Abstract: As an important network function in the 5G core network, UDM has a high security level within the core network. However, the security of UDM is under the jeopardies of certain security threats exist in the mutual access process between 5G core network function. This paper analyzes the security problems in service access process and the security threats to UDM, and further proposes a scheme to improve UDM security on this basis. The given scheme applied the zero-trust security to the core network and designed an UDM security enhancement scheme including monitoring the flow of access to UDM and NRF, performing dynamic trust evaluation on the network function of accessing UDM, and performing a fine-grained control of NF access permission on the basis of strict authentication.