  In this letter, we concentrate on the very recently proposed Measurement
Device Independent Quantum Key Distribution (MDI QKD) protocol by Lo, Curty and
Qi (PRL, 2012). We study how one can suitably present an eavesdropping strategy
on MDI QKD, that is in the direction of the fundamental CNOT attack on BB84
protocol, though our approach is quite different. In this strategy, Eve will be
able to know expected half of the secret bits communicated between Alice and
Bob with certainty (probability 1) without introducing any error. Further, for
the remaining bits, where Eve will only be able to predict the bit values as in
random guess (with probability 1/2), she will certainly find out whether her
interaction induced an error in the secret bits between the communicating
parties. Given the asymmetric nature of the CNOT attack, we also introduce
Hadamard gates to present a symmetric version. Though our analysis does not
refute the security claims in MDI QKD, adapting the CNOT attack in this
scenario requires nontrivial approach using entanglement swapping.
