Shaikh Saalih al-Fawzaan hafidhahullah was asked a question.
Question: May Allah be good to you, the questioner says: I live in an apartment complex and many neighbors have the DSL line and it is an internet line.
Question Reciter: the line DSL
Shaikh: I work…
Question Reciter: …I live in an apartment complex and in the apartment complex…
Question Reciter: …many of neighbors have the DSL line and it is internet. Some of them set a password and some of them do not set a password. I have a device that pulls up all the networks, whether they are locked or open. My question is: is there sin on me if I use a network that is open when searching for the internet, knowing that they[the owners of the line] are capable of setting a password, however they leave the matter as it is?
Shaikh: This is not allowed for you. First, their secrets become apparent to you, if there are with them secrets. Secondly, you used their property without their permission. This is not allowed.
[Taken from here]
Faisal Ibn Abdul Qaadir Ibn Hassan