.NET Core and ASP.NET Core are completely open source. .NET Core is a part of the .NET Foundation that can be found on GitHub at https://github.com/dotnet, and is open to contributions.

Its main repositories are as follows:

• coreclr: This is the cross-platform Common Language Runtime (CLR). It is the virtual machine that runs every .NET-based application.
• corefx: This repository contains the Base Class Library (BCL) and the Framework Class Library (FCL).

ASP.NET Core has its own repositories on GitHub at https://github.com/aspnet. It is also open for contributions from the community.

Its main repositories are as follows:

• MVC: The ASP.NET MVC source code
• EntityFrameworkCore: The Entity Framework Core project source code
• DependencyInjection: ASP.NET Core dependency injection infrastructure implementation

If you're interested in reading the source code or contributing to the development efforts, visit these GitHub pages to learn how.