If the Ukrainians were not complicit - which is the impression they're giving - then he's NOT being held "legally". And if in fact the Ukrainians were complicit, was there extradition proceedings? Seems to me that if there weren't, it's still not exactly legal!
Wounded Israelis in suspected car-ramming attack in West Bank are soldiers (Haaretz)
from the article: Israel to indict Palestinian engineer with alleged info on Shalit