GHSA-9FPW-C9X7-CV3J Mattermost allows remote actor to set arbitrary RemoteId values for synced users

Mattermost versions 9.9.x = 9.9.0 and 9.5.x = 9.5.6 fail to validate the source of sync messages and only allow the correct remote IDs, which allows a malicious remote to set arbitrary RemoteId values for synced users and therefore claim that a user was synced from another remote...