Discourse is an open source platform for community discussion. In affected versions a user logged as an administrator can call arbitrary methods on the `SiteSetting` class, notably `#clear_cache!` and `#notify_changed!`, which when done on a multisite instance, can affect the entire cluster resulting in a denial of service. Users not running in multisite environments are not affected. This issue is patched in the latest stable, beta and tests-passed versions of Discourse. Users are advised to upgrade. There are no known workarounds for this vulnerability.