CVE-2026-18551 in Mattermost
Summary
by MITRE • 10/16/2026
Mattermost versions 11.10.x <= 11.10.0, 11.7.x <= 11.7.9, 11.9.x <= 11.9.1, 11.8.x <= 11.8.5 fail to validate the combination of guest and admin scheme roles which allows an authenticated attacker to escalate a guest account to team or channel admin via a crafted PUT request to the member roles API endpoint.. Mattermost Advisory ID: MMSA-2026-00752
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Analysis
by VulDB Data Team • 10/16/2026
The vulnerability identified in Mattermost versions 11.10.x through 11.8.5, specifically designated as advisory MMSA-2026-00752, represents a critical authorization bypass flaw rooted in insufficient validation of role combinations within the application's access control logic. This issue affects multiple release branches including 11.10.0 and earlier, 11.9.1 and earlier, 11.8.5 and earlier, as well as all versions up to 11.7.9. The core technical flaw lies in the server-side handling of member role assignments via the PUT request endpoint for team or channel members. When an authenticated user attempts to modify their own roles or those of other users within a scope they have permission to manage, the application fails to enforce mutual exclusivity constraints between guest-level permissions and administrative privileges. Specifically, the system does not properly validate that a user holding guest status cannot simultaneously be assigned admin-level roles such as team administrator or channel administrator. This logical error allows an attacker who possesses valid credentials for a low-privilege guest account to manipulate API parameters in order to elevate their own privilege level without triggering appropriate security checks or rejection responses from the backend service.
From a technical perspective, this vulnerability is classified under CWE-269 Improper Privilege Management and aligns with MITRE ATT&CK technique T1078 Valid Accounts, as it relies on legitimate authentication credentials to perform unauthorized actions. The exploitation vector involves crafting specific HTTP PUT requests that target the member roles API endpoint while manipulating role identifiers associated with administrative functions. Because Mattermost processes these updates without verifying whether the resulting state violates predefined security policies regarding guest restrictions, the attacker can successfully overwrite their existing low-privilege status with high-privilege configurations. This bypass of access control mechanisms enables the transformation of a restricted user into an entity capable of performing sensitive operations typically reserved for trusted administrators. The absence of proper input validation at this stage highlights a gap in the application's security model where role assignments are treated as independent attributes rather than part of a cohesive permission hierarchy that must adhere to strict hierarchical constraints.
The operational impact of this vulnerability is severe, granting an authenticated attacker full administrative control over affected teams or channels depending on which endpoint was exploited. Once escalated to team administrator status, the compromised account gains the ability to manage all aspects of the workspace including creating new users, modifying global settings, accessing sensitive data across multiple channels, and potentially installing plugins that could further compromise system integrity. If channel administration privileges are obtained instead, the attacker can control specific communication streams by deleting messages, inviting external parties, or altering channel configurations in ways that disrupt organizational workflows and expose confidential information to unauthorized individuals. This level of access undermines the fundamental principle of least privilege upon which secure collaboration platforms rely, allowing a single compromised guest account to serve as a pivot point for broader network infiltration if integrated with other internal systems via webhooks or API integrations commonly configured by administrators.
Mitigation strategies require immediate action from system administrators and DevOps teams responsible for maintaining Mattermost instances. The primary remediation step is to upgrade the affected software versions to patched releases that address this role validation logic error as outlined in advisory MMSA-2026-00752. Organizations should verify their current deployment version against the listed vulnerable ranges including 11.10.x, 11.9.x, and 11.8.x series to determine exposure status. In environments where immediate patching is not feasible due to operational constraints, temporary mitigations may include restricting API access through network-level controls such as firewalls or reverse proxies that limit PUT requests on member role endpoints to trusted IP addresses only. Additionally implementing strict monitoring for unusual privilege escalation patterns and reviewing audit logs for unexpected changes in user roles can help detect potential exploitation attempts before significant damage occurs. Regular security assessments focusing on authorization logic flaws are recommended to identify similar weaknesses across other enterprise communication platforms where guest-to-admin transitions might be improperly validated.