CVE-2026-81911 in Concrete CMS
Summary
by MITRE • 09/11/2026
Concrete CMS versions 9.0.0 to 9.5.2 is vulnerable to Stored XSS in Board Custom Slot dialog. The custom_slot save_template endpoint authorizes the request only against the target board instance (canEditBoardContents()) and then persists the client-supplied selectedTemplateOption[collection] verbatim, rather than rebuilding the content object collection server-side and verifying that each item belongs to the authorized board's data pool. A user with permission to edit the contents of at least one board instance can therefore store a forged summary object whose description field carries a JavaScript-bearing HTML payload. The default summary template renders the description field without output encoding, so the payload executes in the browser of any user who views the affected board slot, including anonymous front-end visitors and dashboard users who preview the resulting rule or block. This can enable session or action takeover and escalation toward an administrator. Concrete CMS versions below 9 do not include the Boards feature and are not affected. The Concrete CMS security team gave this vulnerability a CVSS v4.0 score of 5.8 with vector CVSS:4.0/AV:N/AC:L/AT:P/PR:H/UI:P/VC:H/VI:L/VA:L/SC:N/SI:N/SA:N. Thanks riodrwn for reporting.
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Analysis
by VulDB Data Team • 09/11/2026
Concrete CMS versions 9.0.0 through 9.5.2 contain a critical stored cross-site scripting vulnerability within the Boards feature, specifically located in the Board Custom Slot dialog functionality. This flaw arises from an insufficient authorization check and improper input validation during the template saving process. The application's backend logic for the custom_slot save_template endpoint performs an authorization verification solely against the target board instance by invoking the canEditBoardContents method. While this ensures that a user has permission to edit content within at least one specific board, it fails to validate the integrity of the data structure being persisted. Specifically, the system accepts and stores client-supplied values for selectedTemplateOption[collection] verbatim without reconstructing the content object collection server-side or verifying that each item in the collection actually belongs to the authorized board's designated data pool. This architectural oversight allows an attacker with edit permissions on any single board instance to inject arbitrary data structures into the system, bypassing logical access controls intended to restrict cross-board data manipulation.
The technical exploitation of this vulnerability involves crafting a forged summary object where the description field contains malicious JavaScript code embedded within HTML tags. Because the application persists these values directly without sanitization or server-side reconstruction and validation, the payload is stored in the database as part of the board's configuration. The severity of this issue is significantly amplified by how the default summary template renders data on the front end. When a user views the affected board slot, the description field is rendered using an output encoding mechanism that fails to properly escape special characters or script tags. Consequently, any browser rendering this view will execute the injected JavaScript payload in the context of the current session. This execution occurs regardless of whether the viewer is an authenticated dashboard user previewing a rule and block, or an anonymous visitor accessing the public-facing site, thereby expanding the attack surface considerably beyond just administrative accounts.
The operational impact of this stored cross-site scripting vulnerability extends far beyond simple defacement or minor script injection. Since the payload executes in the context of any viewer's session, it enables attackers to perform session hijacking by stealing authentication cookies or tokens transmitted via HTTP headers or local storage. This capability facilitates immediate action takeover, allowing the attacker to impersonate legitimate users and access restricted areas of the application. Furthermore, if an administrator views a compromised board slot, the vulnerability can lead to privilege escalation, granting the attacker full administrative control over the Concrete CMS instance. The ability to execute arbitrary code in this manner compromises the confidentiality, integrity, and availability of user data and system operations. Although versions prior to 9.0 are not affected due to the absence of the Boards feature, all supported releases within the specified range remain vulnerable until patched.
From a classification perspective, this vulnerability aligns with CWE-79 Improper Neutralization of Input During Web Page Generation commonly known as Cross-site Scripting and specifically falls under CWE-284 Improper Access Control due to the flawed authorization logic that permits unauthorized data structure injection. In terms of offensive security frameworks, it maps to MITRE ATT&CK technique T1059 Command and Scripting Interpreter for payload execution and potentially T1078 Valid Accounts if used in conjunction with credential theft or social engineering to gain initial access. The Concrete CMS security team has assigned this vulnerability a CVSS v4.0 score of 5.8, characterized by the vector AV:N/AC:L/AT:P/PR:H/UI:P/VC:H/VI:L/VA:L/SC:N/SI:N/SA:N. This scoring reflects that while the attack requires high privileges to initiate and involves user interaction for execution, it results in a high impact on confidentiality due to session takeover capabilities. Mitigation strategies should focus on immediate patching to the latest secure version of Concrete CMS where these authorization checks have been hardened. Additionally, implementing strict output encoding for all dynamic content rendered from database sources is essential to prevent script execution even if input validation fails. Security teams should also audit existing board configurations for any signs of prior exploitation and enforce principle of least privilege by ensuring that edit permissions are granted only to necessary users.