CVE-2026-9859 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 enforce PermissionManageBoardRoles on the channelId field of the batch endpoint, which allows an authenticated board editor to relink any board they can edit to an arbitrary channel via a crafted PATCH request. Mattermost Advisory ID: MMSA-2026-00686
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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 authorization bypass within the platform's board management subsystem. Specifically, this flaw resides in the batch endpoint handling logic for boards, where the server fails to properly validate the PermissionManageBoardRoles privilege against the channelId field included in incoming PATCH requests. This oversight allows an authenticated user who holds editor permissions on a specific board to manipulate the underlying data structure of that board without adequate access control checks. The core technical deficiency is a lack of proper object-level authorization verification, where the system trusts client-supplied identifiers for channel associations rather than verifying whether the requesting user has administrative or management rights over the target channel itself.
From an operational perspective, this vulnerability enables an authenticated attacker to relink any board they can edit to an arbitrary channel within the Mattermost instance. By crafting a malicious PATCH request that specifies a channelId belonging to a different team, workspace, or restricted group, the user effectively moves their editable content into a context where it may be visible to unauthorized users or integrated into workflows intended for other departments. This action disrupts the logical isolation between teams and channels, potentially leading to information leakage if sensitive board data is exposed to audiences that were not originally granted access. Furthermore, this manipulation can cause significant operational confusion by polluting unrelated communication streams with content from different organizational units, thereby degrading the integrity of collaborative workspaces.
The security implications align closely with CWE-269, which describes Improper Privilege Assignment, as well as CWE-862, Missing Authorization, because the application fails to enforce required access controls on a protected resource. In terms of offensive security frameworks, this vulnerability maps to MITRE ATT&CK technique T1078, Valid Accounts, specifically in the context of lateral movement or privilege escalation within an application environment. An attacker leveraging this flaw can achieve unauthorized data exposure and potentially disrupt business operations by misplacing critical project management information into inappropriate channels. The severity is heightened by the fact that it requires only authenticated access with basic editing privileges, lowering the barrier for exploitation compared to vulnerabilities requiring administrative rights.
To mitigate this risk, organizations running affected versions of Mattermost must immediately upgrade to patched releases where these authorization checks have been corrected. For those unable to patch instantly due to operational constraints, implementing strict network-level controls such as Web Application Firewalls can help detect and block anomalous PATCH requests that attempt to modify board-channel associations outside expected parameters. Additionally, reviewing audit logs for unusual changes in channel-board mappings can aid in identifying potential exploitation attempts post-vulnerability disclosure. It is also advisable to enforce the principle of least privilege by ensuring that users with editing rights on boards do not possess unnecessary administrative permissions over channels or teams they are not directly associated with, thereby reducing the blast radius if such a vulnerability were ever discovered in other components of the platform.