Mattermost versions 9.9.x <= 9.9.0, 9.5.x <= 9.5.6 fail to properly restrict channel creation which allows a malicious remote to create arbitrary channels, when shared channels were enabled.
Mattermost versions 9.9.x <= 9.9.0, 9.5.x <= 9.5.6, 9.7.x <= 9.7.5, 9.8.x <= 9.8.1 fail to disallow users to set their own remote username, when shared channels were enabled, which allows a user on a remote to set their remote username prop to an arbitrary string, which would be then synced to the local server as long as the user hadn't been synced before.
Mattermost versions 9.9.x <= 9.9.0, 9.5.x <= 9.5.6 fail to properly validate synced reactions, when shared channels are enabled, which allows a malicious remote to create arbitrary reactions on arbitrary posts
Mattermost versions 9.9.x <= 9.9.0, 9.5.x <= 9.5.6, 9.7.x <= 9.7.5, 9.8.x <= 9.8.1 fail to disallow the modification of local users when syncing users in shared channels. which allows a malicious remote to overwrite an existing local user.
Mattermost Mobile Apps versions <=2.16.0 fail to protect against abuse of a globally shared MathJax state which allows an attacker to change the contents of a LateX post, by creating another post with specific macro definitions.
Mattermost Mobile Apps versions <=2.16.0 fail to validate that the push notifications received for a server actually came from this serve that which allows a malicious server to send push notifications with another server’s diagnostic ID or server URL and have them show up in mobile apps as that server’s push notifications.
Mattermost versions 9.8.0, 9.7.x <= 9.7.4, 9.6.x <= 9.6.2, 9.5.x <= 9.5.5 fail to prevent specifying a RemoteId when creating a new user which allows an attacker to specify both a remoteId and the user ID, resulting in creating a user with a user-defined user ID. This can cause some broken functionality in User Management such administrative actions against the user not working.
Mattermost versions 9.5.x <= 9.5.5 and 9.8.0 fail to properly sanitize the recipients of a webhook event which allows an attacker monitoring webhook events to retrieve the channel IDs of archived or restored channels.
Mattermost versions 9.8.x <= 9.8.0, 9.7.x <= 9.7.4, 9.6.x <= 9.6.2 and 9.5.x <= 9.5.5, when shared channels are enabled, fail to use constant time comparison for remote cluster tokens which allows an attacker to retrieve the remote cluster token via a timing attack during remote cluster token comparison.
Mattermost versions 9.5.x <= 9.5.5 and 9.8.0, when using shared channels with multiple remote servers connected, fail to check that the remote server A requesting the server B to update the profile picture of a user is the remote that actually has the user as a local one . This allows a malicious remote A to change the profile images of users that belong to another remote server C that is connected to the server A.