CVE-2026-6341: Mattermost Plugins Auth Bypass Vulnerability

CVE-2026-6341 Overview

CVE-2026-6341 is a broken access control vulnerability in Mattermost Plugins that allows authenticated users to bypass group-level restrictions. The flaw stems from missing API-level authorization checks on which groups a user can create issues for or attach comments to. A user who belongs to multiple groups can submit direct API requests to create issues in a locked group, bypassing the intended interface restrictions. This issue is tracked under Mattermost Advisory ID MMSA-2026-00602 and is classified as [CWE-863] Incorrect Authorization.

Critical Impact

Authenticated users can create issues in locked groups they should not have write access to, resulting in integrity loss across collaboration workflows.

Affected Products

• Mattermost Plugins versions <=11.5
• Mattermost Plugins versions 11.1.5
• Mattermost Plugins versions 10.13.11 and 11.3.4.0

Discovery Timeline

• 2026-05-18 - CVE-2026-6341 published to NVD
• 2026-05-18 - Last updated in NVD database

Technical Details for CVE-2026-6341

Vulnerability Analysis

The vulnerability resides in the Mattermost Plugins API layer that handles issue creation and comment attachment. The application enforces group membership restrictions at the user interface level but fails to revalidate those constraints when requests arrive directly at the API. An attacker who is already authenticated and is a member of multiple groups can craft API calls targeting a locked group. The server processes the request without verifying that the caller has permission to write to that specific group context.

This is a classic incorrect authorization weakness mapped to [CWE-863]. The server trusts client-side or partial authorization signals rather than performing complete access control checks on every privileged operation. Exploitation requires only standard user credentials and network access to the Mattermost API endpoint.

Root Cause

The root cause is the absence of server-side authorization checks on group-scoped write operations. The plugin code paths handling issue and comment creation accept the requested group identifier from the client without validating whether the authenticated principal is permitted to write to a locked group. Authorization decisions are inconsistent between the UI layer and the API layer.

Attack Vector

An attacker leverages valid Mattermost credentials and group membership in at least one group. The attacker sends a crafted HTTP request directly to the plugin API, specifying the identifier of a locked group as the target. The server creates the issue or attaches the comment despite the locked state. No user interaction is required beyond the attacker's own authenticated session.

No verified public exploit code is available for this issue. See the Mattermost Security Updates advisory for vendor-supplied technical details.

Detection Methods for CVE-2026-6341

Indicators of Compromise

• Issue or comment creation events in audit logs where the target group is marked as locked or restricted.
• API requests to plugin issue-creation endpoints from users whose UI sessions would not display the targeted group.
• Sudden appearance of issues authored by users who lack documented write permissions to the affected group.

Detection Strategies

• Compare audit log entries for issue and comment creation against the group access control list to flag mismatches.
• Alert when API calls bypass typical UI-driven request flows, such as missing referrer headers or non-browser user agents on plugin endpoints.
• Correlate authentication context with target group identifiers to detect privilege boundary violations.

Monitoring Recommendations

• Enable verbose audit logging on Mattermost Plugins API endpoints handling issue and comment operations.
• Forward Mattermost audit logs to a centralized SIEM for retention and analytics.
• Review group membership changes and locked-group write attempts on a recurring basis.

How to Mitigate CVE-2026-6341

Immediate Actions Required

• Upgrade Mattermost Plugins to a fixed version as published in the Mattermost security advisory.
• Audit existing issues and comments in locked groups for unauthorized entries created prior to patching.
• Restrict plugin API access to trusted network segments where feasible.

Patch Information

Mattermost has published remediation guidance under advisory MMSA-2026-00602. Administrators should consult the Mattermost Security Updates page for the specific fixed versions and apply updates that supersede 11.5, 11.1.5, 10.13.11, and 11.3.4.0.

Workarounds

• Reduce the number of users with simultaneous membership in multiple groups where one is locked.
• Implement reverse proxy rules that inspect plugin API requests and reject calls referencing locked group identifiers from non-authorized principals.
• Increase audit log review cadence until patches are deployed across all Mattermost instances.

# Configuration example
# Verify installed Mattermost Plugin version
mattermost plugin list

# After upgrade, confirm fixed version is active
mattermost version