Concrete CMS up to 9.5.2 Express Form Block improper authorization

Summaryinfo

A vulnerability was found in Concrete CMS up to 9.5.2. It has been declared as problematic. This vulnerability affects unknown code of the component Express Form Block. Executing a manipulation can lead to improper authorization. The identification of this vulnerability is CVE-2026-18426. The attack may be launched remotely. There is no exploit available.

Detailsinfo

A vulnerability classified as problematic was found in Concrete CMS up to 9.5.2. This vulnerability affects some unknown processing of the component Express Form Block. The manipulation with an unknown input leads to a improper authorization vulnerability. The CWE definition for the vulnerability is CWE-285. The product does not perform or incorrectly performs an authorization check when an actor attempts to access a resource or perform an action. As an impact it is known to affect confidentiality, integrity, and availability. CVE summarizes:

Concrete CMS 9.0.0 through 9.5.2 did not enforce a block-level edit-permission check on the Express Form block's control-management actions, which relied solely on CSRF token validation. Because the token is bound to the user and action rather than to a specific block, page, or form, an authenticated user with edit access to one Express Form could reuse a validly obtained token to add, modify, or delete controls on Express Forms they were not authorized to edit, including injecting a control whose value is later rendered as HTML to achieve stored XSS. The Concrete CMS security team gave this vulnerability a CVSS v4.0 score of 2.0 with vector CVSS:4.0/AV:N/AC:H/AT:P/PR:H/UI:P/VC:L/VI:N/VA:N/SC:N/SI:N/SA:N. Thanks Yat Wu for reporting.

The advisory is shared for download at documentation.concretecms.org. This vulnerability was named CVE-2026-18426 since 07/30/2026. The exploitation appears to be easy. The attack can be initiated remotely. Additional levels of successful authentication are needed for exploitation. Successful exploitation requires user interaction by the victim. There are neither technical details nor an exploit publicly available. The current price for an exploit might be approx. USD $0-$5k (estimation calculated on 09/15/2026). The MITRE ATT&CK project declares the attack technique as T1548.002.

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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