CVE-2021-38503
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.
Does this matter?
High impact if exploited, but EPSS currently rates exploitation as unlikely (4.12%). Schedule it in the normal patch cycle and watch for a rise in EPSS or a public exploit.
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.
- CVSS 3.1
- 10.0 CRITICALCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:H
- EPSS
- 4.12% probability · 91th percentile
- Public exploits
- None in Exploit-DB
- CISA KEV
- Not listed
- Weakness
- CWE-863
- Affected
- mozilla/firefox · mozilla/firefox esr · mozilla/thunderbird · debian/debian linux
- Source
- security@mozilla.org
EPSS 4.1% since 26 October 2026; no change recorded yet. Points are recorded when the score first appears and whenever it moves by a percentage point or more.
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
Source: NVD record, EPSS from FIRST.org, KEV from CISA, exploits from Exploit-DB. Refreshed daily. Download this record as JSON.