Improper Access Control

Mattermost is vulnerable to improper access control. The vulnerability is due to insufficient sanitization and access restrictions on team email addresses, which allows an authenticated user to exploit the GET /api/v4/channels/channelid/commonteams endpoint to view sensitive team email informatio...

CVE-2026-20796

Mattermost versions 10.11.x = 10.11.9 fail to properly validate channel membership at the time of data retrieval which allows a deactivated user to learn team names they should not have access to via a race condition in the /commonteams API endpoint.. Mattermost Advisory ID: MMSA-2025-00549...

Mattermost doesn't properly validate channel membership at the time of data retrieval

Mattermost versions 10.11.x = 10.11.9 fail to properly validate channel membership at the time of data retrieval which allows a deactivated user to learn team names they should not have access to via a race condition in the /commonteams API endpoint.. Mattermost Advisory ID: MMSA-2025-00549...

Time-of-check Time-of-use (TOCTOU) Race Condition

Overview Affected versions of this package are vulnerable to Time-of-check Time-of-use TOCTOU Race Condition via a race condition in the /commonteams API endpoint. An attacker can gain unauthorized access to team names by exploiting the timing of channel membership validation during data retrieva...

GHSA-2XF7-HMF6-P64J Mattermost doesn't properly validate channel membership at the time of data retrieval

Mattermost versions 10.11.x = 10.11.9 fail to properly validate channel membership at the time of data retrieval which allows a deactivated user to learn team names they should not have access to via a race condition in the /commonteams API endpoint.. Mattermost Advisory ID: MMSA-2025-00549...

CVE-2026-20796

Mattermost versions 10.11.x = 10.11.9 fail to properly validate channel membership at the time of data retrieval which allows a deactivated user to learn team names they should not have access to via a race condition in the /commonteams API endpoint.. Mattermost Advisory ID: MMSA-2025-00549...

CVE-2026-20796 Time-of-check time-of-use vulnerability in common teams API

Mattermost versions 10.11.x = 10.11.9 fail to properly validate channel membership at the time of data retrieval which allows a deactivated user to learn team names they should not have access to via a race condition in the /commonteams API endpoint.. Mattermost Advisory ID: MMSA-2025-00549...

CVE-2026-20796

Mattermost CVE-2026-20796 affects version 10.11.x up to 10.11.9, due to improper validation of channel membership at data retrieval. A race condition in the /common_teams API endpoint can allow a deactivated user to learn team names they should not access. Root cause: insufficient validation duri...

CVE-2026-20796 Time-of-check time-of-use vulnerability in common teams API

Mattermost versions 10.11.x = 10.11.9 fail to properly validate channel membership at the time of data retrieval which allows a deactivated user to learn team names they should not have access to via a race condition in the /commonteams API endpoint.. Mattermost Advisory ID: MMSA-2025-00549...

CVE-2026-20796 Time-of-check time-of-use vulnerability in common teams API

Mattermost versions 10.11.x = 10.11.9 fail to properly validate channel membership at the time of data retrieval which allows a deactivated user to learn team names they should not have access to via a race condition in the /commonteams API endpoint.. Mattermost Advisory ID: MMSA-2025-00549...

SUSE CVE-2025-12559

Mattermost versions 11.0.x = 11.0.2, 10.12.x = 10.12.1, 10.11.x = 10.11.4, 10.5.x = 10.5.12 fail to sanitize team email addresses to be visible only to Team Admins, which allows any authenticated user to view team email addresses via the GET /api/v4/channels/channelid/commonteams endpoint...

Mattermost Unauthorized Access

The vulnerability of the Mattermost instant messaging application is related to insufficient protection of sensitive data during the processing of the /api/v4/channels/channelid/commonteams endpoint. Exploiting this vulnerability could allow an attacker to gain unauthorized access to protected...

CVE-2025-12559

Mattermost versions 11.0.x = 11.0.2, 10.12.x = 10.12.1, 10.11.x = 10.11.4, 10.5.x = 10.5.12 fail to sanitize team email addresses to be visible only to Team Admins, which allows any authenticated user to view team email addresses via the GET /api/v4/channels/channelid/commonteams endpoint...

EUVD-2025-199831

Mattermost versions 11.0.x = 11.0.2, 10.12.x = 10.12.1, 10.11.x = 10.11.4, 10.5.x = 10.5.12 fail to sanitize team email addresses to be visible only to Team Admins, which allows any authenticated user to view team email addresses via the GET /api/v4/channels/channelid/commonteams endpoint...

GHSA-4G87-9X45-CX2H Mattermost fails to sanitize team email addresses

Mattermost versions 11.0.x = 11.0.2, 10.12.x = 10.12.1, 10.11.x = 10.11.4, 10.5.x = 10.5.12 fail to sanitize team email addresses to be visible only to Team Admins, which allows any authenticated user to view team email addresses via the GET /api/v4/channels/channelid/commonteams endpoint...

Information Exposure

Overview Affected versions of this package are vulnerable to Information Exposure via the GET /api/v4/channels/channelid/commonteams endpoint. An attacker can access team email addresses intended to be visible only to Team Admins by making authenticated requests to this endpoint. Remediation...

CVE-2025-12559

Mattermost versions 11.0.x = 11.0.2, 10.12.x = 10.12.1, 10.11.x = 10.11.4, 10.5.x = 10.5.12 fail to sanitize team email addresses to be visible only to Team Admins, which allows any authenticated user to view team email addresses via the GET /api/v4/channels/channelid/commonteams endpoint...

CVE-2025-12559

Mattermost suffers an information-disclosure vulnerability CVE-2025-12559 where certain versions are not sanitizing team email addresses. Affected: Mattermost Server 11.0.x <= 11.0.2; 10.12.x <= 10.12.1; 10.11.x <= 10.11.4; 10.5.x

CVE-2025-12559 Information Disclosure in Common Teams API

Mattermost versions 11.0.x = 11.0.2, 10.12.x = 10.12.1, 10.11.x = 10.11.4, 10.5.x = 10.5.12 fail to sanitize team email addresses to be visible only to Team Admins, which allows any authenticated user to view team email addresses via the GET /api/v4/channels/channelid/commonteams endpoint...

CVE-2025-12559 Information Disclosure in Common Teams API

Mattermost versions 11.0.x = 11.0.2, 10.12.x = 10.12.1, 10.11.x = 10.11.4, 10.5.x = 10.5.12 fail to sanitize team email addresses to be visible only to Team Admins, which allows any authenticated user to view team email addresses via the GET /api/v4/channels/channelid/commonteams endpoint...