The source code for Umbraco is available under MIT License

Umbraco is open source free to use by anyone as detailed by the license below.

MIT License

Copyright (c) 2005-present

Umbraco

Permission is hereby granted, free of charge, to any person
obtaining a copy of this software and associated documentation files (the
"Software"), to deal in the Software without restriction, including without
limitation the rights to use, copy, modify, merge, publish, distribute,
sublicense, and/or sell copies of the Software, and to permit persons to whom
the Software is furnished to do so, subject to the following
conditions:

The above copyright notice and this permission notice shall
be included in all copies or substantial portions of the Software.

The software is provided "as is", without warranty of any kind, express or implied,
including but not limited to the warranties of merchantability, fitness for a
particular purpose and noninfringement. In no event shall the authors or
copyright holders be liable for any claim, damages or other liability, whether
in an action of contract, tort or otherwise, arising from, out of or in
connection with the software or the use or other dealings in the software.