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CVE-2026-81900

Concrete CMS before 9.5.3 is vulnerable to Stored XSS in the YouTube block (vWidth/vHeight)

CVSS 4.0 7.3 HIGHCheck source for fixed versionsKEV status unknown

Original advisory

cve-program · CVE-2026-81900

Published

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.

Read the publisher's advisory

Affected versions, risk and source history

cve-program · CVE-2026-81900

Published

Concrete CMS

https://github.com/concretecms/concretecms

Publisher's version information

Shown as published. These rules are not interpreted as NuGet package version ranges.

{
                    "vendor": "Concrete CMS",
                    "product": "Concrete CMS",
                    "collectionURL": "https://github.com/concretecms/concretecms",
                    "repo": "https://github.com/concretecms/concretecms",
                    "versions": [
                        {
                            "status": "affected",
                            "version": "5.0.0",
                            "lessThanOrEqual": "9.5.2",
                            "versionType": "git"
                        }
                    ],
                    "defaultStatus": "unaffected"
                }

Original source record · Cite this exact observation · Complete original publisher JSON