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Fencer Intel
critical 10.0

CVE-2021-38503 json

The iframe sandbox rules were not correctly applied to XSLT stylesheets, allowing an iframe to bypass restrictions such as executing script…

published 4y ago modified 4mo ago also EUVD-2021-24955

cvss
10.0
version 3.1, NVD
cisa kev
Not listed
not in CISA's catalog
epss
4.1%
EPSS 4.1% chance of exploitation in 30 days, percentile 90%
public exploits
0
none known

Summary

The iframe sandbox rules were not correctly applied to XSLT stylesheets, allowing an iframe to bypass restrictions such as executing scripts or navigating the top-level frame. This vulnerability affects Firefox < 94, Thunderbird < 91.3, and Firefox ESR < 91.3.

Patch / fix

  • Mozilla Firefox 94
  • Mozilla Thunderbird 91.3
  • Mozilla Firefox ESR 91.3
  • firefox-esr Debian 14, Debian 13, Debian 12 91.3.0esr-1
  • thunderbird Debian 14, Debian 13, Debian 12 1:91.3.0-1

debian advisories DEBIAN-CVE-2021-38503

  • firefox Ubuntu 18.04 LTS 94.0+build3-0ubuntu0.18.04.1
  • firefox Ubuntu 20.04 LTS 94.0+build3-0ubuntu0.20.04.1
  • firefox Ubuntu 22.04 LTS 94.0+build3-0ubuntu1
  • thunderbird Ubuntu 18.04 LTS 1:91.5.0+build1-0ubuntu0.18.04.1
  • thunderbird Ubuntu 20.04 LTS 1:91.5.0+build1-0ubuntu0.20.04.1
  • thunderbird Ubuntu 22.04 LTS 1:91.3.1+build1-0ubuntu1

ubuntu advisories UBUNTU-CVE-2021-38503

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 91.3.0-2.el7_9
  • thunderbird RHEL 8 91.3.0-2.el8_4
  • firefox RHEL 8 91.3.0-1.el8_4
  • firefox RHEL 7 91.3.0-1.el7_9
  • thunderbird RHEL 8.1 EUS 91.3.0-2.el8_1
  • thunderbird RHEL 8.2 EUS 91.3.0-2.el8_2
  • firefox RHEL 8.2 EUS 91.3.0-1.el8_2
  • firefox RHEL 8.1 EUS 91.3.0-1.el8_1

red hat advisories RHSA-2021:4133RHSA-2021:4132RHSA-2021:4134RHSA-2021:4605RHSA-2021:4130RHSA-2021:4607

  • MozillaThunderbird SLE Workstation Extension 15 SP3, SLE Workstation Extension 15 SP2 91.4.0-8.45.2
  • MozillaFirefox SLE Module for Desktop Applications 15 SP3, SLE Module for Desktop Applications 15 SP2 91.3.0-152.6.1
  • MozillaFirefox SLES for SAP 12 SP5, SLES for SAP 12 SP4, SLES for SAP 12 SP3 +11 more 91.3.0-112.80.2
  • MozillaFirefox SLES for SAP 15 SP1, SLES for SAP 15, SLES 15-LTSS +7 more 91.3.0-150.6.1

suse advisories SUSE-SU-2021:4150-1SUSE-SU-2021:3745-1SUSE-SU-2021:3721-1SUSE-SU-2021:3651-1

  • firefox-esr openSUSE Tumbleweed 128.5.1-1.1
  • MozillaThunderbird openSUSE Tumbleweed 91.3.0-1.1
  • MozillaFirefox openSUSE Tumbleweed 94.0.1-1.1
  • MozillaThunderbird openSUSE Leap 15.2 91.4.0-lp152.2.52.1
  • MozillaThunderbird openSUSE Leap 15.3 91.4.0-8.45.2
  • MozillaFirefox openSUSE Leap 15.3 91.3.0-152.6.1

opensuse advisories openSUSE-SU-2024:14572-1openSUSE-SU-2024:11607-1openSUSE-SU-2024:11614-1openSUSE-SU-2021:1635-1openSUSE-SU-2021:4150-1openSUSE-SU-2021:3745-1

  • thunderbird AlmaLinux 8 91.3.0-2.el8_4.alma
  • thunderbird AlmaLinux 8 91.3.0-2.el8_4.alma.plus
  • firefox AlmaLinux 8 91.3.0-1.el8_4.alma

almalinux advisories ALSA-2021:4130ALSA-2021:4123

  • thunderbird Rocky Linux 8 91.3.0-2.el8_4
  • thunderbird Rocky Linux 8 91.3.0-2.el8
  • firefox Rocky Linux 8 91.3.0-1.el8

rocky linux advisories RLSA-2021:4130RLSA-2021:4123

  • firefox Alpine 3.23, Alpine 3.22, Alpine 3.21 +6 more 94.0-r0
  • librewolf Alpine 3.24, Alpine 3.23, Alpine 3.22 +1 more 94.0-r0
  • firefox-esr Alpine 3.23, Alpine 3.22, Alpine 3.21 +6 more 91.3.0-r0
  • thunderbird Alpine 3.23, Alpine 3.22, Alpine 3.21 +5 more 91.3.2-r0

alpine advisories ALPINE-CVE-2021-38503

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 94.0-1
  • thunderbird Arch Linux 91.3.0-1

arch linux advisories AVG-2511AVG-2518

Is CVE-2021-38503 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-2021-38503 updated cvss 10.0

Mozilla: iframe sandbox rules did not apply to XSLT stylesheets

  • Red Hat Enterprise Linux 6 firefox, thunderbird
    affected: no fix planned “Out of support scope”
  • Red Hat Enterprise Linux AppStream (v. 8) firefox 91.3.0-1.el8_4, thunderbird 91.3.0-2.el8_4
    fixed: vendor fix RHSA-2021:4123
  • Red Hat Enterprise Linux AppStream EUS (v. 8.1) firefox 91.3.0-1.el8_1, thunderbird 91.3.0-2.el8_1
    fixed: vendor fix RHSA-2021:4607
  • Red Hat Enterprise Linux AppStream EUS (v. 8.2) firefox 91.3.0-1.el8_2, thunderbird 91.3.0-2.el8_2
    fixed: vendor fix RHSA-2021:4605
  • Red Hat Enterprise Linux Client (v. 7) firefox 91.3.0-1.el7_9, thunderbird 91.3.0-2.el7_9 Red Hat Enterprise Linux Client Optional (v. 7) firefox 91.3.0-1.el7_9 Red Hat Enterprise Linux Server (v. 7) firefox 91.3.0-1.el7_9 Red Hat Enterprise Linux Server Optional (v. 7) firefox 91.3.0-1.el7_9, thunderbird 91.3.0-2.el7_9 Red Hat Enterprise Linux Workstation (v. 7) firefox 91.3.0-1.el7_9, thunderbird 91.3.0-2.el7_9 Red Hat Enterprise Linux Workstation Optional (v. 7) firefox 91.3.0-1.el7_9
    fixed: vendor fix RHSA-2021:4116

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

4.12% chance of exploitation in the next 30 days, higher than 90% of all scored CVEs (FIRST EPSS, 2026-11-09).

CVSS scores

assigned by version score vector
NVD 3.1 10.0 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:H
NVD 2.0 7.5 AV:N/AC:L/Au:N/C:P/I:P/A:P

Affected products

  • Mozilla Firefox
  • Mozilla Thunderbird
  • Mozilla Firefox ESR

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.