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Fencer Vulns
medium 6.5

CVE-2020-26967 json

When listening for page changes with a Mutation Observer, a malicious web page could confuse Firefox Screenshots into interacting with elem…

published 2020-12-09 5 y ago modified 2026-06-17 4 mo ago also EUVD-2020-19491

cvss
6.5
version 3.1, NVD
cisa kev
Not listed
not in CISA's catalog
epss
0.9%
EPSS 0.9% chance of exploitation in 30 days, percentile 58%
public exploits
0
none known

Summary

When listening for page changes with a Mutation Observer, a malicious web page could confuse Firefox Screenshots into interacting with elements other than those that it injected into the page. This would lead to internal errors and unexpected behavior in the Screenshots code. This vulnerability affects Firefox < 83.

Patch / fix

  • mozilla firefox 83.0
  • 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

ubuntu advisories UBUNTU-CVE-2020-26967

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-esr openSUSE Tumbleweed 128.5.1-1.1
  • MozillaFirefox openSUSE Tumbleweed 92.0-1.2

opensuse advisories openSUSE-SU-2024:14572-1openSUSE-SU-2024:10600-1

  • 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

alpine advisories ALPINE-CVE-2020-26967

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

Exploit prediction

0.85% chance of exploitation in the next 30 days, higher than 58% of all scored CVEs (FIRST EPSS, 2026-11-08).

CVSS scores

assigned by version score vector
NVD 3.1 6.5 CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:H/A:N
NVD 2.0 4.3 AV:N/AC:M/Au:N/C:N/I:P/A:N

Affected products

  • Mozilla Firefox

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.