Stealing is haraam, and is a major sin. The Prophet (peace and blessings of Allaah be upon him) said: “No thief is a believer at the time when he is stealing.” Narrated by al-Bukhaari (5578) and Muslim (57).
Undoubtedly hacking into banks’ websites and transferring money from them is haraam, and consuming people’s wealth unlawfully.
As to whether it is something that incurs the punishment of amputation, that is to be determined by the shar’i judge to whom the case is referred.
There are conditions which must be met before the thief’s hand is amputated, such as that the property that is stolen should have been taken from a proper storage place (in which wealth is usually kept).
Imam al-Qurtubi (may Allaah have mercy on him) said:
The majority of scholars are agreed that amputation can only be carried out on one who took from a proper storage place that for which amputation is deserved.
A proper storage place is that which is usually set up to store people’s wealth, which varies from one case to another.
Ibn al-Mundhir said: There is no definite report or scholarly opinion concerning that, rather it is like consensus among the scholars. End quote.
Tafseer al-Qurtubi (6/162).
The scholars of the Standing Committee were asked:
What are the conditions for amputating the hand in the case of theft?
For the hand of the thief to be cut off, nine conditions must be met:
1. Stealing, which is taking wealth or property by stealth. If he seizes it forcibly or snatches it, then there is no amputation.
2. The thief should be accountable; there is no amputation for a child or one who is insane.
3. The stolen item should be above a certain threshold; there is no amputation for anything less than that. The threshold is one quarter of a dinar of gold, or the equivalent value of other things.
4. The stolen item should be something that is customarily kept and stored.
5. The stolen item should be something concerning which there is no doubt.
6. It should be stolen from a proper storage place.
7. It should be taken out of the storage place.
8. Proof of the theft should be established before the judge, with two witnesses of good character or a confession from the thief.
9. The owner of the stolen item should come forward and claim it.
Examination of these conditions and the decision as to whether they apply to a case of theft, is something that should be referred to the shar’i courts. End quote.
Shaykh ‘Abd al-‘Azeez ibn Baaz, Shaykh Saalih al-Fawzaan, Shaykh ‘Abd al-Azeez Aal al-Shaykh, Shaykh Bakr Abu Zayd.
Fataawa al-Lajnah al-Daa’imah (22/223, 224)
And Allaah knows best.