GHSA-vg6q-84p8-qvqh Mattermost allows a user on a remote to set their remote username prop to an arbitrary string

• Go github.com/mattermost/mattermost/server/v8: >= 9.5.0, < 9.5.7 → fixed in 9.5.7 • Go github.com/mattermost/mattermost/server/v8: >= 9.7.0, < 9.7.6 → fixed in 9.7.6 • Go github.com/mattermost/mattermost/server/v8: >= 9.8.0, < 9.8.2 → fixed in 9.8.2 • Go github.com/mattermost/mattermost/server/v8: >= 9.9.0, < 9.9.1 → fixed in 9.9.1

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.