Mattermost fails to use consistent error responses when handling the /mute command

Mattermost versions 11.3.x = 11.3.0, 11.2.x = 11.2.2, 10.11.x = 10.11.10 fail to use consistent error responses when handling the /mute command which allows an authenticated team member to enumerate private channels they are not authorized to know about via differing error messages for nonexisten...

EUVD-2026-12437

Mattermost versions 11.3.x = 11.3.0, 11.2.x = 11.2.2, 10.11.x = 10.11.10 fail to use consistent error responses when handling the /mute command which allows an authenticated team member to enumerate private channels they are not authorized to know about via differing error messages for nonexisten...

GHSA-5MR9-CRCG-8WH2 Mattermost fails to use consistent error responses when handling the /mute command

Mattermost versions 11.3.x = 11.3.0, 11.2.x = 11.2.2, 10.11.x = 10.11.10 fail to use consistent error responses when handling the /mute command which allows an authenticated team member to enumerate private channels they are not authorized to know about via differing error messages for nonexisten...

Information Exposure

Overview Affected versions of this package are vulnerable to Information Exposure via inconsistent error handling in the /mute command. An attacker can infer the existence of private channels they are not authorized to access by analyzing the differences in error messages returned for nonexistent...

Missing Authorization

Overview Affected versions of this package are vulnerable to Missing Authorization in the channel search API endpoint. An attacker can access information about all public channels within a private team by querying the API after being removed from the team. Remediation Upgrade...

Missing Authorization

Overview Affected versions of this package are vulnerable to Missing Authorization in the channel search API endpoint. An attacker can access information about all public channels within a private team by querying the API after being removed from the team. Remediation Upgrade...

Mattermost allows a removed team member to enumerate all public channels within a private team

Mattermost versions 11.3.x = 11.3.0, 11.2.x = 11.2.2, 10.11.x = 10.11.10 fail to properly validate team membership when searching channels which allows a removed team member to enumerate all public channels within a private team via the channel search API endpoint. Mattermost Advisory ID:...

GHSA-679F-WMRG-QF57 Mattermost allows a removed team member to enumerate all public channels within a private team

Mattermost versions 11.3.x = 11.3.0, 11.2.x = 11.2.2, 10.11.x = 10.11.10 fail to properly validate team membership when searching channels which allows a removed team member to enumerate all public channels within a private team via the channel search API endpoint. Mattermost Advisory ID:...

EUVD-2026-12409

Mattermost versions 11.3.x = 11.3.0, 11.2.x = 11.2.2, 10.11.x = 10.11.10 fail to properly validate team membership when searching channels which allows a removed team member to enumerate all public channels within a private team via the channel search API endpoint.. Mattermost Advisory ID:...

Memory Allocation with Excessive Size Value

Overview github.com/mattermost/mattermost/server/channels/app is a private-cloud Slack alternative Affected versions of this package are vulnerable to Memory Allocation with Excessive Size Value in the integration action endpoints. An attacker can exhaust server memory and disrupt service...

Improper Validation of Specified Type of Input

Overview github.com/mattermost/mattermost/server/channels/app is a private-cloud Slack alternative Affected versions of this package are vulnerable to Improper Validation of Specified Type of Input due to improper validation of User-Agent header tokens. An attacker can trigger a panic in the...

CVE-2026-21386

Mattermost versions 11.3.x = 11.3.0, 11.2.x = 11.2.2, 10.11.x = 10.11.10 fail to use consistent error responses when handling the /mute command which allows an authenticated team member to enumerate private channels they are not authorized to know about via differing error messages for nonexisten...

CVE-2026-21386

Mattermost versions 11.3.x = 11.3.0, 11.2.x = 11.2.2, 10.11.x = 10.11.10 fail to use consistent error responses when handling the /mute command which allows an authenticated team member to enumerate private channels they are not authorized to know about via differing error messages for nonexisten...

CVE-2026-21386 Private channel enumeration via /mute slash command

Mattermost versions 11.3.x = 11.3.0, 11.2.x = 11.2.2, 10.11.x = 10.11.10 fail to use consistent error responses when handling the /mute command which allows an authenticated team member to enumerate private channels they are not authorized to know about via differing error messages for nonexisten...

CVE-2026-21386

CVE-2026-21386 (Mattermost): The Mattermost server variants 11.3.x <= 11.3.0, 11.2.x <= 11.2.2, and 10.11.x

CVE-2026-21386 Private channel enumeration via /mute slash command

Mattermost versions 11.3.x = 11.3.0, 11.2.x = 11.2.2, 10.11.x = 10.11.10 fail to use consistent error responses when handling the /mute command which allows an authenticated team member to enumerate private channels they are not authorized to know about via differing error messages for nonexisten...

CVE-2026-2458

Mattermost versions 11.3.x = 11.3.0, 11.2.x = 11.2.2, 10.11.x = 10.11.10 fail to properly validate team membership when searching channels which allows a removed team member to enumerate all public channels within a private team via the channel search API endpoint.. Mattermost Advisory ID:...

CVE-2026-2458 Unauthorized channel enumeration in private teams after member removal

Mattermost versions 11.3.x = 11.3.0, 11.2.x = 11.2.2, 10.11.x = 10.11.10 fail to properly validate team membership when searching channels which allows a removed team member to enumerate all public channels within a private team via the channel search API endpoint.. Mattermost Advisory ID:...

CVE-2026-2458

Mattermost versions 11.3.x = 11.3.0, 11.2.x = 11.2.2, 10.11.x = 10.11.10 fail to properly validate team membership when searching channels which allows a removed team member to enumerate all public channels within a private team via the channel search API endpoint.. Mattermost Advisory ID:...

CVE-2026-2458 Unauthorized channel enumeration in private teams after member removal

Mattermost versions 11.3.x = 11.3.0, 11.2.x = 11.2.2, 10.11.x = 10.11.10 fail to properly validate team membership when searching channels which allows a removed team member to enumerate all public channels within a private team via the channel search API endpoint.. Mattermost Advisory ID:...