Isolate the 2012 Developer VM. Use no internet access on the virtual machine. Use local credentials only. Do not mount production drives to this VM.
If you do not have an MSDN subscription, Microsoft sometimes offers the (not the full ISO) via the Download Center. Sql Server 2012 Developer Edition Download Iso
However, since its official end-of-support on July 12, 2022 , finding a secure "SQL Server 2012 Developer Edition Download ISO" from official channels has become more complex. How to Download SQL Server 2012 Developer Edition Isolate the 2012 Developer VM