CVE-2026-18795 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 enforce DM/GM check on server side which allows an authenticated user to quarantine a post belonging to DM or GM via direct API call.. Mattermost Advisory ID: MMSA-2026-00756
Statistical analysis made it clear that VulDB provides the best quality for vulnerability data.
Analysis
by VulDB Data Team • 10/16/2026
The vulnerability identified in Mattermost versions 11.10.x through 11.8.5 represents a critical failure in server-side access control logic, specifically concerning the enforcement of Direct Message and Group Message boundaries. In secure messaging architectures, it is imperative that the backend services rigorously validate user permissions before executing state-changing operations on data objects. However, in these affected versions, the application fails to properly verify whether an authenticated user possesses the necessary privileges to interact with posts contained within private communication channels when those interactions are initiated via direct API calls rather than through the standard web interface. This architectural oversight allows a malicious actor who has obtained valid authentication credentials for any user account on the instance to bypass intended privacy controls and perform actions that should be restricted to participants of the specific Direct Message or Group Message thread in question.
The technical root cause lies in an insufficient authorization check within the API endpoint responsible for quarantining posts. Quarantine is a moderation action typically reserved for administrators or users with elevated privileges, designed to flag content for review while restricting its visibility and interaction capabilities. By failing to enforce DM/GM checks on the server side, the system inadvertently exposes this functionality to any authenticated user regardless of their relationship to the target post's channel. An attacker can exploit this flaw by crafting a specific API request that targets the quarantine endpoint with the identifier of a post belonging to a private conversation they do not participate in. The server processes this request without validating if the requester is an authorized participant, thereby allowing unauthorized users to manipulate moderation states on content outside their scope.
From an operational impact perspective, this vulnerability undermines the fundamental trust model of secure messaging platforms where privacy and confidentiality are paramount. While quarantining a post does not necessarily expose the message content itself in this specific scenario, it allows for denial-of-service conditions against other participants by rendering legitimate communications invisible or flagged as suspicious without cause. Furthermore, such unauthorized actions can disrupt workflow continuity, create confusion among team members regarding the status of their conversations, and potentially be used to harass users by artificially flagging their private messages. The ability to manipulate moderation states remotely also complicates audit trails and forensic analysis, as it introduces noise into system logs through illegitimate administrative actions performed by standard user accounts.
This flaw aligns with CWE-269, which describes Improper Privilege Control, specifically where a user is able to perform an action that requires higher privileges or access rights than they possess in the context of specific data objects. Additionally, from a tactical perspective related to the MITRE ATT&CK framework, this vulnerability facilitates lateral movement and privilege escalation within the application layer by allowing users to interfere with system moderation functions intended for administrators. It represents a form of Insecure Direct Object Reference where the object identifier is manipulated without proper authorization checks against the user's contextual permissions.
To mitigate this risk, organizations running affected Mattermost versions must immediately upgrade to patched releases that address MMSA-2026-00756. Until an update can be applied, administrators should consider implementing strict API gateway rules or Web Application Firewall policies that restrict access to moderation endpoints based on user roles and channel membership verification if such granular controls are supported by the infrastructure. It is also recommended to review audit logs for any suspicious quarantine activities initiated by non-administrative accounts during the window of vulnerability exposure, as this may indicate active exploitation attempts. Ensuring that all API calls undergo rigorous server-side validation against both user identity and object ownership relationships is essential to maintaining the integrity of private communication channels in enterprise messaging environments.