For a remote MCP server using HTTP, OAuth lets a client obtain an access token for that specific server and present it with each request. The server validates the token and decides whether its permissions cover the requested operation. MCP calls this layer authorization: authentication establishes identity, while authorization determines what an identified user or client may access. Authorization is optional across MCP implementations; the specification’s OAuth flow applies to HTTP-based transports, not STDIO.
Who does what in MCP authorization?
Think of the flow as three roles, which may be operated by different systems:
- MCP client: Acts on behalf of the resource owner, obtains a token for the intended MCP server, and sends it with HTTP requests.
- Authorization server: Authenticates or otherwise interacts with the user, applies its authorization policy, and issues access tokens. It may be hosted alongside the MCP server or separately. Its internal implementation is outside the MCP authorization specification.
- Protected MCP server: Acts as an OAuth resource server. It validates the token, including that it is intended for this server, then handles the authorized MCP request.
The MCP server does not necessarily issue the token, and MCP itself is not the token issuer. The authorization server issues it; the protected server checks it.
How the OAuth flow works for a remote MCP server
- Find the server’s authorization server. The client contacts the protected MCP server. The server implements OAuth 2.0 Protected Resource Metadata, which identifies its associated authorization server or servers; the client uses that metadata for discovery.
- Discover authorization endpoints. The client obtains endpoint and capability information from the authorization server through OAuth Authorization Server Metadata or OpenID Connect Discovery. An authorization server must provide at least one of those discovery mechanisms, and a conforming client must support both.
- Obtain a client ID. Before starting the authorization flow, the client needs an ID. It can use a Client ID Metadata Document (CIMD), pre-registration, or Dynamic Client Registration (DCR). CIMD is preferred by the current specification; DCR remains available for compatibility but is deprecated.
- Request access for the intended resource. The client includes the
resourceparameter in both the authorization request and the token request. It identifies the intended MCP server using that server’s canonical URI. - Complete authorization and get a token. The authorization server may ask the user to sign in or approve access, then issues an authorization code for the client to exchange for tokens. The precise user interaction and authorization policy are determined by the authorization server, not by MCP.
- Call the MCP server with the token. The client sends the access token in
Authorization: Bearer <access-token>on every HTTP request to the server. - Validate before serving the request. The MCP server accepts only valid tokens intended for its own resources. It must not accept or relay unrelated tokens. A missing, invalid, or expired token is an HTTP 401 case.
How resource binding and scopes protect access
Bind the token to the server it is for
The resource parameter tells the authorization server which MCP resource the client is asking to access. Requiring it in both authorization and token requests, and validating that the resulting token is intended for the server, helps prevent a token issued for one service from being replayed at a different MCP server. A server should validate the token’s audience or equivalent resource binding, not merely whether the token is syntactically valid.
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Send bearer tokens only in the Authorization header
Bearer tokens are credentials: anyone who obtains one may be able to use it within its permissions. Send them in the HTTP Authorization header on every request, never in a URI query string, where they can be exposed through logs, browser history, or other URL handling.
Use scopes for least privilege
A server should include a scope parameter in its WWW-Authenticate challenge to indicate the permissions needed for an operation. Clients should request scopes appropriate to the operation rather than asking for broad access by default. Treat the challenge scopes as authoritative for that operation; do not assume they must correspond in a particular way to the authorization server’s advertised scopes_supported list.
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Distinguish missing credentials from insufficient permission
- HTTP 401: The token is missing, invalid, or expired. The client generally needs to obtain or renew valid authorization before retrying.
- HTTP 403: The token is valid, but does not grant enough permission for the requested operation. The server should return a Bearer challenge that describes the required scope. A client may seek step-up authorization and should preserve previously granted scopes that are still needed.
Choose a client registration approach
Registration tells an authorization server about a client, including its name and redirect URI. This matters in an open ecosystem, where a client may connect to servers whose authorization systems have not previously registered it. The current MCP specification describes three approaches:
| Approach | How client identity is supplied | Registration endpoint required? | Current status |
|---|---|---|---|
| Client ID Metadata Documents (CIMD) | The client publishes identity metadata in a document identified by its client ID. | No DCR endpoint is needed for this mechanism. | Preferred by the current specification. |
| Pre-registration | The client is registered with the authorization server in advance. | No dynamic registration endpoint is required for the pre-registered client. | Still described as an option. |
| Dynamic Client Registration (DCR) | The client registers with the authorization server dynamically. | Yes; the authorization server needs to support a registration endpoint. | Deprecated, but retained for backward compatibility where CIMD is not supported. |
The MCP project’s 2026-07-28 specification release also describes clients binding registered credentials to the issuer that minted them and registering again if the resource moves to a different authorization server. For DCR, clients declare application_type; this is intended to prevent an authorization server from misclassifying a desktop or CLI client as a web client and rejecting localhost redirects.
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Prevent authorization-server mix-ups
A client must keep track of which authorization server it selected from validated metadata. Under the 2026-07-28 specification, if the authorization response includes the RFC 9207 iss parameter, the client compares it with the recorded issuer before sending the authorization code to a token endpoint. If the metadata says the authorization server supports iss but the response omits it, the client rejects that response. This check helps prevent an authorization code from being sent to the wrong authorization server in a mix-up attack.
Do not assume refresh tokens are issued
An authorization server may or may not issue a refresh token. A client must not assume one will be available; if it requests and receives one, it must protect the token both in transit and in storage. Refresh tokens are longer-lived credentials than many access tokens, so their handling belongs in the client’s credential-protection design, not just its request code.
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How Enterprise-Managed Authorization differs
Enterprise-Managed Authorization (EMA) is a separate MCP extension, announced as stable on June 18, 2026. Rather than relying on a user to approve access separately at each server, it lets an organization govern MCP access through a trusted identity provider (IdP), using group membership, roles, and policy. In the announced flow, the client obtains an identity assertion during single sign-on and exchanges it for an MCP-server access token. EMA therefore requires compatible support in the IdP, client, and server; it is not simply a different name for baseline HTTP authorization.
| Question | Standard per-server OAuth | EMA |
|---|---|---|
| Who controls access? | The authorization server and the applicable user-consent or access policy. | The organization, through IdP policy such as groups and roles. |
| How is access obtained? | The client follows the server’s OAuth authorization flow, which may involve user approval for that server. | The client uses an identity assertion obtained through organizational sign-in and exchanges it for a server access token. |
| What deployment support is needed? | HTTP authorization support in the client and protected server, with an authorization server. | Support for the EMA extension in the IdP, client, and MCP server. |
The June 18, 2026 announcement identified Okta as the first supported IdP and named Anthropic and Visual Studio Code among client implementations, with Asana, Atlassian, Canva, Figma, Granola, Linear, and Supabase among server adopters at that time. Those are dated project-announcement examples, not a guarantee that every product version or deployment supports EMA. The announcement does not establish a neutral performance or cost comparison between EMA and per-server OAuth.
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What changes for STDIO MCP servers?
The MCP authorization specification covers HTTP-based transports. For STDIO implementations, credentials should be obtained from the environment rather than by applying the HTTP authorization flow. Do not assume that HTTP metadata discovery, bearer-header handling, or OAuth redirects apply unchanged to a local STDIO connection.
Implementation checklist
- Decide whether the deployment needs authorization; MCP does not require every implementation to use it.
- For protected HTTP servers, implement Protected Resource Metadata and validate tokens for the server’s own resource.
- Support both OAuth Authorization Server Metadata and OpenID Connect Discovery in clients; ensure the authorization server offers at least one.
- Use CIMD where supported; retain pre-registration or DCR only as appropriate for the deployment and compatibility needs.
- Include the canonical resource URI in both authorization and token requests.
- Send the access token in the Authorization bearer header on every HTTP request, never in a URL.
- Handle 401 and 403 distinctly, using Bearer challenges and scopes to guide reauthorization when needed.
- Validate the authorization response issuer as specified, protect any refresh token, and do not assume one is issued.
- For STDIO, obtain credentials from the environment; for centrally governed enterprise access, verify EMA support across all required components.
The MCP authorization specification dated 2026-07-28 is the relevant version for these requirements. The specification defines the protocol behavior, while individual authorization-server implementations and client or server product support can vary.
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