CVE-2026-6046: Mattermost Server Information Disclosure

CVE-2026-6046 Overview

CVE-2026-6046 affects Mattermost Server versions 11.6.x <= 11.6.1, 11.5.x <= 11.5.4, 10.11.x <= 10.11.15, and 10.11.x <= 10.11.16. The flaw stems from missing validation that a username returned during bot registration belongs to a bot account [CWE-200]. An unprivileged attacker can pre-register a regular user account using a predictable plugin bot username. When a plugin later issues direct messages intended for that bot, the attacker receives them instead. Mattermost tracks this issue as advisory MMSA-2026-00649.

Critical Impact
Unprivileged attackers can intercept private messages sent by plugins via direct message channels, exposing sensitive plugin-to-user communications.

Affected Products

Mattermost Server 11.6.x up to and including 11.6.1
Mattermost Server 11.5.x up to and including 11.5.4
Mattermost Server 10.11.x up to and including 10.11.15 and 10.11.16

Discovery Timeline

2026-06-12 - CVE-2026-6046 published to NVD
2026-06-18 - Last updated in NVD database

Technical Details for CVE-2026-6046

Vulnerability Analysis

The vulnerability resides in the Mattermost bot registration workflow. When a plugin registers a bot, the server returns a username without confirming that the resolved account is a bot rather than a regular user. Plugins then trust this returned identity when opening direct message (DM) channels for sensitive workflows such as notifications, credentials, or interactive prompts.

Because plugin bot usernames follow predictable patterns, an attacker can register an ordinary user account ahead of plugin initialization and claim the username the plugin expects. The plugin's subsequent direct messages are routed to the attacker-controlled account instead of an authentic bot. This breaks the confidentiality boundary between plugins and their intended bot identities.

The issue is categorized as Information Exposure [CWE-200]. Confidentiality is the only directly impacted property; integrity and availability remain unaffected.

Root Cause

The server's bot registration handler fails to assert that the account associated with a requested username has the bot flag set. The validation gap allows a non-bot account to satisfy the lookup, returning a legitimate-looking username to the plugin without enforcing the expected account type.

Attack Vector

Exploitation requires network access to the Mattermost instance and low-privilege authenticated access sufficient to register a user account. The attacker must predict or enumerate the plugin bot username and register it before the plugin runs. Once positioned, the attacker passively receives any direct messages the plugin sends to that bot identity.

No verified public exploit code is available. The vulnerability mechanism is documented in the Mattermost security advisory.

Detection Methods for CVE-2026-6046

Indicators of Compromise

User accounts whose usernames match known plugin bot naming patterns but lack the IsBot attribute in the database.
Plugin-initiated direct message channels whose recipient is a standard user account rather than a registered bot.
Account registration events for usernames associated with installed or planned plugins occurring shortly before plugin deployment.

Detection Strategies

Query the Mattermost user table for accounts whose username matches the *-bot or plugin-specific naming convention and verify the IsBot flag is set to true.
Audit Bots and Users tables for username collisions where a non-bot account shares a namespace reserved for plugin bots.
Review plugin audit logs for bot registration responses to confirm the resolved account ID corresponds to a bot record.

Monitoring Recommendations

Enable Mattermost audit logging and forward events to a centralized log platform for correlation of registration and plugin activity.
Alert on creation of user accounts using reserved or plugin-related usernames.
Monitor direct message channels created by plugins for unexpected human participants.

How to Mitigate CVE-2026-6046

Immediate Actions Required

Upgrade Mattermost Server to a fixed release as listed in the Mattermost Security Updates advisory for MMSA-2026-00649.
Inventory user accounts that share names with plugin bots and disable or rename any non-bot accounts squatting on those identifiers.
Rotate any secrets, tokens, or sensitive data that may have been transmitted through plugin direct messages prior to remediation.

Patch Information

Mattermost has released fixed versions addressed under advisory MMSA-2026-00649. Administrators should consult the Mattermost Security Updates page for the exact patched build corresponding to their deployment branch (11.6.x, 11.5.x, or 10.11.x) and upgrade according to vendor guidance.

Workarounds

Restrict account creation to trusted users by disabling open sign-up and requiring administrator approval until patches are applied.
Pre-create all plugin bot accounts as bots before installing or enabling the corresponding plugin to prevent username squatting.
Reserve plugin bot username prefixes via administrative controls and deny their use for regular user registration.

bash

# Configuration example: disable open signup in config.json
# Edit /opt/mattermost/config/config.json
"TeamSettings": {
    "EnableOpenServer": false,
    "EnableUserCreation": false,
    "RestrictCreationToDomains": "example.com"
}
# Restart the service after applying changes
sudo systemctl restart mattermost