Mattermost versions 10.1.x <= 10.1.2, 10.0.x <= 10.0.2, 9.11.x <= 9.11.4, 9.5.x <= 9.5.12 fail to properly validate the type of callProps which allows a user to cause a client side (webapp and mobile) DoS to users of particular channels, by sending a specially crafted post.