This paper argues that agentic AI and API security now constitute a distinct fourth pillar of cybersecurity, alongside endpoint, network, and cloud security. As AI agents, MCP servers, and API-driven systems proliferate, APIs have become the primary execution layer for business logic and AI actions, yet remain poorly inventoried and weakly protected by existing tools. Traditional security pillars cannot see or govern machine-to-machine API traffic, encrypted payloads, or agent-driven workflows, creating a major blind spot. The paper shows how attackers exploit APIs through business logic abuse, agent chaining, and data exfiltration, and concludes that effective protection in the AI era requires dedicated API security capabilities for discovery, posture governance, and runtime behavioral de

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