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Fencer Vulns
medium 6.5

CVE-2020-26976 json

When a HTTPS pages was embedded in a HTTP page, and there was a service worker registered for the former, the service worker could have int…

published 2021-01-07 5 y ago modified 2026-06-17 4 mo ago also EUVD-2020-19500

cvss
6.5
version 3.1, NVD
cisa kev
Not listed
not in CISA's catalog
epss
1.6%
EPSS 1.6% chance of exploitation in 30 days, percentile 76%
public exploits
0
none known

Summary

When a HTTPS pages was embedded in a HTTP page, and there was a service worker registered for the former, the service worker could have intercepted the request for the secure page despite the iframe not being a secure context due to the (insecure) framing. This vulnerability affects Firefox < 84.

Patch / fix

  • Mozilla Firefox 84
  • firefox-esr Debian 14, Debian 13, Debian 12 78.7.0esr-1
  • thunderbird Debian 14, Debian 13, Debian 12 1:78.7.0-1

debian advisories DEBIAN-CVE-2020-26976

  • 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-2020-26976

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 78.7.0-1.el7_9
  • thunderbird RHEL 8 78.7.0-1.el8_3
  • firefox RHEL 7 78.7.0-2.el7_9
  • firefox RHEL 8 78.7.0-2.el8_3
  • thunderbird RHEL 8.1 EUS 78.7.0-1.el8_1
  • thunderbird RHEL 8.2 EUS 78.7.0-1.el8_2
  • firefox RHEL 8.2 EUS 78.7.0-2.el8_2
  • firefox RHEL 8.1 EUS 78.7.0-2.el8_1

red hat advisories RHSA-2021:0397RHSA-2021:0297RHSA-2021:0298RHSA-2021:0299RHSA-2021:0290RHSA-2021:0289

  • MozillaFirefox SLE Module for Desktop Applications 15 SP2 78.7.0-8.26.2
  • MozillaThunderbird SLE Workstation Extension 15 SP2 78.7.0-8.9.1
  • MozillaFirefox SLES for SAP 15 SP1, SLES 15 SP1-LTSS, SLES 15 SP1-BCL +7 more 78.7.0-3.128.2
  • MozillaThunderbird SLE Workstation Extension 15 SP1 78.7.0-3.119.1
  • MozillaFirefox SLES for SAP 12 SP5, SLES for SAP 12 SP4, SLES for SAP 12 SP3 +15 more 78.7.0-112.45.1
  • MozillaFirefox SLES 11 SP4-LTSS 78.7.0-78.114.1

suse advisories SUSE-SU-2021:0259-1SUSE-SU-2021:0257-1SUSE-SU-2021:0246-1SUSE-SU-2021:0245-1SUSE-SU-2021:14609-1SUSE-SU-2021:0241-1

  • 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 78.7.0-lp152.2.43.1
  • MozillaFirefox openSUSE Leap 15.1 78.7.0-lp151.2.88.2
  • MozillaThunderbird openSUSE Leap 15.2 78.7.0-lp152.2.29.1
  • MozillaThunderbird openSUSE Leap 15.1 78.7.0-lp151.2.69.1

opensuse advisories openSUSE-SU-2024:14572-1openSUSE-SU-2024:10600-1openSUSE-SU-2024:10601-1openSUSE-SU-2021:0223-1openSUSE-SU-2021:0222-1openSUSE-SU-2021:0209-1

  • 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 78.7.0-r0
  • thunderbird Alpine 3.23, Alpine 3.22, Alpine 3.21 +5 more 78.7.0-r0

alpine advisories ALPINE-CVE-2020-26976

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 84.0-1
  • thunderbird Arch Linux 78.7.0-1

arch linux advisories AVG-1362AVG-1496

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-2020-26976 updated cvss 6.1

Mozilla: HTTPS pages could have been intercepted by a registered service worker when they should not have been

  • Red Hat Enterprise Linux 6 firefox, thunderbird
    affected: no fix planned “Out of support scope”
  • Red Hat Enterprise Linux AppStream (v. 8) firefox 78.7.0-2.el8_3, thunderbird 78.7.0-1.el8_3
    fixed: vendor fix RHSA-2021:0288
  • Red Hat Enterprise Linux AppStream EUS (v. 8.1) firefox 78.7.0-2.el8_1, thunderbird 78.7.0-1.el8_1
    fixed: vendor fix RHSA-2021:0285
  • Red Hat Enterprise Linux AppStream EUS (v. 8.2) firefox 78.7.0-2.el8_2, thunderbird 78.7.0-1.el8_2
    fixed: vendor fix RHSA-2021:0289
  • Red Hat Enterprise Linux Client (v. 7) firefox 78.7.0-2.el7_9, thunderbird 78.7.0-1.el7_9 Red Hat Enterprise Linux Client Optional (v. 7) firefox 78.7.0-2.el7_9 Red Hat Enterprise Linux Server (v. 7) firefox 78.7.0-2.el7_9 Red Hat Enterprise Linux Server Optional (v. 7) firefox 78.7.0-2.el7_9, thunderbird 78.7.0-1.el7_9 Red Hat Enterprise Linux Workstation (v. 7) firefox 78.7.0-2.el7_9, thunderbird 78.7.0-1.el7_9 Red Hat Enterprise Linux Workstation Optional (v. 7) firefox 78.7.0-2.el7_9
    fixed: vendor fix RHSA-2021:0290

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

1.64% chance of exploitation in the next 30 days, higher than 76% 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.