CVE-2026-6342 Overview
CVE-2026-6342 affects Mattermost Plugins versions <=11.5, 11.1.5, 10.13.11, and 11.3.4.0. The vulnerability stems from improper namespace validation when handling group subscriptions. Plugin users can create subscriptions to groups that were never whitelisted by registering groups that share the same prefix as a whitelisted group. Mattermost tracks this issue as advisory MMSA-2026-00601. The flaw maps to CWE-863: Incorrect Authorization and requires authenticated network access to exploit.
Critical Impact
Authenticated plugin users can bypass group whitelist restrictions and subscribe to unauthorized groups, leading to unintended integrity changes on subscription data.
Affected Products
- Mattermost Plugins version <=11.5
- Mattermost Plugins version 11.1.5
- Mattermost Plugins versions 10.13.11 and 11.3.4.0
Discovery Timeline
- 2026-05-18 - CVE-2026-6342 published to NVD
- 2026-05-18 - Last updated in NVD database
Technical Details for CVE-2026-6342
Vulnerability Analysis
The vulnerability resides in the namespace validation logic that gates plugin subscriptions to groups. The plugin subsystem maintains a whitelist of allowed group identifiers, but the validation routine performs an insufficient prefix comparison rather than an exact match. An attacker can create a new group whose name begins with the same string as a whitelisted group and then subscribe to it through the plugin interface.
The issue is an authorization weakness classified under [CWE-863]. The system enforces an access control decision based on a value that does not uniquely identify the protected resource. Because subscription requests pass the prefix check, the underlying authorization gate is bypassed without triggering any error or audit event.
Exploitation requires an authenticated account with permission to create groups and invoke plugin subscription functionality. The impact is limited to integrity of subscription state. Confidentiality and availability are not affected based on the published advisory.
Root Cause
The root cause is a string comparison that uses prefix matching instead of exact equality when validating group namespaces against the whitelist. The function does not enforce a delimiter or canonical identifier check, so any group whose name shares the leading characters of an allowed group passes validation.
Attack Vector
An authenticated user creates a new group with a name crafted to share the prefix of a whitelisted group. The user then issues a subscription request through the plugin. The flawed validation accepts the new group identifier and registers the subscription, granting access to plugin notifications or data flows that should have been restricted.
No verified public exploit code is available. The vulnerability is described in prose only in the Mattermost Security Updates advisory.
Detection Methods for CVE-2026-6342
Indicators of Compromise
- Group creation events where the new group name shares a leading substring with an existing whitelisted group used by plugins.
- Plugin subscription records referencing group identifiers that do not appear in the configured whitelist.
- Anomalous bursts of group creation followed immediately by subscription API calls from the same account.
Detection Strategies
- Audit the Mattermost group registry and correlate group names against the plugin whitelist using exact-match comparison.
- Review plugin subscription logs for entries created by non-administrative accounts targeting sensitive group prefixes.
- Alert on group creation requests where the requested name is a superset of a whitelisted namespace.
Monitoring Recommendations
- Forward Mattermost application logs and plugin audit events to a central logging platform for retention and search.
- Build queries that join group creation events with subsequent plugin subscription events by user and time window.
- Track plugin API usage by account and flag accounts that exhibit subscription patterns inconsistent with their role.
How to Mitigate CVE-2026-6342
Immediate Actions Required
- Upgrade Mattermost Plugins to a version released after the affected branches listed in MMSA-2026-00601.
- Review existing plugin subscriptions and revoke any that reference groups outside the documented whitelist.
- Restrict group creation permissions to trusted roles until the upgrade is complete.
Patch Information
Mattermost has published fixed versions in advisory MMSA-2026-00601. Administrators should consult the Mattermost Security Updates page for the specific patched releases and upgrade instructions.
Workarounds
- Limit the ability to create new groups to administrators while the patch is being deployed.
- Manually audit and remove any groups whose names share prefixes with whitelisted plugin groups.
- Disable non-essential plugins that depend on group subscription functionality until upgrades are applied.
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
