First of all, you should ask a rav who knows both choshen mishpat and the relevant technologies.
Here’s my two cents as a systems guy and a programming student.
1- These days when the setup utilities usually prompt you to create a password, the network would probably have the din of a bench in front of someone’s house which, according to the gemara in Bava Kama, falls under the rubric of zeh neheneh v’zeh lo chaser. I.e., if you were to use it lightly enough that your usage wouldn’t impact the network, it’s not a problem. If you were to use it to seed torrents or play online games that would be the equivalent of popa bar abba sitting on the bench in the gemara. (Sorry popa, couldn’t resist. :))
2- WEP encryption should have a din of an unsecured network because it really is that easy.
3- Even if you got a heter, it’s still unethical.
4- Even if you got a heter and don’t have any objections ethically (mitzad yashrus) it’s not a very smart thing to do from a security standpoint.