Secure an MCP server by limiting what it can reach and what it can do, then adding protections for the way it runs and how an agent uses its tools. A server that can read public information needs different safeguards from one that can delete records, send messages, expose credentials, or administer infrastructure. “Blast radius” is a practical way to make that distinction—not an official MCP risk score.
Start by mapping what each server can affect
An MCP connection can expose tool names, descriptions, schemas, and results to a model that may choose actions based on natural-language context. The risk is not limited to a malicious server: poisoned tool descriptions, malicious returned content, tool shadowing, and data exfiltration through otherwise legitimate tool calls are among the documented concerns. Treat tool metadata and content from users, databases, and external services as untrusted input. See the OWASP MCP Security Cheat Sheet and Google Cloud’s MCP security guidance.
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For every server, record its owner and purpose, the data it can access, the actions it can take, and the identity and permissions it uses. Include whether actions are reversible, where the server executes, and whether a person reviews consequential actions. This comparison is a planning aid based on least privilege and agent-operation guidance, not an MCP-defined tier system.
| Example access | What misuse could reach | Guardrail emphasis |
|---|---|---|
| Read-only access to low-sensitivity public information | Incorrect or misleading answers; exposure of any additional data returned by the tool | Review tool definitions and outputs; limit the server to the data it needs |
| Read access to private records or credentials | Disclosure of sensitive information through outputs or tool calls | Use narrow, server-specific access; scrutinize what data can be returned and to whom |
| Write access, messaging, deletion, or other consequential actions | Unauthorized or mistaken changes, including actions that may be difficult or impossible to reverse | Restrict available operations and require meaningful human review for high-impact actions |
| Host files, processes, or infrastructure administration | Credential exposure, code execution, data loss, or broader system changes | Constrain execution and permissions; closely review installation, configuration, and authorization boundaries |
These examples are not formal risk ratings. An agent’s human-approval step can reduce risk, but it cannot guarantee safety: a person may approve a destructive or maliciously framed suggestion, and agent-only operation can be affected by prompt injection, unsafe tool chaining, or error-handling failures. Google discusses these agent and action risks in its MCP security and safety guidance.
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Apply controls to every MCP deployment
Limit permissions to the task
Remove unused tools and permissions, and give each server only the access its stated purpose requires. Prefer narrow, server-specific credentials and OAuth scopes; use short-lived credentials where feasible rather than broad, long-lived personal access tokens. These practices limit the reach of compromised credentials or an unintended tool call. See the OWASP MCP Security Cheat Sheet.
Review the tool interface and its changes
Before approving a server, inspect its tool names, descriptions, parameter schemas, and return schemas. These are part of the surface an agent interprets, not merely documentation. Consider pinning reviewed definitions and triggering a review when they change. A stable definition does not prove that the underlying server code or behavior is unchanged.
Keep content separate from instructions
Instruct the agent to treat user-provided text and database-derived or external content as information to analyze, not as instructions to obey. Clear delimiters and explicit instructions can help keep data separate from directions, but they do not replace authorization checks or narrow permissions. Google describes this as a defense-in-depth measure in its MCP security guidance.
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Require review where actions have high impact
Use meaningful human confirmation before high-impact operations, such as sending messages, deleting records, or changing infrastructure. Make the proposed action and its target clear enough to check; an approval prompt that hides the consequence is not a meaningful safeguard. Approval supplements, rather than replaces, authorization and access limits.
Secure remote servers that use OAuth
Remote MCP servers need an authorization boundary that ensures a token is valid for the server receiving the request. The MCP authorization guidance says the server must validate incoming tokens and must not forward the client’s token to an upstream API. Instead, obtain a separate token intended for that API. The specification states: “The MCP server MUST NOT pass through the token it received from the MCP client.” See the dated MCP Authorization Security Considerations.
- Use the MCP authorization flow’s
resourceparameter to identify the intended resource for which the client is requesting a token. - Validate each access token before processing a tool request, and accept only tokens issued for the MCP server.
- Use HTTPS for authorization-server endpoints, validate redirect URIs against exact registered values, and use PKCE. Clients technically capable of it must use the S256 challenge.
- If the MCP server proxies a third-party API, handle user consent for each client. The MCP guidance describes a confused-deputy risk when a static client ID and dynamic client registration are combined without proper consent.
These requirements and considerations are detailed in the MCP project’s Authorization Security Considerations. Use tested authentication middleware or a library for token validation rather than implementing validation from scratch; Microsoft Learn also warns that validation mistakes can expose a server to unauthorized callers. Its steps are specific to Microsoft Entra ID: Secure an MCP server with Microsoft Entra ID.
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Treat local server setup as host execution
A local MCP server runs on the user’s machine. Its startup command and configuration can affect the host, and other local processes may be able to reach an insecure server. Malicious or poorly reviewed packages, environment variables, or payloads can create routes to code execution, credential exposure, or data loss.
- Check package provenance and inspect the startup command, environment variables, and requested filesystem and network access before installing or approving the server.
- Where practical, sandbox the process and grant access only to the specific directories, credentials, and processes it needs.
- Do not treat localhost as a security boundary by itself. The MCP project’s Security Best Practices discusses risks from insecure local servers accessible to other processes, including DNS rebinding scenarios.
Protect stateful workflows from handle substitution
Some implementations retain state between requests and use a cart, workflow, or other handle to refer to it. A handle identifies stored state; it does not establish who is making the request. The MCP project’s Security Best Practices states: “MCP servers MUST NOT treat possession of a state handle as authentication.”
- Verify authorization on every request that uses stored state.
- Generate unpredictable handles and bind each stored state object to the authenticated user on the server side.
- Reject a handle presented by a different user, and consider expiring handles when they are no longer needed.
Keep the control set proportional to the server’s reach
A practical review should connect each permission to a specific task, identify who can invoke the server, and check what could be read, changed, sent, or triggered if a tool call or credential were misused. Add stronger execution constraints and human review as the potential impact rises. The MCP security materials provide requirements, attack descriptions, and mitigations; they do not establish a quantified incident rate or a measured effectiveness score for these controls.
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