Acknowledge within hours and say thanks
Tell the reporter you received it, that you take it seriously and who will follow up. Do not argue about severity in the first reply.
Move it out of the general queue
Forward it to whoever owns security, mark the ticket private and restrict who can see it. Do not discuss it in public channels or shared threads.
Do not ask for more than you need
Ask only for the steps to reproduce and the affected area. Do not ask them to test further, and never threaten legal action over good-faith reports.
Tell them what happens next
Give a realistic timeline for an update. Even a short note that you are still investigating keeps the reporter from going public.
Close the loop and record it
Tell the reporter when it is fixed, thank them again and keep an internal record. Add a security contact to your site so the next report goes to the right place.