Person piggybacking wireless, can they affect your computer?

We recently set up a home wireless network (unsecured then, but now secure) one of our home laptops has yahoo messenger. One day, someone else's account was open on our messenger (I could read their messages, see all their contacts, and could have pretended I was them). However I just deleted all their contacts in retribution for them using our wireless. I'm not very tech-savvy. How exactly did their info get on our computer?

Public Comments

1. http://news.bbc.co.uk/1/hi/magazine/6960304.stm
2. It's too many details to go into, but if you have a particular port open, it leaves you EXTREMELY vulernable to someone being able to log into your computer and do whatever they want with it. Luckily for you, this was some kid who thought he was 'kewl' and just wanted to use your computer. On unsecured wireless networks, you're even more vulnerable. Most everyone leaves settings at default with no security. It wouldn't be difficult to connect to your network (even if you have WEP installed), find your computer, and browse through your files. Again, leaving settings at default, it would be pretty easy for someone to login to your router. If someone does that, they could lock you out of the router, put encryption on it (which would close you out of it) and they could put QoS (Quality of Service) settings on it that would make it work MUCH better for them and not for you. They could also setup a DMZ (demilitarized zone), where it allows them to connect to there computer (as long as they're on the network) from anywhere in the world. Regards, Brandon
3. Yes they can. If you have file sharing turned on they are essentially connected to your network and could easily browse any files you've shared. It's also possible to take control of a computer and install malware to send reports on everything you do on that computer. The only sure means is to have the laptop that was compromised wiped and reinstalled. It is possible to clean it by other means but that can be time consuming, reinstalling is quicker and safer As for your wireless WEP can be cracked within minutes. Use WPA2 with a long password with mixed upper and lower case letters, numbers, and special symbols such as @$#%^. It will make it more difficult but not impossible to crack. Also turn on MAC address access discrimination so only computers with their MAC's on the list can connect to the router. Turn off your SSID broadcast and change the SSID to something other than default. You may have to manually enter the SSID into your computer then but at least it's not screaming to the world that it's there to connect to. This doesn't mean they couldn't sniff your traffic passively just makes things more difficult and reduces the number of people capable of cracking it. If you really want to protect your computers turn off wireless and plug in an ethernet cable.
4. >>However I just deleted all their contacts in retribution for them using our wireless.<< Did it even occur to you that the person was "not very tech-savvy", either, and accidentally used your wireless connection? If, for example, both you and your neighbor have networks with the same common default name, neither his computers nor yours will know which is which. Would you like it if he messed around with your applications "in retribution" for YOU accidentally using HIS network?