How can I switch to sa account from guest session in 2012 sql server?

I have a situation where I can log into my server as guest, and see that there is no password on the sa account. However when I try to login as sa with null password over the network it is rejected.

My question is, since there is no sa password, how can I do the equivalent of a unix sudo or su command, while logged in as guest? I looked into runas and changing passwords but guest lacks the very priviledge I am trying to gain. ('alter login sa..' doesnt work.)

Also, why does the server refuses login when there is no sa password?

Thanks

edit: I forgot to be more specific, this is SQL Server 2012 sp1 web edition 64bit 11.0.3153.0.