In Dubai Islamic Affairs Charitable Department has published a Fatwa for those who use others WiFi without their permission. As per details a citizen has asked the usage of anyone’s WiFi without his/her permission on the department’s website. The question was asked in the Fatwa section of Islamic Affairs of Charitable Department.
The published Fatwa is that using someone’s property without his/her permission is “Mumnoh”, so the WiFi usage only should be when you get your service. It was also mentioned in the reply that with more people using one WiFi also effects the speed of Internet.