MCP Server Security (OWASP LLM07:2025)

What this checks

Prevents MCP tool handlers from being exploited via malicious inputs, hardcoded secrets, unrestricted file access, or shell injection. A compromised MCP server gives attackers direct access to the host environment.

Vulnerable patterns

• MCP tool handler that opens or reads a file path supplied directly by the caller with no allowlist or root confinement
• Tool handler that runs a shell command built from tool input with shell expansion enabled
• Hardcoded API key, token, or credential in the handler body or tool definition
• Tool schema declaring a string parameter with no length cap, pattern, or enum constraint

Fix immediately

Flag the vulnerable code and explain the risk. Then suggest a fix that establishes these properties:

1. Tool schemas constrain every string parameter — maximum length, regex pattern, enumerated values, or rejection of additional properties. A bare string type is a blank check. The schema is the first line of defense because the model generated the input.
2. Paths are canonicalized and confined to an allowed root. The handler resolves the supplied filename against a fixed base directory and verifies the result stays inside that directory — defeating traversal, symlink escape, and absolute paths.
3. Shell execution uses argument lists, never a shell string with interpolated input. If the tool must run a command, the command name comes from a static allowlist and arguments are passed as discrete argv elements.
4. Secrets come from the environment or a secrets manager — never hardcoded literals in the handler or tool definition. A leaked MCP server config should not leak credentials.
5. Every invocation is audit-logged with the tool name and sanitized inputs before the side-effect runs. For broader MCP tool design guidance, see the insecure-plugin-design skill.

Translate each principle to the MCP SDK and language of the audited handler. Use the SDK's documented schema-validation, path-resolution, and process-execution APIs — do not hand-roll equivalents.

Verification

Confirm the response:

• No secrets hardcoded — all credentials loaded from environment variables or a secrets manager
• File paths resolved and confined to an allowed base directory
• Shell calls use argument lists, not a shell string with interpolated input
• Every schema string parameter has a length cap, pattern, or enum constraint

References

• CWE-284 (Improper Access Control)
• CWE-78 (OS Command Injection)
• CWE-200 (Exposure of Sensitive Information)
• OWASP LLM07:2025 Insecure Plugin Design