The text does not explicitly say. Please note that there is an essential subtext to this entire paragraph; that is, despite the seduction of the prostitute and the mind of the beast, someone is orchestrating the action from behind. The beast “was and is not” (= finite) and goes to destruction (Revelation 17:8). Someone has “a mind with wisdom” so as to explain the mystery (Revelation 17:9). The seven kings “must” remain only a little while (Revelation 17:10). The eighth “goes to destruction,” whether it wants to or not (Revelation 17:11). Now someone permits the ten kings to “receive authority,” but for only one hour. Behind the scene, sovereignly controlling it all, is the mighty hand of sovereign God (Revelation 17:17). It follows that God himself permits these kings to receive their authority.
12 And the ten horns that you saw are ten kings who have not yet received royal power, but they are to receive authority as kings for one hour, together with the beast.