The security of Cobra-H64/128 block cipher under related-key differential cryptanalysis was studied.For Cobra-H64

we use related-key differential attack to get the whole 128 bits key with probability 1 based on the new constructed related-key differential and loopholes in CP box

and the corresponding computational complexity is 2^40.5 Cobra-H64 encryptions

the data complexity is 2^40.5 chosen-plaintexts

the memory complexity is 2^22 bits.To recover the whole 256 bits key of Cobra-H128

we need a computational complexity of 2^76 Cobra-H128 encryptions

a data complexity of 2^76 chosen-plaintexts

a memory complexity of 2^16.2 bits.The analysis results show that Cobra-H64/128 are unsafe under related-key differential attacks.