Architectural flexibility in 5G networks enabled by flexible splitting of base station functions facilitates optimal management of network slice resources to support divergent applications. However, this approach is vulnerable to a novel stealthy slice selection attack (namely the S3 attack which we formulate and analyze in this work) by manipulating the slice selection decisions of local radio access network (RAN) controllers. The S3 attacker intentionally maps service requests from users to inappropriate network slices in order to damage cloud radio access network (CRAN) utilization and the quality of experience (QoE) of users. We also present a countermeasure to tackle the S3 attack using a novel protocol called verifiable privacy-preserving protocol for slice selection in 5G networks (VACCINE). VACCINE ensures privacy-preserving slice selection by local RAN controllers and distributed anonymous verification of the slice selection decisions by network functions of target network slices. We also prove the strength of VACCINE against the chosen-ciphertext attack (CCA). VAC-CINE’s computational cost is lower than that of a recently proposed privacy-preserving group anonymous protocol for different security levels.