Mattermost versions 9.5.x <= 9.5.3, 9.6.x <= 9.6.1, 8.1.x <= 8.1.12 fail to restrict the audience of the "custom_playbooks_playbook_run_updated" webhook event, which allows a guest on a channel with a playbook run linked to see all the details of the playbook run when the run is marked by finished.