I could find source for ckeditor at https://dev.ckeditor.com/browser and https://github.com/ckeditor but I couldn't find the sources for CKBuilder. All I could find are the binaries.
On http://ckeditor.com/about/license it says ckeditor is released under GPL/ LGPL/MPL. Does it mean this is not true for CKBuilder.
I have an open source project where we could be interested in exploring using ckeditor but we need to verify the full stack is open source before proceeding! :)
Thanks a lot