The Tool Desk
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An MCP tool is a delegated operation. The model can choose when to call it, combine several calls, and pass content between them. The model’s plan or a system prompt is therefore not an access-control boundary. The official MCP security policy warns: “The LLM may invoke tools in ways the user did not explicitly request.” Treat the model as an untrusted planner and make security decisions deterministically.
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Start with the right security model
An MCP client connects an AI host to servers that may read files, query databases, call APIs, send messages or use credentials. A tool call can do whatever the server implementation and its credentials permit. A harmless-looking request can also become a chain: one tool finds a record, another exports it, and a third posts the result elsewhere.
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Model Context Protocol documentation assigns different responsibilities to server developers, client developers and operators. Servers are responsible for access controls, input validation and least privilege. Clients should explain capabilities, provide consent mechanisms and show tool or resource access when appropriate. Operators must choose trusted servers, review configuration and restrict access. These duties complement one another; none can be replaced by a prompt instruction. See the MCP Security Policy.
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The controls below are defense in depth. No single setting, transport or approval dialog prevents every leak.
Map the trust boundaries before enabling a server
Make an inventory for each server and tool before connecting it to an AI host. Record what identity reaches the server, which files or records it can access, which credentials it uses, where results go, and whether it can make outbound requests.
- Identity boundary: identify the human, service account, tenant and client session associated with each call.
- Data boundary: list files, database tables, rows, API resources and fields that the server can read or change.
- Action boundary: separate read, create, update, delete, export, message-sending and administrative operations.
- Network boundary: identify reachable hosts, metadata endpoints, internal services and public destinations.
- Credential boundary: distinguish end-user tokens, application credentials and credentials used only to reach the MCP server.
- Output boundary: determine what enters the model context, client interface, downstream tools and logs.
Review tool descriptions, annotations, prompts, resources, errors and results as untrusted content. A description can be misleading, and a returned document can contain instructions designed to influence a later call.
Apply least privilege to tools, data and runtime
Expose the smallest useful capability rather than a broad connection that happens to contain everything. A server that only needs to read invoices should not receive write access to the accounting system or unrestricted access to every customer record.
| Control area | Safer design | Risky design |
|---|---|---|
| Tools | Expose separate, narrowly named operations such as get_invoice_summary and list_invoice_ids. |
Expose a generic SQL, shell or “run anything” tool. |
| Data | Restrict paths, rows, columns, tenants and API scopes to the task. | Give the server a whole home directory, database or organization. |
| Identity | Bind every decision to verified user, client and tenant context. | Authorize everything from one connection-level permission. |
| Runtime | Use a restricted account, read-only mounts and a minimal environment. | Run with the operator’s full privileges and inherited secrets. |
| Network | Allow only documented destinations and protocols. | Permit unrestricted outbound internet access. |
| Results | Return the fields and number of records needed for the task. | Return raw records, credentials or complete documents by default. |
Authorize the actual operation after parsing its arguments. For example, “read customer file” is not enough: the policy should check the normalized path, tenant, file classification and requested fields. Re-check authorization for every operation, including calls made after an earlier tool result.
Separate capabilities instead of hiding them in prompts
Prefer distinct read and write tools, and distinct tools for ordinary and privileged records. A policy can then deny a write capability entirely, rather than hoping the model will never select a dangerous argument. If a task needs a temporary export, issue a narrowly scoped capability that expires when the task ends.
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Keep authorization outside the model
Use the model to plan, not to decide who is allowed to do what. The server should obtain a verified identity from the authenticated session, parse the structured arguments, evaluate policy, and only then perform the operation.
- Authenticate the caller. Establish the client and user identity through the MCP authorization flow or a trusted local process boundary. Do not infer identity from a username supplied in tool arguments.
- Normalize arguments. Canonicalize paths, decode and validate identifiers, apply size limits, and reject ambiguous encodings before policy evaluation.
- Evaluate policy for that operation. Check user, tenant, tool, action, resource, sensitivity, time and purpose. A broad session scope must not silently authorize every tool.
- Apply approval rules. Require an explicit, visible confirmation when the action is external, destructive, costly, privileged or likely to disclose sensitive data.
- Execute with constrained credentials. Use the minimum server-side identity needed for the approved operation.
- Filter the result. Remove fields and records that the caller is not allowed to receive before returning anything to the client.
This approach follows the practical guidance in the MCP Security Best Practices reference. That community reference is an operational guide, not an official compliance certification.
Validate inputs and make outputs safe
Use closed, structured schemas
Define required fields, types, enumerations, length limits and numeric bounds. Reject unknown fields rather than silently accepting them. A schema should express whether a path is a file identifier or a path, whether a query is read-only, and the maximum number of records or bytes that may be returned.
Never concatenate model-controlled values into shell commands, SQL, file paths, URLs or templates. Use parameterized database queries, argument arrays for processes, allow-listed path resolution and URL policies that block unexpected schemes, hosts, ports and redirects. Validate again at the server boundary even if the client advertises a schema.
Minimize and classify results
Return a summary, selected fields or an opaque reference when the model does not need the raw value. Enforce row, byte and time limits. Mask account numbers, personal identifiers, access tokens and unrelated notes. If a result is too sensitive for model context, provide a user-visible action or a short-lived reference that can be redeemed only after a separate authorization check.
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Protect OAuth tokens and authorization sessions
MCP authorization has specific requirements in the authorization security considerations. An MCP server must accept only tokens intended for that server. The specification states: “MCP servers MUST only accept tokens specifically intended for themselves and MUST reject tokens that do not include them in the audience claim or otherwise verify that they are the intended recipient of the token.”
Bind a token to its resource
MCP clients must include the resource parameter, and servers must validate that the token was issued for that resource. Reject a token issued for another API, even when its signature and scopes look valid. Audience validation prevents a confused-deputy path in which an MCP server misuses a token meant for a different service.
Do not pass the MCP token downstream
An MCP client’s access token is for the MCP server. Do not forward it to an upstream API. When the server needs to call another service, it should act as its own OAuth client and obtain a separate upstream token with the smallest required scope. The official best-practices text is explicit: “MCP servers MUST NOT accept any tokens that were not explicitly issued for the MCP server.”
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- Use HTTPS for authorization and transport endpoints in production.
- Use PKCE with the
S256method when technically capable. - Register exact redirect URIs; do not use broad wildcard redirects.
- Prefer short-lived access tokens and narrowly scoped refresh behavior.
- Encrypt server-side credential caches and restrict who can read the encryption keys.
- Never log authorization headers, access tokens, authorization codes, client secrets or refresh tokens.
The MCP authorization tutorial covers these implementation details. Rotate and revoke credentials when a server, client or operator is no longer trusted.
Make human approval meaningful
Approval is useful only when the person can understand the exact operation. For a sensitive call, show the tool name, destination, affected account or tenant, normalized arguments, data categories leaving the system, and whether the action is reversible. “Allow this server?” is weaker than “Send these 12 invoice numbers to api.example.test using the accounting tenant.”
Set approval policies by risk:
- Automatic: low-risk, read-only operations limited to already authorized data.
- Confirm once per task: a bounded batch of similar reads with a fixed destination and output limit.
- Confirm every call: external transmission, privilege changes, destructive actions, financial operations and access to highly sensitive records.
- Disallow: operations outside the user’s policy, unknown destinations, unbounded exports and attempts to retrieve credentials.
Do not allow the model to manufacture an approval, reuse an old confirmation for changed arguments, or suppress a warning by altering a tool description.
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Keep secrets out of context and logs
Design tools so the model does not need to see credentials. Store API keys and client secrets in a managed secret store or protected process environment, inject them only into the server component that needs them, and return a success status or redacted metadata instead of the secret value.
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Log decisions, not private payloads
Useful audit records include timestamp, tool identifier and version, actor and tenant, decision outcome, policy rule, correlation ID, destination and a hash or reference to the request. Redact authorization headers, tokens, secrets, full file contents, query results and unnecessary personal data. Apply the same redaction to debug logs, traces, exception messages and support bundles.
Keep audit access restricted and define retention. You need enough information to investigate an incident without creating a second database of sensitive tool output. The authorization tutorial describes secure handling and redaction expectations.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Isolate local and remote MCP servers
A local stdio server runs with the privileges and ambient environment of its launching process. “Local” describes location, not trust. A malicious or compromised local server can read accessible files, inspect environment variables or use network access unless the host constrains it.
| Deployment | Primary trust boundary | Controls to prioritize |
|---|---|---|
| Local stdio | The operating-system account, process environment and host filesystem. | Review provenance and configuration; use a sandbox or restricted account; remove inherited secrets; mount only required paths; restrict egress; require approval for sensitive actions. |
| Remote HTTP | The network endpoint, authentication flow, tenant policy and server infrastructure. | Use HTTPS; validate resource audiences; restrict reachable internal destinations; protect metadata discovery from SSRF; enforce per-operation authorization and rate limits. |
For local deployments, do not launch a server with an administrator account merely because installation is convenient. Clear environment variables containing cloud credentials, mount sensitive directories read-only or not at all, disable unnecessary device access, and place the process in a container, sandbox or dedicated operating-system account where feasible.
For remote deployments, separate the MCP endpoint from internal management networks, restrict outbound destinations, and treat URL fetching or metadata discovery as an SSRF risk. Network controls should back up application authorization rather than replace it.
A deployment procedure you can reuse
- Classify the data. Mark files, tables, fields and API resources as public, internal, confidential or highly restricted.
- Define the task. Write the smallest user job the server must perform and the exact outputs the model needs.
- Design narrow tools. Split read and write operations, use structured schemas, and set row, byte, time and batch limits.
- Create an authorization matrix. Map user or service identity, tenant, tool, action, resource and sensitivity to allow, deny or approval-required outcomes.
- Implement server-side checks. Authenticate, normalize arguments, authorize, obtain constrained credentials, execute, then filter the result.
- Configure OAuth safely. Register exact redirects, use PKCE, include
resource, validate audience, and obtain separate upstream tokens. - Remove ambient authority. Run with a restricted account, minimal environment, limited filesystem mounts and an egress allow-list.
- Design the approval screen. Display the real tool, arguments, destination, data category and scope of the action.
- Harden observability. Redact secrets and payloads, retain decision metadata, and restrict audit access.
- Test abuse paths. Attempt path traversal, SQL injection, shell metacharacters, oversized results, cross-tenant identifiers, token substitution, malicious tool descriptions and chained exfiltration.
- Prepare revocation. Document how to disable a server, revoke tokens, rotate secrets, preserve relevant audit records and notify affected users.
What is normative and what is operational guidance?
The MCP specification and official security documents contain normative requirements, while broader security references add practical defense-in-depth measures. Keep that distinction clear when writing policy or claiming conformance.
| Area | Official requirement or statement | Operational practice |
|---|---|---|
| Token audience | Servers must accept only tokens intended for themselves and reject tokens for another resource. | Validate issuer, audience and resource consistently; alert on failed audience checks. |
| Downstream calls | The MCP client token must not be passed to an upstream API. | Use a separate upstream OAuth client and narrowly scoped token. |
| Model behavior | The security policy recognizes that models may invoke tools users did not explicitly request. | Use argument-aware authorization, approval and output controls. |
| Transport | Authorization flows define HTTPS, PKCE and redirect requirements for applicable deployments. | Use network segmentation, egress allow-lists and SSRF defenses. |
| Local servers | The security policy assigns access-control, consent, sandboxing and configuration duties across developers, clients and operators. | Use restricted accounts, minimal mounts, provenance review and secret scrubbing. |
Documentation paths include version dates and can change. Check the current MCP specification and the documentation for the client and server versions you deploy.
Test for leakage before production
Security testing should verify both direct disclosure and multi-step behavior. Use synthetic records that look like real secrets but cannot harm anyone, then check every channel: model-visible results, client displays, server logs, traces, caches, error responses and downstream requests.
- Can a user read another tenant’s record by changing an identifier?
- Can a relative path escape the approved directory after normalization?
- Can an unexpected schema field select a privileged operation?
- Does an oversized query return an unbounded export?
- Does a tool result containing instructions cause an automatic second call?
- Does the server accept a valid token whose audience is another service?
- Does any upstream request contain the MCP client’s authorization header?
- Can a local process read inherited cloud credentials or reach an unapproved host?
- Do logs or error pages contain tokens, authorization codes or private payloads?
When a test fails, fix the deterministic boundary that allowed it. Do not rely on adding a stronger system prompt or asking the model to behave better.
Respond when exposure is suspected
- Disable the affected tool or server and block its network egress.
- Revoke and rotate MCP, upstream and application credentials that may have been exposed.
- Preserve restricted audit records, policy decisions and infrastructure evidence without copying additional private payloads.
- Determine which identities, tenants, resources and downstream destinations were reachable.
- Notify the appropriate security, privacy and affected-user contacts under your incident procedure.
- Rebuild the server from a trusted source, tighten its permissions and retest the abuse paths before restoring access.
The bottom line
Secure MCP deployments assume that model-generated plans, tool metadata and returned content can be untrusted. Keep authorization and validation in deterministic server and infrastructure code; minimize capabilities and outputs; bind tokens to the correct resource; never pass MCP tokens downstream; require clear approval for high-impact actions; keep secrets out of context and logs; and isolate both local processes and remote network paths. Layered controls make a data leak harder to cause, easier to detect and less damaging when one control fails.
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