploitable use-after-free condition. This vulnerability affects Firefox < 82.0.3, Firefox ESR < 78.4.1, and Thunderbird < 78.4.2.

Patch / fix

  • mozilla firefox 82.0.3
  • mozilla firefox_esr 78.4.1
  • mozilla thunderbird 78.4.2
  • firefox-esr Debian 14, Debian 13, Debian 12 78.4.1esr-1
  • thunderbird Debian 14, Debian 13, Debian 12 1:78.4.2-1

debian advisories DEBIAN-CVE-2020-26950

  • firefox Ubuntu 16.04 LTS 82.0.3+build1-0ubuntu0.16.04.1
  • firefox Ubuntu 18.04 LTS 82.0.3+build1-0ubuntu0.18.04.1
  • firefox Ubuntu 20.04 LTS 82.0.3+build1-0ubuntu0.20.04.1
  • 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-2020-26950

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 6 78.4.3-1.el6_10
  • thunderbird RHEL 7 78.4.3-1.el7_9
  • thunderbird RHEL 8 78.4.3-1.el8_3
  • firefox RHEL 7 78.4.1-1.el7_9
  • firefox RHEL 6 78.4.1-1.el6_10
  • firefox RHEL 8 78.4.1-1.el8_3
  • thunderbird RHEL 8.2 EUS 78.4.3-1.el8_2
  • firefox RHEL 8.0 E4S 78.4.1-1.el8_0

red hat advisories RHSA-2020:5164RHSA-2020:5163RHSA-2020:5162RHSA-2020:5146RHSA-2020:5138RHSA-2020:5135

  • MozillaThunderbird SLE Workstation Extension 15 SP2, SLE Workstation Extension 15 SP1 78.4.2-3.103.2
  • MozillaFirefox SLES for SAP 12 SP5, SLES for SAP 12 SP4, SLES for SAP 12 SP3 +15 more 78.4.1-112.32.1
  • MozillaFirefox SLE Module for Desktop Applications 15 SP2 78.4.1-8.14.1
  • MozillaFirefox SLE Module for Desktop Applications 15 SP1 78.4.1-3.116.1
  • MozillaFirefox SLES 11 SP4-LTSS 78.4.1-78.102.1

suse advisories SUSE-SU-2020:3418-1SUSE-SU-2020:3331-1SUSE-SU-2020:3312-1SUSE-SU-2020:3311-1SUSE-SU-2020:14542-1

  • 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 78.4.2-lp151.2.56.1
  • MozillaThunderbird openSUSE Leap 15.2 78.4.2-lp152.2.16.1
  • MozillaFirefox openSUSE Leap 15.2 78.4.1-lp152.2.27.1
  • MozillaFirefox openSUSE Leap 15.1 78.4.1-lp151.2.76.1

opensuse advisories openSUSE-SU-2024:14572-1openSUSE-SU-2024:10600-1openSUSE-SU-2024:10601-1openSUSE-SU-2020:2133-1openSUSE-SU-2020:2022-1openSUSE-SU-2020:1919-1

  • firefox Alpine 3.23, Alpine 3.22, Alpine 3.21 +9 more 82.0.3-r0
  • mozjs78 Alpine 3.16, Alpine 3.15, Alpine 3.14 +1 more 78.4.1-r0
  • librewolf Alpine 3.24, Alpine 3.23, Alpine 3.22 +1 more 82.0.3-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-2020-26950

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 82.0.3-1

arch linux advisories AVG-1265

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-2020-26950 updated cvss 8.8

Mozilla: Write side effects in MCallGetProperty opcode not accounted for

  • Red Hat Enterprise Linux 5 firefox, thunderbird
    affected: no fix planned “Out of support scope”
  • Red Hat Enterprise Linux AppStream (v. 8) firefox 78.4.1-1.el8_3, thunderbird 78.4.3-1.el8_3
    fixed: vendor fix RHSA-2020:5100
  • Red Hat Enterprise Linux AppStream E4S (v. 8.0) firefox 78.4.1-1.el8_0, thunderbird 78.4.3-1.el8_0
    fixed: vendor fix RHSA-2020:5138
  • Red Hat Enterprise Linux AppStream EUS (v. 8.1) firefox 78.4.1-1.el8_1, thunderbird 78.4.3-1.el8_1
    fixed: vendor fix RHSA-2020:5135
  • Red Hat Enterprise Linux AppStream EUS (v. 8.2) firefox 78.4.1-1.el8_2, thunderbird 78.4.3-1.el8_2
    fixed: vendor fix RHSA-2020:5139
  • Red Hat Enterprise Linux Client (v. 7) firefox 78.4.1-1.el7_9, thunderbird 78.4.3-1.el7_9 Red Hat Enterprise Linux Client Optional (v. 7) firefox 78.4.1-1.el7_9
    fixed: vendor fix RHSA-2020:5099
  • Red Hat Enterprise Linux Desktop (v. 6) firefox 78.4.1-1.el6_10, thunderbird 78.4.3-1.el6_10 Red Hat Enterprise Linux HPC Node Optional (v. 6) firefox 78.4.1-1.el6_10 Red Hat Enterprise Linux Server (v. 6) firefox 78.4.1-1.el6_10
    fixed: vendor fix RHSA-2020:5104
  • Red Hat Enterprise Linux Server (v. 7) firefox 78.4.1-1.el7_9
    fixed: vendor fix RHSA-2020:5099
show 4 more
  • Red Hat Enterprise Linux Server Optional (v. 6) thunderbird 78.4.3-1.el6_10
    fixed: vendor fix RHSA-2020:5164
  • Red Hat Enterprise Linux Server Optional (v. 7) firefox 78.4.1-1.el7_9, thunderbird 78.4.3-1.el7_9
    fixed: vendor fix RHSA-2020:5099
  • Red Hat Enterprise Linux Workstation (v. 6) firefox 78.4.1-1.el6_10, thunderbird 78.4.3-1.el6_10
    fixed: vendor fix RHSA-2020:5104
  • Red Hat Enterprise Linux Workstation (v. 7) firefox 78.4.1-1.el7_9, thunderbird 78.4.3-1.el7_9 Red Hat Enterprise Linux Workstation Optional (v. 7) firefox 78.4.1-1.el7_9
    fixed: vendor fix RHSA-2020:5099

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

  1. First public exploit (Metasploit exploit/multi/browser/firefox_jit_use_after_free)
  2. Published

Exploit prediction

42.33% chance of exploitation in the next 30 days, higher than 98% of all scored CVEs (FIRST EPSS, 2026-11-08).

Exploits and checks

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 9.3 AV:N/AC:M/Au:N/C:C/I:C/A:C

Affected products

  • Mozilla Firefox
  • Mozilla Firefox ESR
  • Mozilla Thunderbird

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.