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An MCP server is a security boundary: an AI application can use it to reach tools, data, and external services. Secure it by authenticating callers, authorizing each operation, limiting permissions, isolating execution, reviewing server packages and tool definitions, treating retrieved content as untrusted, validating inputs and outputs, requiring approval for consequential actions, and monitoring calls. Which controls apply depends on whether the server runs locally or remotely and which MCP and authorization versions your clients support.
Why MCP servers need security controls
MCP connects an AI application to tools and data, so the risk is not limited to whether the server’s network endpoint is exposed. A model may choose a tool based on its name, description, arguments, and returned content. An attacker may try to influence that choice through a malicious tool description or poisoned webpage, document, or email; a compromised package; or an overly privileged server. A proxy can also act as a confused deputy, using its own broader authority for an action the initiating user should not be allowed to perform.
These are plausible attack paths, not evidence that MCP is inherently insecure or that any particular product is universally safe. The MCP project’s Security Best Practices page, documented for specification version 2026-07-28 and accessed September 29, 2026, and OWASP’s MCP Security Cheat Sheet, accessed September 29, 2026, describe controls for reducing these risks. Neither source establishes an incident rate. Treat security as a set of boundaries to enforce and verify, rather than a property conferred by adopting MCP, a gateway, or a prompt filter.
How to secure an MCP server: a control order
- Identify the boundary. Inventory the clients, servers, tools, data stores, upstream services, credentials, and network paths involved. Decide who can reach each server and what identity it should see.
- Authenticate and authorize at the server. Verify the caller and check that the verified principal may perform the specific operation. Do not rely on the model, client UI, or an upstream system to enforce the server’s access policy.
- Reduce authority. Give each server and operation only the permissions it needs. Prefer read-only access where possible, narrow scopes, and separate credentials over broad reusable access.
- Constrain and isolate execution. Review package provenance and startup configuration. Restrict process, filesystem, and network access; separate servers from one another and from the host where practical.
- Protect the model’s decision boundary. Review tool names, descriptions, schemas, and changes to them. Treat retrieved pages and tool results as untrusted data, not instructions with authority.
- Validate and gate actions. Validate arguments before execution and results before returning them. Apply deterministic policy checks and require meaningful human approval for consequential actions.
- Monitor use and test recovery. Record enough context to investigate who invoked which operation and what decision was made, without logging secrets. Test denials, failures, and revocation paths.
Authenticate callers and enforce authorization
Authentication answers who is calling; authorization answers what that caller may do. Enforce the second question at the MCP server boundary for every operation. A tool description saying “only administrators may use this” is not an access control. Nor is an instruction to the model to avoid a tool.
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Bind tokens to the server that accepts them
The MCP authorization security considerations for specification version 2026-07-28 say a server should validate incoming access tokens and accept only tokens issued for that server. Do not accept a token simply because it is valid for some other service, return data to an unauthorized caller, or pass the MCP client’s token through to an upstream API. Obtain and use the appropriate upstream credential instead, and authorize the requested operation against the verified caller.
Token handling details matter. The same versioned guidance calls for secure token storage, HTTPS for authorization endpoints, PKCE to protect authorization-code flows, exact matching of registered redirect URIs, and state validation where appropriate. Use established authentication libraries or middleware rather than writing token validation yourself. For an organization already using Microsoft’s identity stack, Microsoft’s guide, Secure a Model Context Protocol (MCP) server with Microsoft Entra ID, provides one vendor-specific implementation path; its recommendations should not be mistaken for a universal MCP requirement.
Check compatibility before choosing an OAuth flow
Authorization is version-sensitive. The MCP project’s announcement for the 2026-07-28 specification says Dynamic Client Registration (DCR) is formally deprecated in favor of Client ID Metadata Documents, while remaining supported for backward compatibility in that version’s description. That does not establish that every client, MCP server, or authorization server supports the replacement. Confirm the versions and discovery behavior on all three sides before selecting a flow or planning a migration. Recheck the current specification when deploying because the project documentation is mutable.
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A server that has broad access can unintentionally exercise its own authority on behalf of a less-privileged user. This risk grows when many tools share a service credential, when OAuth scopes are broader than the task, or when a proxy does not preserve whose request it is handling. A legitimate tool channel can also be misused to disclose data if its permissions are excessive.
- Use a separate credential and narrowly scoped permissions for each server or workload where feasible.
- Separate read and write capabilities instead of giving all tools one broad credential.
- Keep the user’s identity and consent context through the request path, and check that identity against the requested operation.
- Require explicit authorization for access to sensitive records, external recipients, or high-impact actions.
- Never treat a client’s possession of a token for one audience as permission to use it at another service.
When reviewing a proxy or shared OAuth client design, ask whether each client receives appropriate consent and whether the proxy can distinguish the user behind every operation. Shared or static client arrangements combined with weak per-client consent can create a confused deputy.
Defend against prompt injection and tool poisoning
Indirect prompt injection can arrive in content a system retrieves, such as a webpage, document, or email. Tool poisoning instead places manipulative instructions in tool names, descriptions, or other metadata, potentially steering the model toward an unintended call. Microsoft for Developers described the risk of unintended tool calls and possible data exposure in its April 28, 2025 article, Protecting against indirect prompt injection attacks in MCP.
There is no reliable prompt-only fix: asking a model to ignore malicious instructions does not prevent it from seeing them or guarantee it will behave as intended. Use layered controls:
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- Review tool names, descriptions, schemas, and metadata changes, including changes made during an update.
- Keep tool capabilities narrow and constrain which arguments, destinations, and records they can access.
- Treat tool results and retrieved content as data, even when that content appears to address the model directly.
- Enforce identity, permission checks, and approval requirements outside the model’s interpretation of a prompt.
OWASP’s MCP checklist also calls out schema integrity, input and output validation, and prompt injection through tool return values. Those measures limit what untrusted content can cause, even when a model’s behavior cannot be predicted perfectly.
Secure local and remote deployments differently
Neither local nor remote deployment is automatically safer. Compare the actual exposure, identity model, permissions, isolation, integrity, and safeguards rather than choosing by transport alone.
| Review area | Questions to answer |
|---|---|
| Exposure | For local stdio, which user account launches the process, and what can it reach on that machine? For a remote HTTP service, which networks and machines can connect, and how is the endpoint protected? |
| Identity | Does the server act with each user’s delegated identity or a shared service identity? Can it preserve user-specific consent and authorization context? |
| Permission scope | Are tools read-only or write-capable? Are permissions scoped per operation, or does the server hold a broad credential? |
| Isolation | Can a process access host files, other processes, or another MCP server? Are filesystem and network boundaries enforced? |
| Integrity | Who reviewed the package and startup configuration? Are versions pinned, updates controlled, and tool metadata changes reviewed? |
| Execution safeguards | Are inputs and outputs validated? Are sensitive calls approved, rate-limited, and logged for audit? |
| Protocol compatibility | Which MCP and OAuth discovery behaviors do the client, server, and authorization server actually support? |
Extra checks for local servers
A local server executes on a user’s machine and may have access to local data, processes, or credentials. Scrutinize startup configuration and the source of its executable or package. Run it with the least host privilege possible; sandbox it or isolate it where feasible; restrict filesystem and outbound network access; and keep secrets out of broadly readable files and logs. Review whether other local processes or remote content could influence its startup or configuration. The MCP security guidance also notes exposure risks involving insecure local servers reachable through DNS rebinding.
Extra checks for remote servers
For a remote HTTP service, limit network reachability, use transport security, and authenticate and authorize every request at the server. Check that upstream calls use credentials intended for the upstream service rather than forwarding the caller’s MCP token. If a gateway or proxy is involved, verify that it preserves the user’s identity and consent context instead of turning a shared credential into blanket authority.
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Tool schemas describe expected arguments, but a schema alone does not establish that a request is safe or authorized. The server should validate argument types, allowed values, size or range bounds, and any destination or object identifier before acting. After an operation, validate the result before returning it to the model or user; avoid disclosing fields the caller is not authorized to see.
Best Value
For sending messages, changing records, deleting data, executing code, or other high-impact actions, combine server-side policy checks with a meaningful human confirmation. The approval should make clear what will happen and to which target. Model-generated confidence or a natural-language claim that the user approved is not a substitute for an authorization decision enforced by the application.
Monitor, audit, and test failure paths
Keep audit records that let an operator investigate the principal, tool, operation, authorization outcome, and relevant request context. Redact access tokens, credentials, and sensitive payloads; logging everything can turn logs into another data-exposure path. Apply rate limits where appropriate, alert on unusual denials or call patterns, and make sure access can be revoked without relying on a model instruction.
Before release, test both allowed and denied cases: a caller without permission, a token intended for another audience, malformed or out-of-range arguments, an altered tool definition, a failed upstream call, and a sensitive action with approval absent. For local deployment, include attempts to access files or network destinations outside the intended boundary. Confirm that failures do not return protected data or leave a partial high-impact action behind.
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Troubleshoot common MCP security failures
- A valid token is rejected: check issuer, audience, expiry, signature, and server-side authorization for the requested operation. Confirm that the token was issued for this server, not merely that it is valid elsewhere.
- An OAuth redirect fails: compare the registered redirect URI exactly with the one used, then check PKCE,
state, and whether all parties support the configured authorization flow. - An upstream API rejects a call: do not forward the MCP client’s token as a workaround. Configure an appropriate upstream credential and map the verified caller’s permissions to the operation.
- A tool appears or behaves differently after an update: compare its package version, provenance, schema, description, and startup configuration with the reviewed version. Pause use until the change is understood.
- A model calls an unexpected tool: inspect the retrieved content and tool metadata that influenced the choice, but do not treat prompt wording as the enforcement fix. Restrict the capability and apply a server-side authorization or approval check.
- A local process reaches an unexpected resource: review its launch account and configuration, then tighten process, filesystem, and network isolation.
Where ScreenshotNeo fits—and what its presence does not establish
ScreenshotNeo is a website screenshot API and MCP server from Yorker Media. Its MCP tools are take_screenshot, get_page_info, and capture_pdf. If your AI workflow needs website captures, it is an option to consider: ScreenshotNeo. Its being an MCP server does not, by itself, establish how it meets the security controls in this article; evaluate the deployment, identity, permissions, and data handling relevant to your use before connecting any server.
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For a direct screenshot request, one GET call returns an image or PDF. The following cURL example saves a WebP screenshot of https://stripe.com; see the ScreenshotNeo API documentation for request options.
curl -G "https://api.screenshotneo.com/v1/shot" -d access_key=YOUR_API_KEY --data-urlencode url=https://stripe.com -o shot.webpScreenshotNeo can accept cookie or consent banners and remove more than 60 known consent platforms, newsletter popups, and chat widgets before capture; those steps can be turned off. Bot checks, blank pages, timeouts, failed loads, and cache hits are not billed, and responses report the page verdict and billing status in headers. Its MCP server gives AI agents screenshot, page-info, and PDF tools. The free plan includes 1,000 shots per month without a card; paid plans start at $5 for 3,000 shots. These are ScreenshotNeo product details, not a security certification or a substitute for the controls above. Sign up for 1,000 free screenshots a month, with no card.
Frequently Asked Questions
Is there a single security score that can tell me whether an MCP server is safe?
No universal score is established by the guidance cited here. Assess the actual package, identity model, permissions, isolation, and execution controls in the deployment you plan to use.
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