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Where tool poisoning and prompt injection actually enter
The attack surface is the host and the combined set of tools in a session. It isn’t one server in isolation. Text that reaches the model can carry hostile instructions through several channels:
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- Tool metadata: names, descriptions and parameter descriptions. This is the classic tool-poisoning route, because the model reads this text to decide how to call the tool.
- Server instructions: a free-text “user manual” a server can supply. Some hosts inject it into model context, and others may not.
- Tool results and resources: web pages, tickets, emails, files and database rows returned by a legitimate tool can contain injected instructions.
- Served skills: skill content delivered by a server through the MCP Skills extension.
A connected server is not automatically authoritative. Treat all of the above as untrusted unless a separate verification and policy mechanism establishes otherwise.
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Server instructions are not a control
MCP maintainer Ola Hungerford wrote in the Model Context Protocol Blog post “Server Instructions: Giving LLMs a user manual for your server” (November 3, 2025): “Don’t rely on instructions for any critical actions that need to happen in conjunction with other actions, especially in security or privacy domains. These are better implemented as deterministic rules or hooks.” Hosts decide whether and how to apply instructions, and no instruction guarantees model behavior.
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Tool annotations are hints
Read-only and destructive hints help a client decide what to display or when to ask for approval. They are static metadata. An untrusted server can set them falsely, and the MCP project says they do not make a model resist injection. Treat them as untrusted input and enforce real constraints through authorization, transport or runtime logic.
The MCP project’s guidance here is maintainer commentary. It is not the normative specification, and host behavior varies, so test the hosts you actually support.
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Step 1: Establish and preserve trust boundaries
- Record which server supplied each tool description, resource and result, and keep that provenance visible to the user and in logs.
- Don’t merge remote content into the same apparent authority as system policy, user messages or trusted local material.
- Namespace tools by server so a malicious server can’t shadow or impersonate a trusted tool name.
- Pin or review tool definitions, and require re-approval when a description or schema changes after the user approved it. This is a practical mitigation for definitions that change after approval; the sources reviewed don’t prescribe a specific mechanism.
Step 2: Analyze the session, not just each tool
The MCP project frames risk as a property of the whole session. Three capabilities are dangerous in combination:
- access to private data,
- exposure to untrusted content,
- the ability to communicate externally.
Each tool may look harmless alone. Together, an injected instruction in untrusted content can steer a tool that reads private data, and another tool can send it out. The MCP project’s discussion describes this risk model and an illustrative research demonstration. It isn’t a measured prevalence figure, and no published figure for how often MCP tool-poisoning or injection attacks succeed was established.
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For each deployment, list the tools that can be active together and ask: can untrusted content influence a tool that reads private data or sends data out? If yes, break the chain by removing a capability, splitting sessions, or putting a human approval or policy check at the dangerous step.
Step 3: Enforce least privilege with hard controls
These are practical recommendations that follow from putting guarantees in deterministic controls. MCP doesn’t require any particular sandbox product.
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- Narrow credentials and scopes: give each server only the access its tools need, and use per-user tokens rather than shared, broad service credentials.
- Isolate execution: run servers that touch files, shells or code in a constrained environment.
- Restrict network egress: allow-list destinations so an injected instruction can’t send data anywhere.
- Require explicit authorization for sensitive or externally consequential operations such as sending messages, deleting data or making payments. Make the approval prompt show the real parameters, not a model-written summary.
- Use host-side rules or hooks for security and privacy actions rather than asking the model to behave.
Step 4: Treat MCP-served skills as higher risk
The stable MCP Skills extension specification says served skill text is untrusted and a higher-risk surface than a remote tool invocation. Its Security Considerations state: “Hosts MUST treat MCP-served skill content as untrusted model input, subject to the same prompt-injection defenses applied to any server-provided text.” They also state: “Hosts MUST NOT allow MCP-served skill content to cause host-side code execution without explicit per-skill user approval.”
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- tag served skill content with the host-assigned identity of its originating server and never present it as indistinguishable from a local skill;
- gate execution behind explicit per-skill approval;
- bind resource reads to the skill’s origin, and allow cross-origin reads only for specific servers the user approves;
- prevent a remote skill from silently widening permissions;
- prevent skill-name collisions from shadowing local or other-origin skills.
Step 5: Authorize remote servers at the HTTP boundary
The official MCP Apps authorization guide describes two patterns. Choose by how sensitive your tools are.
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| Pattern | How it works | Use when |
|---|---|---|
| Per-server authorization | Every request must carry a valid bearer token. | All tools are sensitive. |
| Per-tool authorization | Inspect the incoming JSON-RPC request, identify protected tool calls, and enforce auth only for them. | Public and protected tools deliberately coexist. |
For protected calls:
- Verify the bearer token, including signature and issuer. The guide demonstrates JWT validation against an identity provider’s JWKS endpoint and issuer. Adapt that pattern to your identity provider, token format, framework and SDK.
- If credentials are missing or invalid, return HTTP 401 with a
WWW-Authenticateheader pointing to Protected Resource Metadata. Don’t convert an unauthenticated request into a normal tool-level error, because clients rely on the 401 to start authorization. - Pass the verified user identity into the tool handler so it can scope data access to that user.
Authorization limits who can call a tool and what data it can reach. It doesn’t stop injection by itself, but it caps what an injected instruction can do.
Step 6: Check your protocol and SDK version
The MCP project announced specification version 2026-07-28 on July 28, 2026. According to that announcement:
- The core is stateless, with self-describing requests.
- New
Mcp-MethodandMcp-Nameheaders support routing and metering, which helps gateways apply per-tool policy without parsing bodies. - Authorization is hardened: clients validate the OAuth issuer, and credentials are bound to their issuing authorization server.
- Dynamic Client Registration (DCR) is deprecated in favor of Client ID Metadata Documents (CIMD), though DCR remains compatible for now.
Confirm which version each server, client, gateway and SDK implements, and read its migration notes before relying on release-specific behavior. The announcement’s adoption figures (close to half a billion monthly Tier 1 SDK downloads) show reach, not safety.
Deployment checklist
- All server-supplied text is handled as untrusted model input, with the origin kept visible.
- No security rule depends on a prompt, instruction or annotation.
- Tool combinations per session have been reviewed for the private-data, untrusted-content, external-communication trio.
- Credentials are scoped, per-user and short-lived where possible; egress is restricted.
- Sensitive actions need explicit, parameter-accurate approval enforced outside the model.
- Skills need per-skill execution approval and origin-scoped resource access.
- Protected endpoints verify tokens and return 401 with
WWW-Authenticate. - Protocol and SDK versions are known and their migration notes reviewed.
This guide doesn’t audit any particular host, SDK or identity provider, so validate each item in your own environment.
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