What’s Vulnerable › Vulnerabilities

CVE-2021-38503

• Severity: critical (CVSS 10.0 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 4.1% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2021-12-08 · updated 2026-06-17
• Weakness: CWE-863

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.

Affected products

• Debian Linux: 9.0, 10.0, 11.0
• Mozilla Firefox: before 94.0 — fixed in 94.0
• Mozilla Firefox ESR: before 91.3 — fixed in 91.3
• Mozilla Thunderbird: before 91.3 — fixed in 91.3

How to fix

Upgrade to a fixed version: Mozilla Firefox 94.0; Mozilla Firefox ESR 91.3; Mozilla Thunderbird 91.3. Check the vendor advisory for mitigations.

References

• https://bugzilla.mozilla.org/show_bug.cgi?id=1729517
• https://lists.debian.org/debian-lts-announce/2021/12/msg00030.html
• https://lists.debian.org/debian-lts-announce/2022/01/msg00001.html
• https://security.gentoo.org/glsa/202202-03
• https://security.gentoo.org/glsa/202208-14
• https://www.debian.org/security/2021/dsa-5026
• https://www.debian.org/security/2022/dsa-5034
• https://www.mozilla.org/security/advisories/mfsa2021-48/
• https://www.mozilla.org/security/advisories/mfsa2021-49/
• https://www.mozilla.org/security/advisories/mfsa2021-50/
• https://bugzilla.mozilla.org/show_bug.cgi?id=1729517
• https://lists.debian.org/debian-lts-announce/2021/12/msg00030.html

Get alerted when CVE-2021-38503 or the next vulnerability affects your stack