By sending a specially crafted HTTP GET request to a listening Rapid7 Metasploit HTTP handler, an attacker can register an arbitrary regular expression. When evaluated, this malicious handler can either prevent new HTTP handler sessions from being established, or cause a resource exhaustion on the Metasploit server.
https://github.com/rapid7/metasploit-framework/pull/12433
https://github.com/rapid7/metasploit-framework/pull/11966
https://github.com/rapid7/metasploit-framework/commit/5621d200ccf62e4a8f0dad80c1c74f4e0e52d86b