EUVD-2026-15806

Mattermost versions 11.2.x = 11.2.2, 10.11.x = 10.11.10, 11.4.x = 11.4.0, 11.3.x = 11.3.1 fail to properly validate CSRF tokens in the /api/v4/accesscontrolpolicies/policyid/activate endpoint, which allows an attacker to trick an admin into changing access control policy active status via a craft...

CVE-2026-26304

Mattermost versions 11.3.x = 11.3.0, 11.2.x = 11.2.2 fail to verify runcreate permission for empty playbookId, which allows team members to create unauthorized runs via the playbook run API. Mattermost Advisory ID: MMSA-2025-00542...

CVE-2026-25783 Denial of service via malformed User-Agent header in getBrowserVersion

Mattermost versions 11.3.x = 11.3.0, 11.2.x = 11.2.2, 10.11.x = 10.11.10 fail to properly validate User-Agent header tokens which allows an authenticated attacker to cause a request panic via a specially crafted User-Agent header. Mattermost Advisory ID: MMSA-2026-00586...

CVE-2026-2578 Information Disclosure via WebSocket Event When Deleting Unrevealed Burn on Read Posts

Mattermost versions 11.3.x = 11.3.0 fail to preserve the redacted state of burn-on-read posts during deletion which allows channel members to access unrevealed burn-on-read message contents via the WebSocket post deletion event.. Mattermost Advisory ID: MMSA-2026-00579...

CVE-2026-2463 Unauthorized access to invite ID during team creation

Mattermost versions 11.3.x = 11.3.0, 11.2.x = 11.2.2, 10.11.x = 10.11.10 fail to filter invite IDs based on user permissions, which allows regular users to bypass access control restrictions and register unauthorized accounts via leaked invite IDs during team creation.. Mattermost Advisory ID:...

CVE-2026-2456

Mattermost is affected by CVE-2026-2456 due to an unbounded memory allocation when handling responses from integration action endpoints. A authenticated attacker can cause server memory exhaustion and a denial of service by having a malicious integration server return an arbitrarily large respons...