Fog is a computing environment which extends the resources to the end of the network infrastructure. It reduces the latency and reachability to the user. Fog computing supports heterogeneity, mobility, and wireless access. Fog nodes had distributed over the vast geographical area. These features facilitate fog computing for sensitive real-time applications which concerns about low latency, such as smart city, smart home, healthcare, and other industrial applications. Simultaneously, these features of fog computing create a space for virtualization issues, web security issues, internal/external communication issues, data security-related issues, wireless security issues, and malware protection. In this chapter, the various existing security mechanisms in fog computing such as Ciphertext-Policy Attribute-Based Encryption(CP-ABE), Black-Box Traceable CP-ABE(T-CP-ABE), Lightweight Fine-Grained ciphertext search(LFGS), security mechanism based on evolutionary game theory, hybrid scheme for fine-grained search and access authorization, variant of password-authenticated key exchange (vPAKE) protocol, fog computing through blockchain and quantum based steganography protocol for fog are discussed and compared. Next, the proposed techniques for trust and authentication based attacks with the future research challenges are tabulated. Then, the advantages and limitations of the other techniques are discussed. Finally, the existing research challenges in fog security are summarized.