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Fencer Intel
medium 6.1

CVE-2020-26951 json

A parsing and event loading mismatch in Firefox's SVG code could have allowed load events to fire, even after sanitization.

published 5y ago modified 4mo ago also EUVD-2020-19475

cvss
6.1
version 3.1, NVD
cisa kev
Not listed
not in CISA's catalog
epss
1.1%
EPSS 1.1% chance of exploitation in 30 days, percentile 64%
public exploits
0
none known

Summary

A parsing and event loading mismatch in Firefox's SVG code could have allowed load events to fire, even after sanitization. An attacker already capable of exploiting an XSS vulnerability in privileged internal pages could have used this attack to bypass our built-in sanitizer. 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-2020-26951

  • 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-2020-26951

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-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-2020-26951

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

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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-2020-26951 updated cvss 6.1

Mozilla: Parsing mismatches could confuse and bypass security sanitizer for chrome privileged code

  • 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

1.05% chance of exploitation in the next 30 days, higher than 64% of all scored CVEs (FIRST EPSS, 2026-11-09).

CVSS scores

assigned by version score vector
NVD 3.1 6.1 CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N
NVD 2.0 4.3 AV:N/AC:M/Au:N/C:N/I:P/A:N

Affected products

  • Mozilla Firefox
  • Mozilla Firefox ESR
  • Mozilla Thunderbird

Classification

Weakness
  • CWE-79 Improper Neutralization of Input During Web Page Generation

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.