CVE-2021-38503

Name: CVE-2021-38503
Creator: NVD / NIST
Published: 2021-12-08T22:15:08.737+00:00
License: https://creativecommons.org/publicdomain/zero/1.0/

CRITICALCVSS 10/10EPSS 3.83%

Last modified Jun 17, 2026

CVE-2021-38503 is a critical-severity vulnerability rated 10/10 on the CVSS scale. 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.. EPSS estimates a 3.83% chance of exploitation in the next 30 days.

Description

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.

Metrics

CVSS 3.1

10/10

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:H

EPSS Probability

3.83%

88.7th percentile

Probability of exploitation in the next 30 days. Learn more

Weakness Enumeration

CWE-863

Affected Software

Vendor	Product	Versions
Mozilla	Firefox	< 94.0
Mozilla	Firefox Esr	< 91.3
Mozilla	Thunderbird	< 91.3
Debian	Debian Linux	9.0
Debian	Debian Linux	10.0
Debian	Debian Linux	11.0

References

https://bugzilla.mozilla.org/show_bug.cgi?id=1729517Issue Tracking, Permissions Required, Vendor Advisory
https://lists.debian.org/debian-lts-announce/2021/12/msg00030.htmlMailing List, Third Party Advisory
https://lists.debian.org/debian-lts-announce/2022/01/msg00001.htmlMailing List, Third Party Advisory
https://security.gentoo.org/glsa/202202-03Third Party Advisory
https://security.gentoo.org/glsa/202208-14Third Party Advisory
https://www.debian.org/security/2021/dsa-5026Third Party Advisory
https://www.debian.org/security/2022/dsa-5034Third Party Advisory
https://www.mozilla.org/security/advisories/mfsa2021-48/Vendor Advisory
https://www.mozilla.org/security/advisories/mfsa2021-49/Vendor Advisory
https://www.mozilla.org/security/advisories/mfsa2021-50/Vendor Advisory
https://bugzilla.mozilla.org/show_bug.cgi?id=1729517Issue Tracking, Permissions Required, Vendor Advisory
https://lists.debian.org/debian-lts-announce/2021/12/msg00030.htmlMailing List, Third Party Advisory
https://lists.debian.org/debian-lts-announce/2022/01/msg00001.htmlMailing List, Third Party Advisory
https://security.gentoo.org/glsa/202202-03Third Party Advisory
https://security.gentoo.org/glsa/202208-14Third Party Advisory
https://www.debian.org/security/2021/dsa-5026Third Party Advisory
https://www.debian.org/security/2022/dsa-5034Third Party Advisory
https://www.mozilla.org/security/advisories/mfsa2021-48/Vendor Advisory
https://www.mozilla.org/security/advisories/mfsa2021-49/Vendor Advisory
https://www.mozilla.org/security/advisories/mfsa2021-50/Vendor Advisory

Timeline

Published: Dec 8, 2021
Last Modified: Jun 17, 2026
Status: Modified

Frequently Asked Questions

What is CVE-2021-38503?

How severe is CVE-2021-38503?

CVE-2021-38503 has a CVSS score of 10/10 (CRITICAL severity). The EPSS model estimates a 3.83% probability of exploitation in the next 30 days.

How do I fix CVE-2021-38503?

Check the vendor references and advisories linked above for patched versions and mitigation guidance. You can also run a Strix scan to test if your systems are affected.

Are you affected by CVE-2021-38503?

Run a free Strix scan to check your systems for this vulnerability.

Scan your code now

Source: NVD / NIST