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The Model Context Protocol (MCP) is a client-server standard that lets AI applications discover and use capabilities exposed by external servers, including tools, prompts, and resources. It standardizes how those components exchange messages; it does not certify that a client, tool, server, or connected service is safe. MCP security depends on controls at each boundary, from the AI application and transport to the tool’s permissions and any downstream system.
What MCP standardizes—and what it does not
MCP gives an AI application a common way to connect to external context and actions. An MCP client, typically part of a host application, communicates with MCP servers. A server can expose resources (such as data to read), prompts, and tools (actions the client can invoke). Clients can discover and use these capabilities through protocol messages rather than relying on a different integration format for every server.
The protocol standardizes the connection and message exchange, not the quality, correctness, or safety of what a server exposes. A tool may be described as read-only or given an annotation, but that description is not an access-control mechanism. The host application and server still need to enforce what the tool is allowed to do and under whose authority.
What “stateless” means in the 2026-07-28 specification
The Model Context Protocol Basic Specification, version 2026-07-28, describes MCP as stateless: “The Model Context Protocol (MCP) is a stateless protocol: all the information needed to process a request is contained in the request itself.” In that model, a server must not infer the client’s identity, capabilities, or conversation context from earlier requests merely because they arrived over the same connection.
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This does not mean an AI application or service can never store state. It means that state a request depends on must be identified and supplied explicitly, rather than inferred from a persistent process or open stream. A long-lived STDIO process or HTTP connection is not, by itself, a trustworthy user session or proof that two requests belong to the same conversation. Applications that keep task, user, or conversation state need to define and protect that state separately.
Where the security boundaries sit
1. The client, host application, and model
The host application mediates between a model and MCP servers. MCP does not determine whether a model’s choice to call a tool is appropriate, whether the user intended a consequential action, or whether content returned by a server should be trusted. Treat tool descriptions, tool results, and retrieved data as inputs to the application—not as authority to bypass its policy.
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The host should decide which capabilities are available to which users, when user approval is required, and which actions the model may trigger without confirmation. This matters especially when a tool can modify data, send information outside the organization, or access sensitive records. The NSA’s May 2026 Model Context Protocol (MCP): Security Design Considerations discusses risks including overly broad tool access and sensitive information moving through tool workflows; it does not establish that every MCP deployment has those weaknesses.
2. The transport and authentication
MCP authorization is optional at the protocol-wide level. The published authorization profile describes OAuth-based authorization for HTTP transports; it is not a universal authentication flow for every MCP connection. The 2026-07-28 authorization specification directs STDIO implementations to obtain credentials from the environment, while alternate transports should follow their established security practices.
| Transport | Credential and authorization model | Security implication |
|---|---|---|
| HTTP | The MCP authorization specification defines an OAuth-based flow for protected HTTP servers. | Use the HTTP profile’s resource-specific token handling and server-side validation; do not assume that a token is valid for every MCP server. |
| STDIO | The specification says implementations should obtain credentials from the environment. | Protect the process environment and its surrounding host; the HTTP OAuth flow is not the specified STDIO model. |
| Other transports | The authorization specification says to follow the transport’s established security practices. | Choose and document an authentication approach appropriate to that transport rather than assuming MCP supplies one. |
For protected HTTP deployments, the specification’s security considerations call for resource-specific authorization: the client should request a token for the intended MCP server, and that server must verify that the token was issued for it. This is the token’s audience boundary. A token intended for one service should not be accepted by another MCP server.
The same guidance calls for secure token storage, HTTPS authorization endpoints, PKCE for authorization-code flows, and exact validation of registered redirect URIs. It also addresses issuer validation to prevent authorization-server mix-up, recommends short-lived access tokens, and requires public clients to rotate refresh tokens under the referenced OAuth requirements. These protections reduce particular token and redirect risks; they do not decide whether a requested tool action is safe.
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3. The MCP server and its tools
A successful authorization check answers whether a client may access a protected server. It does not, by itself, define which tools that client may call, the data each tool can reach, or whether a tool should be allowed to make a destructive change. Those are server and application-policy decisions.
Enforce permissions at the point where tools execute. Give each tool only the access it needs, validate its inputs, and separate read operations from write or destructive operations where practical. Require additional approval for consequential actions. Treat tool names and annotations as useful metadata, not as substitutes for enforcement.
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An MCP server may call another API or service on the user’s behalf. The MCP security specification warns against passing the token received from an MCP client through to an upstream API: the token may have been issued for the MCP server, not that downstream resource. The server should use appropriately scoped downstream credentials and make authorization decisions that reflect both the principal and the intended resource. It should also map user identity and consent deliberately instead of assuming that access to the MCP server automatically grants equivalent access elsewhere.
Operational isolation remains necessary even though the protocol is stateless. Separate users’ and tasks’ data where required, protect sensitive information in storage and logs, and monitor access and consequential tool activity. The NSA’s May 2026 guidance identifies token and session risks, task and data isolation weaknesses, implementation inconsistency, and broad tool privileges as design concerns. Those observations are reasons to assess a deployment, not evidence that every implementation shares the same flaw.
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- Inventory the connections. Record which clients connect to which servers, which transport each uses, and which specification and SDK versions they support. Do not assume every component implements the same version or authorization behavior.
- Map principals to capabilities. For each user or service identity, document which tools and resources it can access, what those tools can change, and which actions require approval. Check whether tool access reaches sensitive data or external services.
- Trace every credential. For HTTP authorization, confirm that discovery and token requests target the intended server, that the server validates token audience, and that credentials are protected in storage and logs. Confirm that upstream services receive separate, appropriately scoped credentials rather than the MCP client’s token. For STDIO, review how environment credentials are supplied, exposed to processes, and protected on the host.
- Test boundaries, not just connectivity. Verify that a token for a different resource is rejected, that callers cannot invoke tools outside their permissions, that invalid inputs do not produce unintended actions, and that users or tasks cannot access one another’s protected data. Test confirmation and denial paths for consequential operations.
- Maintain the deployment. Track vulnerabilities in implementations and dependencies, monitor tool and data access, and review token rotation and revocation behavior. Recheck compatibility when clients, servers, SDKs, or the MCP specification change.
What changed in the 2026-07-28 specification
The MCP project’s 2026-07-28 release describes a stateless core, an extensions framework, and Tasks and MCP Apps as extensions, alongside authorization hardening and a formal deprecation policy. These are version-specific project details; deployments may support other versions.
The release notes formally deprecate Dynamic Client Registration (DCR) in favor of Client ID Metadata Documents. DCR remains available for backward compatibility and is planned for removal in a future specification version. The same release notes mark Roots, Sampling, Logging, and legacy HTTP+SSE as deprecated and describe an offramp. Check the release notes and the versions supported by your actual client and server before relying on a deprecated feature or migration path.
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