CVE-2026-9816 in Mattermost
Summary
by MITRE • 08/18/2026
Mattermost versions 11.7.x <= 11.7.6, 10.11.x <= 10.11.21, 11.8.x <= 11.8.3 fail to validate BoardMember.Scheme* fields server-side on insert and archive-import paths which allows a board editor or non-guest team member to grant board admin to arbitrary users via POST /api/v2/boards/{boardID}/members and POST /api/v2/teams/{teamID}/archive/import.. Mattermost Advisory ID: MMSA-2026-00685
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Analysis
by VulDB Data Team • 08/18/2026
The vulnerability identified in Mattermost versions 11.7.x through 11.7.6, 10.11.x through 10.11.21, and 11.8.x through 11.8.3 represents a critical server-side request forgery flaw within the platform's board management subsystem. Specifically, the application fails to perform adequate validation on the Scheme* fields associated with BoardMember objects during both direct insertion operations and archive import processes. This lack of input sanitization allows an attacker who possesses the privileges of a board editor or a non-guest team member to manipulate these fields arbitrarily. By exploiting this deficiency, the attacker can escalate their own permissions or those of other users by granting them board administrator status without proper authorization checks from the server side.
From a technical perspective, the core issue lies in the absence of strict type and value validation for the Scheme* attributes when processing POST requests to endpoints such as /api/v2/boards/{boardID}/members and /api/v2/teams/{teamID}/archive/import. In secure software design, any user-supplied data that influences access control decisions must be validated against a whitelist of allowed values or strictly typed constraints on the server side. Here, the application trusts client-provided input for role assignment fields without verifying if the requesting user has the authority to assign such elevated privileges. This constitutes an insecure direct object reference combined with broken access control mechanisms, where the logical flow of privilege escalation is bypassed due to insufficient server-side enforcement.
The operational impact of this vulnerability is significant as it enables unauthorized privilege escalation within Mattermost workspaces. An attacker who gains initial foothold as a board editor or non-guest team member can leverage this flaw to grant themselves or accomplices full administrative control over specific boards. This level of access typically includes the ability to modify board settings, delete content, manage other members, and potentially exfiltrate sensitive data stored within those boards. In enterprise environments where Mattermost is used for critical communications, such an escalation could lead to broader system compromise if these elevated permissions are leveraged to pivot into more privileged areas or disrupt operational continuity through malicious configuration changes.
This vulnerability aligns with Common Weakness Enumeration (CWE) category CWE-269 Improper Privilege Management and CWE-862 Missing Authorization, as the application fails to enforce proper access control policies for sensitive operations. Furthermore, it maps to MITRE ATT&CK technique T1078 Valid Accounts, where an attacker uses legitimate credentials with insufficiently restricted permissions to escalate privileges within a system. The exploitation path also reflects aspects of CWE-915 Improper Control of Dynamically-Managed Code Resources if the imported archives contain malicious payloads that rely on these elevated permissions for execution or data access.
To mitigate this risk, organizations running affected versions must immediately upgrade Mattermost to patched releases where server-side validation for BoardMember.Scheme* fields has been implemented and enforced strictly during insert and archive import operations. Until patching is complete, administrators should review board membership configurations regularly to detect any unauthorized elevation of privileges. Additionally, implementing strict role-based access control policies that limit the ability of lower-privileged users to modify member roles can provide a compensating control. Security teams should also monitor audit logs for unusual patterns in POST requests targeting board and team archive endpoints to identify potential exploitation attempts in real-time.