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Secure an MCP server by limiting what it can reach and what it can do, then adding controls in proportion to the consequences of misuse. A read-only server exposing public information needs a different level of protection from one that can delete records, send messages, access credentials, or administer infrastructure. “Blast radius” is a useful way to make that decision; it is not a formal MCP risk score or standard.
How do you assess an MCP server’s blast radius?
Start with the server’s purpose, execution location, identity, and every action its tools can take. Assess the possible impact if a tool is misused, its output is malicious, or its credentials or host process are compromised. MCP and security guidance describe the relevant risks and controls, but do not publish a universal risk tier or quantified measure of blast radius.
| Example server | What to examine | Controls to prioritize |
|---|---|---|
| Read-only access to public information | Whether tools can reach anything beyond the public data, and whether returned content could influence later tool calls. | Review tool definitions and outputs; keep access limited to the stated purpose. |
| Access to sensitive or private data | Which records are readable, which identity grants access, and whether data can be sent elsewhere through an otherwise legitimate tool. | Use narrow, server-specific permissions and credentials; review destinations and tool behavior. |
| Tools that write, delete, send, or administer | Whether actions are reversible, what systems they affect, and whether an agent can act without a person checking the specific action. | Restrict permissions, require meaningful human review for high-impact actions, and scrutinize changes to tool definitions. |
| Local server running on a user’s machine | Which files, credentials, processes, and network resources the process can reach. | Review startup configuration and provenance; sandbox and limit host access where practical. |
This comparison applies least privilege and agent-operation guidance to deployment decisions; it is not an official MCP classification. The Google Cloud guidance on AI security and MCP notes that MCP actions can include non-reversible changes. The OWASP MCP Security Cheat Sheet and the MCP Security Best Practices describe risks involving tools, credentials, and local execution.
What can go wrong when a model uses MCP tools?
A connected model may receive tool names, descriptions, schemas, and results, then choose actions based on natural language. That creates an attack surface beyond the server’s API boundary: a malicious tool description or result can influence the model, and a tool call that is valid according to the server can still disclose data to an unintended destination.
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- Tool poisoning: hostile or misleading descriptions and schemas can steer a model toward unsafe use.
- Malicious returned content: text from a user, database, or external source can contain instructions intended to manipulate the model.
- Tool shadowing and definition changes: a tool’s name, description, or schema can mislead a user or model about what it does. A review of metadata does not prove that the underlying server code is unchanged.
- Data exfiltration: sensitive information can leave through a legitimate tool call if the server or agent is permitted to send it somewhere.
- Unsafe tool chaining: an agent can combine individually permitted actions in a harmful way, particularly when it operates without a person checking each consequential action.
These risks are described in the OWASP guidance and Google Cloud’s MCP security guidance. Treat tool metadata and content returned by tools as untrusted input, not as proof that an action is safe.
Which controls should every MCP deployment have?
Build a server-by-server inventory before granting access. Record an owner and purpose, the data each server can read, the operations it can perform, the credentials it uses, and whether an agent or person initiates consequential actions. Remove unused tools and permissions rather than relying on instructions to prevent their use.
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- Apply least privilege: give each server only the access required for its task. Prefer scoped, per-server credentials, narrow OAuth scopes, and short-lived credentials where feasible over broad shared access or long-lived personal access tokens. See the OWASP MCP Security Cheat Sheet.
- Review tool interfaces: inspect tool names, descriptions, parameter schemas, and return schemas before approving a server. Consider pinning reviewed definitions and requiring review when they change. This can reveal metadata changes, but cannot establish that the server’s code or behavior has not changed.
- Keep data separate from instructions: tell the model to treat user-provided and database-derived text as material to analyze, not as instructions to follow. Clear delimiters and explicit instructions are defense in depth, not substitutes for authorization and permission limits; see Google Cloud’s guidance.
- Make approval meaningful: require a person to review high-impact actions, including what will change and where information will go. An approval prompt reduces risk only if the reviewer can understand the action and checks the proposal; a person can still approve a malicious or destructive suggestion.
How should remote MCP servers handle OAuth?
For remote servers using OAuth, authorization must ensure that a token is valid for the server receiving it. The MCP authorization guidance also sets out protections for authorization endpoints, redirects, and client flows. Use an established authentication library or middleware for token validation rather than writing validation logic from scratch; Microsoft Learn’s MCP guidance warns that validation mistakes can leave a server open to unauthorized callers.
- Validate each incoming access token before processing a tool request, and accept only tokens intended for that MCP server.
- Request tokens for the intended resource. MCP clients use the
resourceparameter to identify the resource for which a token is requested. - Do not forward the client’s MCP token to an upstream API. Obtain and use a separate token issued for that API. The specification states: “The MCP server MUST NOT pass through the token it received from the MCP client.” See the MCP Authorization Security Considerations.
- Protect the authorization flow: use HTTPS for authorization-server endpoints, register redirect URIs and validate them exactly, and use PKCE. Clients technically capable of it must use the S256 challenge method.
- Handle delegated access carefully: if the server proxies a third-party API, obtain user consent for the client. The MCP authorization guidance identifies a confused-deputy risk when a static client ID and dynamic client registration are combined without proper consent.
These requirements and cautions are set out in the MCP authorization security considerations. Do not interpret a token’s validity as permission to use it for a different service.
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What extra precautions do local MCP servers need?
A local MCP server runs on the user’s machine, so installation and startup configuration can have consequences beyond the MCP client. Review the package’s provenance, the command used to launch it, environment variables, and its filesystem and network access before running it. Malicious configuration or payloads can expose credentials, execute code, or cause data loss.
- Run the server with access only to the directories, credentials, and processes required for its task; sandbox it where practical.
- Review changes to startup commands and environment variables, not only the tool list displayed by the client.
- Do not treat localhost as a security boundary. The MCP Security Best Practices discusses risks from insecure local servers accessible to other processes, including DNS rebinding scenarios.
The OWASP MCP guidance also describes how excessive host access can enable traversal, credential theft, or arbitrary code execution.
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How should a server protect state that persists between calls?
If a server stores a cart, workflow, or other state across requests, a handle identifying that state must not stand in for authentication. The MCP project’s security guidance states: “MCP servers MUST NOT treat possession of a state handle as authentication.” See MCP Security Best Practices.
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- Check authorization on every request that accesses or changes stored state.
- Bind the state to the authenticated user on the server side and reject a handle presented by a different user.
- Generate unpredictable handles and consider expiration so an exposed or abandoned handle is less useful.
How much protection is enough?
Increase safeguards as the reachable data becomes more sensitive, the available actions become less reversible, or a compromised server gains broader access. A read-only server is not automatically harmless if its output can steer an agent or expose private data; a confirmation step is not enough if the underlying credentials can still perform unrestricted actions. Match permissions, token boundaries, host restrictions, and human review to the specific impact of misuse. The sources describe attacks and mitigations, not a quantified estimate of MCP incidents or a measured guarantee that any one control prevents them.
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