FlexAEAD is a block cipher candidate submitted to the NIST Lightweight Cryptography standardization project, based on repeated application of an Even-Mansour construction. In order to optimize performance, the designers chose a relatively small number of rounds, using properties of the mode and bounds on differential and linear characteristics to substantiate their security claims. Due to a forgery attack with complexity $2^{46}$, FlexAEAD was not selected to the second round of evaluation in the NIST project.

In this paper we present a practical key recovery attack on FlexAEAD, using clusters of differentials for the internal permutation and the interplay between different parts of the mode. Our attack, which was fully verified in practice, allows recovering the secret subkeys of FlexAEAD-64 with a time complexity of less than $2^{31}$ encryptions (with an experimental success rate of $75,%$). This is the first practical key recovery attack on a candidate of the NIST standardization project.

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