Assalamualaikum ustaz. Is it permissible for us to use Wi-Fi that isn’t password protected? Thank you.
Waalaikumussalam wrt. wbt.,
Alhamdulillah, praise and gratitude to Allah SWT for His countless blessings for us all. Praise and salutations to our beloved Prophet Muhammad PBUH, his family, companions, and all those who follow his footsteps until the Final day.
The definition of Wi-Fi
Wi-Fi is a wireless technology used to connect computers, tablets, smartphones and other devices to the Internet. In other words, Wi-Fi is a radio signal sent from a wireless router to a nearby device, which translates the signal into data that you can see and use. The device sends a radio signal back to a router, which connects to the internet via wires or cables. 
In general, our society today is accustomed to the use of the internet and Wi-Fi in life. Usually, Wi-Fi owners will put a password for their device or Wi-Fi. However, some owners do not put a password in their settings.
Basically, we are prohibited from using or eating the wealth of another unjustly. This is clear according to the statement of Allah SWT:
يَا أَيُّهَا الَّذِينَ آمَنُوا لَا تَأْكُلُوا أَمْوَالَكُمْ بَيْنَكُمْ بِالْبَاطِلِ
“O you who have believed, do not consume one another’s wealth unjustly,” 
Syeikh al-Sa’di when commenting on this verse stated that Allah SWT prohibited His believing slaves from eating the wealth of another unjustly. This includes any type of wealth that is taken forcefully, stolen or through gambling. This may also mean spending excessively, for it is included as unbeneficial matters and not from correct matters. Subsequently, Allah SWT permits them to eat from the wealth they obtain through trade or business and income that doesn’t have any barriers and fulfil the conditions such as both parties agreed and others. 
Furthermore, al-Maraghi stated that the word بَاطِلِ also means unlawfully. However, according to the Islamic perspective, the terminology means taking the wealth of another without paying the appropriate price or the consent of the owner. 
Likewise, Dr Wahbah al-Zuhaili said the meaning of ‘consuming the wealth of another unjustly’ means obtaining something through impermissible means by syarak such as through usury, gambling, force and deception in trade (weighing of goods). 
Moreover, there is a hadith from Anas bin Malik RA, where the Prophet PBUH said:
لَا يَحِلُّ مَالُ امْرِئٍ مُسْلِمٍ إِلَّا بِطِيبِ نَفْسِهِ
“Impermissible is the property of a man except with his goodwill.” 
Concurrently, there is another hadith from ‘Abd al-Rahman bin Abu Bakrah, from his father, the Prophet PBUH said:
فَإِنَّ دِمَاءَكُمْ، وَأَمْوَالَكُمْ، وَأَعْرَاضَكُمْ، بَيْنَكُمْ حَرَامٌ، كَحُرْمَةِ يَوْمِكُمْ هَذَا، فِي شَهْرِكُمْ هَذَا، فِي بَلَدِكُمْ هَذَا
“Your blood and your property and your honor are forbidden for you to violate, like the sacredness of this day of yours, in this month of yours, in this city of yours.” 
According to the above arguments and discussions, in our opinion, it is impermissible to use Wi-Fi without a password except after getting permission from its owner. The reason is it is included in taking the property of another unjustly because one didn’t obtain the permission of its owner. However, it is a different situation if the Wi-Fi is intended for general use placed in a public place and it is known that the owner permits its usage, then in this situation, it is permissible.
May Allah SWT grant us all a clear understanding in religion and distance us from matters that He dislikes. Amin.
 Surah al-Nisa’: 29
 See Tafsir al-Sa‘di, 1/175.
 See Tafsir al-Maraghi, 3/1168.
 See al-Tafsir al-Munir, 5/31.
 Narrated by al-Daraqutni (2885)
 Narrated by Bukhari (67); Muslim (1679)