Objection 1. It would seem unlawful to steal through stress of need. For penance is not imposed except on one who has sinned. Now it is stated (Extra, De furtis, Cap. Si quis): "If anyone, through stress of hunger or nakedness, steal food, clothing or beast, he shall do penance for three weeks." Therefore it is not lawful to steal through stress of need.
Objection 2. Further, the Philosopher says (Ethic. ii, 6) that "there are some actions whose very name implies wickedness," and among these he reckons theft. Now that which is wicked in itself may not be done for a good end. Therefore a man cannot lawfully steal in order to remedy a need.
Objection 3. Further, a man should love his neighbor as himself. Now, according to Augustine (Contra Mendac. vii), it is unlawful to steal in order to succor one's neighbor by giving him an alms. Therefore neither is it lawful to steal in order to remedy one's own needs.
On the contrary, In cases of need all things are common property, so that there would seem to be no sin in taking another's property, for need has made it common.
I answer that, Things which are of human right cannot derogate from natural right or Divine right. Now according to the natural order established by Divine Providence, inferior things are ordained for the purpose of succoring man's needs by their means. Wherefore the division and appropriation of things which are based on human law, do not preclude the fact that man's needs have to be remedied by means of these very things. Hence whatever certain people have in superabundance is due, by natural law, to the purpose of succoring the poor. For this reason Ambrose [Loc. cit., 2, Objection 3] says, and his words are embodied in the Decretals (Dist. xlvii, can. Sicut ii): "It is the hungry man's bread that you withhold, the naked man's cloak that you store away, the money that you bury in the earth is the price of the poor man's ransom and freedom."
Since, however, there are many who are in need, while it is impossible for all to be succored by means of the same thing, each one is entrusted with the stewardship of his own things, so that out of them he may come to the aid of those who are in need. Nevertheless, if the need be so manifest and urgent, that it is evident that the present need must be remedied by whatever means be at hand (for instance when a person is in some imminent danger, and there is no other possible remedy), then it is lawful for a man to succor his own need by means of another's property, by taking it either openly or secretly: nor is this properly speaking theft or robbery.
Reply to Objection 1. This decretal considers cases where there is no urgent need.
Reply to Objection 2. It is not theft, properly speaking, to take secretly and use another's property in a case of extreme need: because that which he takes for the support of his life becomes his own property by reason of that need.
Reply to Objection 3. In a case of a like need a man may also take secretly another's property in order to succor his neighbor in need.
Source: Summa Theologica (New Advent)