From their FAQ at
https://massgrave.dev/faq
Is MAS safe? How can I know if there is any malware?
MAS is fully
open-source. It has over 100K stars on
GitHub and is used by millions of people. You can open the batch files with Notepad and verify the code, or take help from ChatGPT if you don't understand something in batch scripts. You can even perform the activation manually, check the Docs section for instructions.
For what it's worth, I've used this for many machines over 3-4 years and never faced any issues. It is a PowerShell script that makes Windows think your copy is genuine. It works by using the same system Microsoft uses to check if Windows is properly activated. When you run it, it sends information to Microsoft's servers that makes them recognize your computer as having a valid Windows license.
It's similar to what happens when you officially upgrade Windows or when you need to reactivate it after changing computer parts. The script is available on GitHub (which Microsoft owns), and even though Microsoft could take it down, they haven't done so.