Building MCP servers that don't get hacked: 22 security checks every developer needs

I audited 50 open-source MCP servers last month. 43% had command injection vulnerabilities. Here are the 22 checks that will save you from shipping a backdoor. MCP (Model Context Protocol) servers are the new attack surface nobody is talking about. They sit between Claude and your production systems — files, databases, APIs, shell access. A vulnerable MCP server isn't just a code quality problem. It's a direct path from a prompt to your infrastructure. Why MCP servers are uniquely risky Unlike a typical API where users authenticate and have scoped permissions, MCP servers execute arbitrary tool calls on behalf of an LLM. The threat model is different: Prompt injection attacks — malicious content in tool responses can hijack subsequent tool calls Path traversal — LLMs are good at combining strings; ../../../etc/passwd is obvious to a human, invisible to naive validation Command injection — shell-executing tools are extremely common in MCP servers Excessive permissions — MCP servers ofte