Discourse vulnerable to DoS through Onebox

Discourse is an open-source discussion platform. Prior to version 3.2.3 on the `stable` branch and version 3.3.0.beta3 on the `tests-passed` branch, Oneboxing against a carefully crafted malicious URL can reduce the availability of a Discourse instance. The problem has been patched in version 3.2.3 on the `stable` branch and version 3.3.0.beta3 on the `tests-passed` branch. There are no known workarounds available for this vulnerability.