Recently, Ren et al. presented two algorithms for outsourcing modular exponentiations [IEEE Transactions on Cloud Computing, 9(1):145-154, 2021 ], which aim to make the remote cloud server help resource-constrained clients securely perform expensive modular exponentiations in cryptography. In this note, we show their algorithms are incorrect due to the misuse of the Euler theorem in the verification step. Moreover, we suggest a remedial measure for the two-server algorithm.