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A secure MCP server must authorize the data and actions behind each tool—not just accept a valid-looking request or describe a tool as safe. For a remote server, verify that each access token was issued for your server, scope every operation to the authenticated user or workload, and enforce authorization both at the HTTP boundary and inside sensitive tool handlers. Pin the MCP specification revision and SDK you implement: the protocol and language-specific options continue to evolve.
Choose the protocol revision and deployment model first
The MCP maintainers announced specification revision 2026-07-28 on July 28, 2026. Its release describes a stateless protocol core, header-based routing, authorization changes, extensions, and deprecations. State which revision and SDK your server supports, then use the matching specification and SDK documentation when configuring transport and authorization. Do not carry session or HTTP+SSE assumptions from an older deployment into a newer one without checking the applicable revision.
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The release describes Mcp-Method and Mcp-Name headers as routing and metering aids for gateways, rate limiters, and web application firewalls. Treat them as request metadata, not proof of identity or permission: the server must still authenticate the caller and authorize the requested operation.
Write down the boundaries
Before implementation, identify the caller, the MCP resource the caller is accessing, each tool’s data and side effects, and any downstream services or files the tool can reach. Decide which operations are public and which require an authenticated principal. This makes it possible to configure the HTTP layer and each handler against the same access policy instead of relying on tool names or model-provided arguments.
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Decide whether authorization applies to every request or selected tools
Official MCP Apps authorization guidance describes two patterns. Choose based on whether any exposed operation is safe to offer without an authenticated identity, and whether your HTTP handler can reliably distinguish protected calls.
| Pattern | When it fits | Required safeguards |
|---|---|---|
| Authorization across the server | All exposed operations require an authenticated identity. | Validate the access token for this MCP resource on every request and apply the caller’s permissions to the requested operation. |
| Authorization for selected tools | Some tools are genuinely public while others expose protected data or actions. | Identify protected calls at the HTTP boundary; reject a protected request without valid authorization using HTTP 401 and an appropriate WWW-Authenticate challenge that references Protected Resource Metadata. |
For either pattern, publish OAuth discovery metadata and validate that the access token was issued specifically for this server. A syntactically valid token intended for another resource must not grant access. Follow the matching specification and verifier documentation to establish how your chosen stack checks that binding.
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Authenticate at the boundary, then authorize in the handler
Use the HTTP boundary to reject unauthenticated requests early, but do not make middleware the only protection for sensitive operations. Official authorization guidance recommends checking authorization again in sensitive tool handlers. That second check limits the impact of a routing or middleware mistake and gives the handler a place to enforce the correct user or workload scope.
- Authenticate the request. Validate the bearer token with the configured verifier and establish the authenticated principal in request context. Do not treat a tool argument, tool description, or routing header as identity.
- Check the resource and operation. Confirm the token is for this server, then determine whether the requested tool and operation are permitted for that principal.
- Scope the data access. Have the handler derive account, tenant, or other user-specific scope from the authenticated context. Do not let model-supplied arguments select another user’s records without an independent authorization check.
- Constrain the operation. Enforce the permission at the point where data is read or changed, and limit any file or network access the tool needs to perform its job.
- Return the correct authentication response. At the HTTP boundary, a protected call without valid authorization should receive HTTP 401 with the applicable
WWW-Authenticatechallenge and Protected Resource Metadata reference.
The account lookup example in the official guidance illustrates the core rule: use authenticated identity when deciding which account’s data a tool can access. A caller’s valid identity does not automatically authorize every tool or every record.
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Apply the 2026 authorization changes in the right component
The July 28, 2026 MCP release describes issuer validation and credential binding changes. Authorization servers should return the iss parameter specified by RFC 9207, and clients must validate it before redeeming an authorization code. Client credentials are bound to the authorization server that issued them and must not be reused with another issuer.
These responsibilities span components. The client’s authorization-code issuer check is not a substitute for the resource server validating incoming access tokens, and server code cannot perform a client-side check on the client’s behalf. Likewise, do not assume token audience or resource binding is enforced merely because an SDK provides an OAuth abstraction; verify the configuration and behavior of the selected SDK and verifier.
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The release formally deprecates Dynamic Client Registration (DCR) in favor of Client ID Metadata Documents (CIMD), while retaining DCR for backward compatibility. When choosing registration behavior, weigh interoperability with existing clients and authorization servers against support for the direction of the current protocol and the capabilities of your selected SDK.
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Do not treat tool descriptions or annotations as security controls
Tool annotations can describe traits such as read-only behavior, destructive effects, idempotency, or effects on the external world. They help clients and users understand a tool, but they do not enforce its behavior. The MCP maintainers warn that clients should treat annotations as untrusted by default: an incorrect or malicious server can mislabel a destructive tool as read-only. As the maintainers put it, “They don’t make the model resist prompt injection.”
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Keep the distinction clear: metadata communicates risk; the server and its runtime enforce policy. If a tool must not make arbitrary network requests or access unrelated files, restrict those capabilities with runtime or infrastructure controls such as sandboxing and network restrictions. Do not expect the model to infer or preserve those boundaries from a description.
Review token forwarding, deputy behavior, and request forgery
Official Go SDK security documentation identifies confused-deputy behavior, token passthrough, and server-side request forgery (SSRF) as security topics for implementers. These are useful threat-model categories beyond Go, but concrete mitigations depend on the SDK and deployment; consult the guidance for the stack you actually use.
- Confused deputy: Ask whether the server could use its own authority to perform an action the caller is not allowed to perform. Bind each operation to the caller’s permissions, not merely to the server’s broad credentials.
- Token passthrough: Do not forward an incoming bearer token to a downstream service without verifying that it is intended for that service and that forwarding is part of the design. A token valid for the MCP server is not automatically valid for another resource.
- SSRF: Assess whether tool inputs or returned content can influence a URL or destination the server contacts. Restrict destinations and network reachability where the tool does not need unrestricted access.
- Untrusted tool output: Treat content fetched from external systems as potentially attacker-controlled. It may influence the agent’s later decisions, so tool descriptions alone cannot guarantee safe handling.
Turn the design into an implementation review
Use this review to check that the policy is enforced at the components that can actually guarantee it:
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- OAuth discovery is published and the verifier checks that access tokens are intended for this MCP resource.
- The HTTP layer either requires authorization on all requests or reliably identifies protected tool calls and returns the specified 401 challenge when authorization is missing or invalid.
- Sensitive handlers independently check permissions and bind data access to the authenticated principal.
- Registration and issuer behavior are compatible with the chosen revision, including the distinction between client-side authorization-code issuer validation and server-side access-token validation.
- Tool annotations are treated as descriptive, while sandboxing, file access, and network policy enforce properties that must hold even if a model or tool description is wrong.
- Reviewers have considered confused-deputy behavior, token passthrough, and SSRF using the detailed security guidance for the selected SDK and deployment.
The official MCP sources establish these design boundaries, not a complete checklist for every cloud, language, or threat model. Verify exact transport and security settings against the pinned specification and SDK documentation before deployment.
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