Concrete CMS up to 9.5.2 YouTube block trim width/height cross site scripting

Summaryinfo

A vulnerability has been found in Concrete CMS up to 9.5.2 and classified as problematic. Affected is the function trim of the component YouTube block. Performing a manipulation of the argument width/height results in cross site scripting. This vulnerability is known as CVE-2026-81900. Remote exploitation of the attack is possible. No exploit is available.

Detailsinfo

A vulnerability classified as problematic has been found in Concrete CMS up to 9.5.2. This affects the function trim of the component YouTube block. The manipulation of the argument width/height with an unknown input leads to a cross site scripting vulnerability. CWE is classifying the issue as CWE-79. The product does not neutralize or incorrectly neutralizes user-controllable input before it is placed in output that is used as a web page that is served to other users. This is going to have an impact on integrity. The summary by CVE is:

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.

It is possible to read the advisory at documentation.concretecms.org. This vulnerability is uniquely identified as CVE-2026-81900 since 08/27/2026. The exploitability is told to be easy. It is possible to initiate the attack remotely. The exploitation requires an enhanced level of successful authentication. It demands that the victim is doing some kind of user interaction. Technical details of the vulnerability are known, but there is no available exploit. The attack technique deployed by this issue is T1059.007 according to MITRE ATT&CK.

There is no information about possible countermeasures known. It may be suggested to replace the affected object with an alternative product.

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Productinfo

Type

• Content Management System

Name

• Concrete CMS

Version

• 9.5.0
• 9.5.1
• 9.5.2

License

• open-source

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