In this paper we cryptanalyse a block cipher mode of operation, called Input Output Chaining (IOC), designed by Recacha and submitted to NIST in 2013 for consideration as a lightweight authenticated encryption mode. We present an existential forgery attack against IOC which makes only one chosen message query, runs in a small constant time, and succeeds with an overwhelming probability 1 - 3 x 2(-n), where n is the block length of the underlying block cipher. Therefore, this attack fully breaks the integrity of IOC.