Photonic quantum key distribution (QKD) is commonly implemented using interferometers, devices that inherently cause the addition of vacuum ancillas, thus enlarging the quantum space in use. This enlargement sometimes exposes the implemented protocol to new kinds of attacks that have not yet been analyzed. We consider several QKD implementations that use interferometers, and analyze the enlargement of the quantum space caused by the interferometers. While we show that some interferometric implementations are robust (against simple attacks), our main finding is that several other implementations used in QKD experiments are totally insecure. This result is somewhat surprising since although we assume ideal devices and an underlying protocol which is proven secure (e.g., the Bennett-Brassard QKD), the realization is insecure. Our novel attack demonstrates the risks of using practical realizations without performing an extensive security analysis of the specific setup in use.