It is almost impossible to hack in to a network like yahoo with just an Internet connection and a software. Telnet was used but not anymore. U need a good hacking knowledge to hack in to those networks + a good knowledge of getting ya ass out before they trace u...or else u can spend da rest of ya life time in da jail!
The stupidest way of hacking which is a wide spread info around the Internet is by fooling the network server in to thinking of a password retrieval. I guarantee you 100% that will not work! Yahoo is not soo stupid enough. The ppl who do this crazy thing asks u for your email address and password as well. And they sell it to other companies.