CVE-2026-68533 in Concrete CMS
Summary
by MITRE • 09/15/2026
Concrete CMS below 9.5.3 conversation attachment uploaded endpoint imported files into the file manager before evaluating the "Add Message Attachments" permission, which was only checked after the file had been stored. A user denied that permission, or an unauthenticated visitor on a guest-posting configuration, could import approved files of allowed types into the file manager. The Concrete CMS security team gave this vulnerability a CVSS v4.0 score of 2.3 with vector CVSS:4.0/AV:N/AC:L/AT:P/PR:L/UI:N/VC:N/VI:L/VA:L/SC:N/SI:N/SA:N. Thanks riodrwn for reporting.
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Analysis
by VulDB Data Team • 09/15/2026
The vulnerability identified in Concrete CMS versions prior to 9.5.3 represents a critical logic flaw within the conversation attachment upload mechanism, specifically classified under CWE-284 Improper Access Control. The core technical issue stems from an incorrect sequence of operations during file ingestion. When a user uploads an attachment through the conversation interface, the system is designed to validate permissions before allowing any persistent changes to the site's storage infrastructure. However, in affected versions, the implementation incorrectly prioritizes the physical importation and registration of files into the central file manager over the evaluation of specific access control policies. This architectural oversight means that the file is successfully stored on disk and indexed by the system before the security subsystem checks whether the requesting user possesses the "Add Message Attachments" permission or if the action aligns with guest-posting configurations.
This inversion of validation logic creates a significant bypass opportunity for unauthorized actors. In standard operational scenarios, access control lists are evaluated at the point of entry to prevent illicit actions from being executed. Here, because the file is imported into the manager first, an attacker who lacks the necessary permissions can still force the system to accept and store files that would otherwise be rejected if checked beforehand. This flaw allows users explicitly denied the "Add Message Attachments" permission, as well as unauthenticated visitors on sites configured for guest posting, to upload approved file types directly into the global file manager. The ability of an unauthorized user to manipulate the file repository undermines the integrity and confidentiality controls established by the CMS administrator, effectively granting them write access to areas they should not be able to reach.
The operational impact of this vulnerability is primarily centered on data integrity and potential downstream exploitation risks associated with stored files. While the CVSS v4.0 score provided by the Concrete CMS security team indicates a low severity rating of 2.3, reflecting limited confidentiality or availability impact in isolation, the implications for system administration are notable. An attacker can populate the file manager with arbitrary content, which may be used to consume disk space, clutter administrative interfaces, or potentially serve as a staging ground if other vulnerabilities exist that allow execution from uploaded files. The ATT&CK framework categorizes such behavior under techniques related to Initial Access and Persistence, specifically where an adversary leverages application logic flaws to establish a foothold within the system's data stores without triggering standard authentication failures.
To mitigate this vulnerability, administrators must upgrade Concrete CMS to version 9.5.3 or later immediately upon availability. The patch corrects the execution order by ensuring that permission checks are performed prior to any file storage operations. Until an upgrade is feasible, site administrators should review their guest-posting configurations and restrict public-facing conversation features where possible to reduce the attack surface. Additionally, implementing strict web application firewall rules can help monitor for anomalous upload patterns, although this is a compensatory control rather than a fix for the underlying logic error. Regular audits of file manager contents may also assist in identifying any unauthorized files that were imported during the window of vulnerability exposure.