This post demonstrates how Docker AI Sandboxes defend against malicious MCP server credential theft attacks targeting AI agents like Kiro. Unlike human-in-the-loop defenses, which require users to recognize suspicious requests, Docker AI Sandboxes provide structural protection by running agents inside isolated microVM environments where host credentials simply do not exist. The post shows that a previously demonstrated multistage AWS credential theft attack fails completely inside the sandbox because the credentials are never mounted. Additionally, configurable network policies block unauthorized outbound traffic, providing a second defensive layer. The post also explains why alternative mitigations like kiroignore are insufficient, since shell execution can bypass file-reading restrictions entirely. While the sandbox does not protect against everything, such as credentials stored within the project directory or prompt injection, it significantly reduces the attack surface and contains potential damage compared to running agents without isolation.