How can I permanently enable multi-user remote desktop on a Windows system by modifying the termsrv.dll file?

First, stop the Remote Desktop Services (`net stop TermService /y`), then replace the original `c:\windows\system32\termsrv.dll` with a modified version. Use a hex editor (like CFF Explorer) to locate the pattern (e.g., for Win7 x64 find `39873C0600000F84`) and replace it with `C7873C060000FFFFFF7F9090`. Restart the service and you'll have permanent multi-user support. Note that the exact pattern varies by OS version, as detailed in the article Penetration Techniques - Multi-user Login for Windows Remote Desktop.