Concrete CMS up to 9.5.0 AddMessage/UpdateMessage attachments authorization

Summaryinfo

A vulnerability labeled as critical has been found in Concrete CMS CMS up to 9.5.0. Affected by this issue is the function AddMessage/UpdateMessage. Such manipulation of the argument attachments leads to authorization. This vulnerability is uniquely identified as CVE-2026-7886. The attack can be launched remotely. No exploit exists.

Detailsinfo

A vulnerability was found in Concrete CMS CMS up to 9.5.0. It has been rated as critical. Affected by this issue is the function AddMessage/UpdateMessage. The manipulation of the argument attachments with an unknown input leads to a authorization vulnerability. Using CWE to declare the problem leads to CWE-639. The system's authorization functionality does not prevent one user from gaining access to another user's data or record by modifying the key value identifying the data. Impacted is confidentiality, integrity, and availability. CVE summarizes:

Concrete CMS 9.5.0 and below is vulnerable to IDOR in AddMessage/UpdateMessage via attachments[] parameter which can lead to file permission bypass. The `AddMessage` and `UpdateMessage` conversation controllers accept user-supplied file attachment IDs and load files directly via `$em->find(File::class, $attachmentID)` without checking per-file permissions (`canViewFile()`). A user who can post in any conversation can reference any file in the CMS file manager by its sequential ID, effectively bypassing the file permission system.  The Concrete CMS security team gave this vulnerability a CVSS v.4.0 score of 2.3 with a vector CVSS:4.0/AV:N/AC:L/AT:P/PR:L/UI:N/VC:L/VI:L/VA:N/SC:N/SI:N/SA:N. Thanks Tristan Mandani for reporting. if a site truly has private files, the owner should set up a private storage location https://documentation.concretecms.org/user-guide/editors-reference/dashboard/system-and-maintenance/files/file-storage-locations outside of the webroot so that permissions can be checked on view as well. That way, even if a authorized user attaches a file, or otherwise links to it, unauthorized users won't be able to view the file.

The weakness was disclosed by Tristan Mandani. The advisory is available at documentation.concretecms.org. This vulnerability is handled as CVE-2026-7886 since 05/05/2026. The exploitation is known to be easy. The attack may be launched remotely. Technical details are known, but there is no available exploit. The structure of the vulnerability defines a possible price range of USD $0-$5k at the moment (estimation calculated on 05/22/2026).

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

Vendor

• Concrete CMS

Name

• CMS

Version

• 9.0
• 9.1
• 9.2
• 9.3
• 9.4
• 9.5.0

License

• open-source

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CPE 2.2info

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