CVE-2026-16048 in Mattermost
Summary
by MITRE • 08/17/2026
Mattermost versions 11.8.x <= 11.8.2, 11.7.x <= 11.7.6, 10.11.x <= 10.11.21 fail to restrict channel member role assignment to channel-scoped roles which allows a channel administrator to gain additional channel permissions via the channel member roles API.. Mattermost Advisory ID: MMSA-2026-00697
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Analysis
by VulDB Data Team • 08/17/2026
The vulnerability identified in Mattermost versions 11.8.x through 11.8.2, 11.7.x through 11.7.6, and 10.11.x through 10.11.21 represents a critical authorization flaw within the platform's channel management subsystem. This issue stems from an improper restriction of role assignment operations via the channel member roles API endpoint. In a properly secured implementation, APIs handling user permissions must strictly validate that any requested role changes are confined to the scope defined by the current context and the privileges granted to the initiating actor. However, in these affected versions, the backend logic fails to enforce this boundary effectively. Consequently, an authenticated user possessing channel administrator privileges can exploit this gap to assign roles or modify permission sets beyond what is permitted for standard channel members. This design flaw allows a channel admin to escalate their own permissions within that specific channel context by manipulating API requests directly rather than using the intended graphical interface controls which may have had additional validation layers.
From a technical perspective, this vulnerability aligns with CWE-269, Improper Privilege Management, specifically where an actor is able to elevate privileges beyond what was originally granted or expected within that specific scope. The core issue lies in the server-side processing of role assignment payloads. When a channel administrator submits a request to update member roles, the system should verify that the target role does not exceed the maximum permissions allowed for that user's current standing and that the action itself is restricted to channel-scoped attributes only. Instead, the application accepts requests that attempt to apply broader or elevated permission sets, effectively bypassing the intended access control model. This lack of strict input validation on privilege-related fields allows an attacker with moderate initial access to significantly expand their influence over a specific communication channel without requiring global administrative rights.
The operational impact of this vulnerability is substantial for organizations relying on Mattermost for secure internal communications. A malicious actor who has compromised or been granted the role of channel administrator can use this flaw to gain additional permissions that typically require higher-level oversight, such as managing other administrators, accessing restricted channels, or altering system-wide settings if those are linked through complex permission hierarchies within a single workspace. This undermines the principle of least privilege and compromises the integrity of access control policies designed to segregate duties and limit blast radius in case of account compromise. It also facilitates potential data exfiltration or disruption by allowing unauthorized users to modify channel configurations, invite external parties without proper vetting processes enforced at higher levels, or alter message visibility settings for other members.
This behavior is consistent with patterns observed in the MITRE ATT&CK framework under techniques related to Privilege Escalation and Defense Evasion. Specifically, it reflects an abuse of valid API endpoints to perform unauthorized administrative actions, which can be categorized under T1078 Valid Accounts if combined with credential theft or simply as a direct exploitation of application logic flaws that allow lateral movement within the platform's permission structure. The ability to manipulate role assignments via API calls rather than UI interactions also highlights risks associated with insufficient separation between client-side validation and server-side enforcement, a common pitfall in modern web applications where frontend restrictions are often relied upon too heavily for security decisions.
To mitigate this risk, organizations running affected versions must immediately upgrade Mattermost to the latest patched release that addresses MMSA-2026-00697. Until an update is applied, administrators should audit channel member roles and restrict API access where possible by implementing network-level controls or Web Application Firewalls (WAF) rules that monitor for anomalous role assignment patterns via the affected endpoints. It is also advisable to review logs for any unusual privilege changes initiated by channel admins in recent weeks to assess potential exploitation history. Future development practices should emphasize rigorous server-side validation of all permission-related inputs, ensuring that API handlers strictly enforce scope boundaries regardless of client-supplied data or UI constraints. Regular security audits and penetration testing focused on authorization logic are recommended to detect similar flaws before they can be exploited in production environments.