If you dont license your mod then nobody has the right to even modify it. License like GPL and CC exist to relax the rules on that. Simply Copyright your mod and obfuscate it. That way you can DMCA any copies
The issue you're stating is about who's the original author and holds copyright, If you code, script, paint, draw, sculpt, take a photo, construct, you hold the copyright to it. End of story. The Licence just dictates how you want your work distributed and what rules that people are expected to follow while using your work.