Nottingham Forest and Hull City have reached the midway point at the City Ground on level terms.
A subdued game burst into life on 34 minutes when Simon Gillett brought down David Meyler inside the 18-yard box.
Koren converted the spot kick with superb precision, finding the bottom-right corner to give goalkeeper Lee Camp no chance.
Lone Forest forward Billy Sharp, who has been starved of decent service for most of the game, levelled the scores with another penalty nine minutes later.
Sharp powerfully found the top-right corner from 12 yards after he had been brought down by Alex Bruce.
