CVE-2026-27769: Mattermost Auth Bypass Vulnerability

CVE-2026-27769 Overview

Mattermost versions 10.11.x <= 10.11.12 fail to validate whether users were correctly owned by the correct Connected Workspace, which allows a malicious remote server connected using the Connected Workspaces feature to change the displayed status of local users via the Connected Workspaces API. This vulnerability represents a missing authorization check (CWE-862) in the Connected Workspaces functionality.

Critical Impact

A malicious remote server can manipulate the displayed status of local users, potentially enabling social engineering attacks or disrupting organizational communication by misrepresenting user availability.

Affected Products

• Mattermost versions 10.11.x through 10.11.12

Discovery Timeline

• 2026-04-15 - CVE CVE-2026-27769 published to NVD
• 2026-04-15 - Last updated in NVD database

Technical Details for CVE-2026-27769

Vulnerability Analysis

This vulnerability stems from missing authorization controls (CWE-862) in the Connected Workspaces API. When remote servers communicate with a Mattermost instance through the Connected Workspaces feature, the application fails to properly validate ownership relationships between users and their respective workspaces. This allows an attacker controlling a malicious remote server to submit API requests that modify user status information without proper authorization checks.

The impact is primarily focused on integrity—attackers cannot read sensitive data or execute code, but they can manipulate the visual status indicators of legitimate users. While the direct technical impact is limited, the potential for social engineering and organizational disruption makes this a noteworthy security concern.

Root Cause

The root cause is a missing authorization check in the Connected Workspaces API endpoint responsible for handling user status updates. When processing status change requests from connected remote servers, the application does not verify that the requesting server has legitimate ownership or authority over the user accounts being modified. This omission allows any connected workspace to submit status changes for any user, regardless of their actual workspace membership.

Attack Vector

Exploitation requires an attacker to have a server connected to the target Mattermost instance via the Connected Workspaces feature. The attack vector is network-based and requires high privileges (administrative access to configure a connected workspace). Once connected, the attacker can:

1. Establish a connection to the target Mattermost instance using the Connected Workspaces feature
2. Send crafted API requests to the Connected Workspaces API endpoint
3. Specify arbitrary local user IDs in the status update requests
4. Modify the displayed status of those users without authorization

The vulnerability does not require user interaction and has a low attack complexity, though the prerequisite of having an established connected workspace limits the practical exploitability.

Detection Methods for CVE-2026-27769

Indicators of Compromise

• Unexpected user status changes that users did not initiate
• Status update API calls originating from connected workspace servers for users not belonging to those workspaces
• Anomalous patterns in Connected Workspaces API logs showing cross-workspace user references

Detection Strategies

• Monitor Connected Workspaces API endpoints for status update requests that reference users from different workspaces than the requesting server
• Implement logging for all user status changes with source identification to detect unauthorized modifications
• Review audit logs for patterns of status changes that don't correlate with actual user activity

Monitoring Recommendations

• Enable detailed logging for the Connected Workspaces API functionality
• Set up alerts for user status changes that originate from remote workspace connections
• Periodically review connected workspace configurations to ensure only trusted servers are connected

How to Mitigate CVE-2026-27769

Immediate Actions Required

• Upgrade Mattermost to a patched version beyond 10.11.12
• Review currently connected workspaces and remove any untrusted or unnecessary connections
• Audit recent user status changes for any suspicious activity

Patch Information

Mattermost has released security updates to address this vulnerability. Administrators should consult the Mattermost Security Updates page for the latest patched versions and detailed upgrade instructions. The fix implements proper ownership validation checks in the Connected Workspaces API to ensure that remote servers can only modify status for users legitimately associated with their workspace.

Workarounds

• Temporarily disable the Connected Workspaces feature if not actively required for business operations
• Restrict connected workspace configurations to only trusted, verified remote servers
• Implement network-level controls to limit which external servers can establish Connected Workspace connections