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CVE-2026-81900 Details

Description

Concrete CMS before 9.5.3 applied only trim() to the YouTube block's stored width and height values and printed them into iframe HTML attributes without escaping or integer casting, resulting in stored cross-site scripting. A user with edit_block permission could inject an event handler that executed script for visitors rendering the page, acting with administrative privileges where the victim was an administrator. The Concrete CMS security team gave this vulnerability a CVSS v4.0 score of 7.3 with vector CVSS:4.0/AV:N/AC:L/AT:P/PR:H/UI:P/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N. Thanks sh4d0byss for reporting.

Metrics

CVSS 3.x Severity and Vector Strings:

NIST: [email protected]CVSS-B:6.1 MEDIUMVector:CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N

References to Advisories, Solutions, and Tools

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URL	Source(s)	Tag(s)
https://documentation.concretecms.org/developers/introduction/version-history/953-release-notes	ConcreteCMS	Broken Link

Weakness Enumeration

CWE-ID	CWE Name	Source
CWE-79	Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')	ConcreteCMS

Affected Products

Product	Versions
concretecms concrete cms	< 9.5.3
CPE cpe:2.3:a:concretecms:concrete_cms:::::::: Remediation No remediation found in references.

Change History

3 change records found show changes

Date	Action	Recorded By
Sep 16, 2026	Initial Analysis	[email protected]
Sep 15, 2026	CVE Modified	CISA-ADP
Sep 14, 2026	New CVE Received	ConcreteCMS