Note: I'm not claiming that I know of a new x-risk, I just want to know about the right policy in this situation
If someone identifies a new existential or catastrophic risk, it seems prudent to avoid publishing it widely as this may constitute an infohazard.
However, it probably doesn't make sense to keep this information to oneself since other people can begin to work on research and mitigation if they are aware of the risk.
Is there a group of people to disclose new x-risks to that can make relevant experts aware of the risk? In general, how and where should someone disclose a new x-risk?