  Recently in 2018, Niu et al. proposed a measurement-device-independent
quantum secure direct communication protocol using Einstein-Podolsky-Rosen
pairs and generalized it to a quantum dialogue protocol (Niu et al., Science
bulletin 63.20, 2018). By analyzing these protocols we find some security
issues in both these protocols. In this work, we show that both the protocols
are not secure against information leakage, and a third party can get half of
the secret information without any active attack. We also propose suitable
modifications of these protocols to improve the security.
