Distributed systems have become the backbone of modern computing environments, enabling scalable, fault-tolerant, and high-performance applications across cloud computing, IoT, and enterprise infrastructures. However, their decentralized nature introduces significant security challenges, including unauthorized access, data breaches, distributed denial-of-service (DDoS) attacks, and inconsistent security policies across nodes. This study provides a comprehensive review of security issues in distributed systems and explores effective threat mitigation techniques to ensure confidentiality, integrity, and availability of data and services. It examines key security mechanisms such as authentication, authorization, encryption, secure communication protocols, and intrusion detection systems. The study also highlights advanced approaches including zero-trust architecture, blockchain-based security, anomaly detection using machine learning, and secure multi-party computation. Furthermore, it discusses challenges such as scalability of security solutions, latency overhead, and coordination across distributed nodes. Emerging trends such as AI-driven security analytics and decentralized identity management are also analyzed. The findings emphasize that a multi-layered and adaptive security approach is essential for protecting distributed systems from evolving cyber threats.