Multi-agent AI systems increasingly power critical applications across industries, from financial trading algorithms to smart infrastructure management. These distributed architectures face unique security challenges that traditional cybersecurity approaches often miss. When compromised, the consequences hit hard—financial losses, operational disruptions, and even physical dangers in critical infrastructure. The interconnected design that makes multi-agent systems powerful also creates vulnerabilities at the intersection of communication, coordination, and emergent behaviors. This guide explores how multi-agent systems are exploited by adversaries and presents a comprehensive defense framework that security teams can implement.