Mattermost versions 11.0.x <= 11.0.4, 10.12.x <= 10.12.2, 10.11.x <= 10.11.6 and Mattermost Calls versions <=1.10.0 fail to implement CSRF protection on the Calls widget page which allows an authenticated attacker to initiate calls and inject messages into channels or direct messages via a malicious webpage or crafted link
Update Mattermost to versions 11.1.0, 11.0.5, 10.12.3, 10.11.7 or higher. Alternatively, update the Mattermost Calls plugin to version 1.11.0 or higher.