CVE-2026-10527 in Mattermost
Summary
by MITRE • 08/17/2026
Mattermost versions 11.7.x <= 11.7.6, 10.11.x <= 10.11.21, 11.8.x <= 11.8.3 fails to reconcile SchemeAdmin flags with a user's current role which allows a user demoted to System Guest to retain Board Admin privileges and perform admin-only operations via the Boards REST API or UI.. Mattermost Advisory ID: MMSA-2026-00691
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Analysis
by VulDB Data Team • 08/17/2026
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 privilege escalation flaw rooted in the application's role reconciliation logic. Specifically, when an administrator demotes a user from a higher-privilege role to System Guest, the system fails to properly revoke or reconcile existing administrative permissions associated with specific features such as Boards. This discrepancy allows users who have been explicitly downgraded to the most restricted access level to retain Board Admin privileges. Consequently, these users can continue to perform operations that require elevated authority, effectively bypassing the intended security controls designed to limit their capabilities within the platform.
From a technical perspective, this issue stems from an incomplete state synchronization between user role assignments and feature-specific permission flags. When a role change occurs, particularly one involving significant privilege reduction like moving to System Guest status, the backend service responsible for managing Boards does not adequately clear or invalidate previous administrative tokens or session states linked to that specific module. This lack of immediate and comprehensive flag reconciliation means that authentication checks against the Boards REST API or user interface continue to validate against outdated permission sets rather than the current global role definition. The flaw is particularly insidious because it persists even after explicit administrative action has been taken to restrict the account, creating a window of opportunity for abuse that relies on state inconsistency within the application's authorization engine.
The operational impact of this vulnerability is severe, as it directly compromises the principle of least privilege and allows unauthorized access to sensitive data or configuration settings managed through Boards. An attacker who gains control of an account demoted to System Guest could exploit this flaw to modify board structures, delete critical information, alter permissions for other users, or execute administrative commands that should be strictly prohibited at that role level. This undermines the integrity of collaborative workspaces and poses a significant risk in enterprise environments where strict access controls are mandated by compliance frameworks such as SOC 2 or ISO 27001. The ability to perform admin-only operations via both REST API and UI ensures that automated exploitation tools can also leverage this flaw, increasing the likelihood of successful attacks against vulnerable instances.
This vulnerability aligns with CWE-863, which describes improper authorization checks where an application fails to enforce proper access control policies during specific actions or states. Furthermore, it maps to MITRE ATT&CK technique T1078, specifically Valid Accounts and potentially T1098 if the attacker uses this privilege escalation to maintain persistent access by modifying board permissions for other compromised accounts. The failure to reconcile role changes with feature-specific flags is a classic example of state management errors that lead to authorization bypasses in complex web applications.
To mitigate this risk, organizations running affected versions must immediately upgrade Mattermost to a patched version where the role reconciliation logic has been corrected to ensure that demoting a user to System Guest fully revokes all associated administrative privileges across all modules including Boards. Until an upgrade is feasible, administrators should manually audit users with Board Admin roles and verify their current global status, removing any discrepancies by reassigning or disabling accounts as necessary. Additionally, implementing strict monitoring for unusual activity in the Boards module, particularly from recently demoted accounts, can help detect potential exploitation attempts while patching efforts are underway. Regular review of role assignments and permission audits is recommended to prevent similar state synchronization issues across other integrated features within the platform.