Mattermost versions 9.10.x <= 9.10.2, 9.11.x <= 9.11.1, 9.5.x <= 9.5.9 and 10.0.x <= 10.0.0 fail to properly authorize the requests to /api/v4/channels  which allows a User or System Manager, with "Read Groups" permission but with no access for channels to retrieve details about private channels that they were not a member of by sending a request to /api/v4/channels.