The .NET Core and ASP.NET Core support policy, including supported versions can be found at the .NET Core Support Policy Page.
Security issues and bugs should be reported privately to the Microsoft Security Response Center (MSRC), either by emailing secure@microsoft.com or via the portal at https://msrc.microsoft.com. You should receive a response within 24 hours. If for some reason you do not, please follow up via email to ensure we received your original message. Further information, including the MSRC PGP key, can be found in the MSRC Report an Issue FAQ.
Reports via MSRC may qualify for the .NET Core Bug Bounty. Details of the .NET Core Bug Bounty including terms and conditions are at https://aka.ms/corebounty.
Please do not open issues for anything you think might have a security implication.