CVE-2026-81926 in Concrete CMS
Summary
by MITRE • 09/15/2026
Concrete CMS 9.4.0 through 9.5.2 did not escape colliding page paths before rendering them in the location panel's duplicate-path confirmation dialog. The panel's check endpoint returned the submitted path unmodified in its JSON response, and client-side JavaScript inserted each value into the dialog as raw HTML, so a crafted page path executed script in the editor's authenticated browser session. The Concrete CMS security team gave this vulnerability a CVSS v4.0 score of 2.0 with vector CVSS:4.0/AV:N/AC:L/AT:P/PR:H/UI:P/VC:L/VI:L/VA:L/SC:N/SI:N/SA:N. Thanks Yonatan Drori from Tenzai for reporting.
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Analysis
by VulDB Data Team • 09/16/2026
The vulnerability identified in Concrete CMS versions 9.4.0 through 9.5.2 represents a classic stored cross-site scripting flaw rooted in insufficient input validation and improper output encoding within the administrative interface. The core technical failure occurs during the page creation or editing workflow, specifically when the system attempts to validate whether a requested URL path is already occupied by another entity. In this process, the backend check endpoint accepts user-supplied path data but fails to sanitize special characters before returning them in the JSON response. This lack of server-side encoding means that malicious payloads embedded within the page path are preserved intact during transmission from the server back to the client browser.
Upon receiving the unmodified JSON payload, the client-side JavaScript responsible for rendering the duplicate-path confirmation dialog proceeds to insert these values directly into the Document Object Model without applying appropriate HTML entity escaping or context-aware encoding. Because the data is treated as raw HTML rather than safe text content, any script tags or event handlers contained within the crafted page path are executed immediately upon display of the dialog box. This execution occurs within the authenticated browser session of a privileged user, such as an administrator or editor, who has access to the CMS backend interface. The vulnerability relies on social engineering tactics where an attacker might trick a legitimate user into creating a page with a maliciously crafted name, thereby triggering the vulnerable code path during the validation phase.
From an operational impact perspective, this flaw allows for session hijacking, credential theft via keylogging overlays, or defacement of the administrative interface if combined with other vectors. Although the CVSS v4.0 score assigned by the Concrete CMS security team is relatively low at 2.0 due to the requirement for high privileges and user interaction, the potential consequences in a compromised environment can be severe. An attacker could leverage this initial access point to escalate privileges further or pivot into more critical systems if the administrative session remains active. The vulnerability highlights the persistent risk associated with client-side rendering of unsanitized server responses, particularly in content management systems where dynamic path generation is common.
This issue aligns closely with CWE-79, which describes Improper Neutralization of Input During Web Page Generation commonly known as Cross-site Scripting (XSS). Specifically, it falls under the sub-category of reflected or stored XSS depending on how long the malicious payload persists in the system state before triggering. In terms of offensive security frameworks, this vulnerability maps to MITRE ATT&CK technique T1059, Command and Control via Browser-based scripting, as well as aspects of T1204, User Execution, since it requires a user to initiate the action by submitting the crafted page path. The attack vector is classified as Network-accessible with Local complexity but requires Privileges High and User Interaction Present, reflecting the authenticated nature of the exploit.
Mitigation strategies for this vulnerability involve both immediate patching and long-term architectural improvements. Administrators should update Concrete CMS to version 9.5.3 or later where these input validation checks have been corrected. From a development perspective, implementing strict output encoding on all dynamic content inserted into HTML contexts is essential to prevent script execution regardless of the source data integrity. Additionally, adopting Content Security Policy headers can provide an additional layer of defense by restricting the sources from which scripts are allowed to execute, thereby mitigating the impact even if similar vulnerabilities exist in other parts of the application. Regular security audits focusing on JSON response handling and client-side DOM manipulation practices will help identify and remediate such encoding flaws before they can be exploited in production environments.