mcp-implementation-security-review

MCP Implementation Security Review

Process

Step 1 — Classify the target

• Check MCP protocol version 2025-03-26 or later (current: 2025-11-25). Flag older versions as a finding but continue the review.
• Determine whether the target is a server or client.
• Classify transport as network-exposed or local-only using the transport reference below.
• Record transport, protocol version, and whether sessions exist.

Completion criterion: Target type, protocol status, and transport are identified.

Step 2 — Filter false positives

• Apply the False Positive Filters before opening findings.
• Keep docs only when they describe the repo's own server behavior, deployment, transport, or auth posture.
• For framework/SDK repositories, scope findings to the default configuration and public API surface.

Completion criterion: Remaining evidence is in-scope code, repo-owned docs, or public API behavior.

Step 3 — Check baseline controls

• For network-exposed servers, check MCP-01 through MCP-05.
• For local/STDIO servers, do not mark baseline controls PASS/FAIL; give best-practice notes and continue to RCE review.
• For clients, only review token/session handling explicitly visible in client code; do not apply the server baseline unless the user asks for client-side risk review.

Completion criterion: Each applicable control has a supported status.

Step 4 — Check RCE vectors

• Review all 7 RCE vectors.
• Mark each vector SAFE, AT RISK, or N/A.
• Prefer direct evidence over inference; the RCE Vectors table below enumerates the patterns to look for.

Completion criterion: Every relevant tool has an RCE result or explicit N/A.

Step 5 — Check OWASP MCP Top 10

• Evaluate all 10 OWASP risks below.
• If a control from Step 3 already fully covers an OWASP risk, reference that result rather than re-checking.
• For local/STDIO servers, mark network-dependent OWASP risks (MCP07, MCP09) as N/A.
• Mark each risk PASS, FAIL, or NEEDS INVESTIGATION.

Completion criterion: All 10 OWASP risks have outcomes supported by observable evidence or referenced from Step 3.

Step 6 — Report

• Use the Compliance Output Format below.
• Include file/line references in every justification.
• Separate code findings from manual follow-ups.
• If evidence is incomplete, use NEEDS INVESTIGATION and name the missing artifact.

Completion criterion: The report includes controls, RCE, optional OWASP, and actions.

Reference

Decision rules

• Network-exposed server: Apply all 5 controls, then run RCE and requested OWASP checks.
• Local/STDIO server: Give best-practice guidance only for the 5 controls; still run RCE because tool input can execute locally.
• Client: Review received-token handling and refusal to trust server-provided session IDs; do not force server controls unless asked.
• Reverse proxy or container exposure: If traffic can reach the server over a network, treat it as network-exposed even if inner binding is localhost.
• Unclear evidence: Do not guess. Mark NEEDS INVESTIGATION and say what must be verified manually.
• Ambiguous auth coverage: Auth middleware exists but it is unclear whether it covers MCP endpoints → mark NEEDS INVESTIGATION.
• Undeterminable transport: If transport cannot be established from code, flag for manual review and do not assume STDIO — defaulting to STDIO would wrongly skip the server controls.

Transport classification

Network-exposed (enforce all controls):

Local-only (best practices only):

Host binding gotchas:

False Positive Filters

Docs describing the repo's own server behavior, transport, auth posture, or deployment are not false positives.

Controls Reference

MCP-01 — Identity isolation

Scope: Remote MCP servers

Condition

• Authenticate every inbound request with a trusted identity provider and enforce authorization at the server boundary; do not infer auth from session IDs, prior requests, or network location.
• Use a unique server-specific application identity and audience/resource identifier; outbound calls use independently scoped service credentials or on-behalf-of flow where required, never the inbound token.
• Unauthenticated discovery endpoints are allowed only for metadata-only OAuth/MCP bootstrapping: /.well-known/oauth-protected-resource, /.well-known/oauth-authorization-server, /.well-known/openid-configuration.

What to check

• Token validation and authorization middleware run on every MCP route; authorization distinguishes tool invoke, read-only, and admin operations if present.
• Identity config shows a dedicated application/client/resource ID and audience; outbound clients acquire their own tokens and never copy inbound Authorization.
• Discovery endpoints return metadata only and cannot execute tools or expose protected data.

Key pitfall: Shared application identities or forwarded caller tokens break identity isolation and create confused-deputy paths.

MCP-02 — Sessions

Scope: Remote MCP servers that support sessions

Applicability

• No session identifiers issued or used anywhere → mark N/A (per-request auth is still required; see MCP-01).
• Sessions managed by the transport/SDK (e.g., Streamable HTTP Mcp-Session-Id) but generation/binding not visible in source → mark NEEDS INVESTIGATION, not FAIL.
• Session identifiers present in code → score PASS/FAIL against the conditions below.

Condition

• Authenticate and authorize every request; session state never substitutes for token validation.
• Session IDs are opaque correlation/continuity tokens only; they do not grant privileges, encode authorization, or bypass auth.
• Session IDs are CSPRNG-generated, unpredictable, bound to an authenticated context, and never embedded in URLs.

What to check

• Middleware validates tokens per request, not only when a session starts.
• Authorization logic never trusts a session ID alone; loss or reuse of a session ID must not grant access.
• Session creation uses random IDs (GUID v4/CSPRNG acceptable; sequential or time-based IDs are not).

Key pitfall: Treating a session ID as a bearer credential turns a correlation token into authentication.

MCP-03 — Rate limits

Scope: MCP servers and tools

Condition

• Enforce rate limits and abuse protection on tool discovery and tool invocation.
• Enforce limits at the MCP server runtime, not only at a gateway; partition by authenticated identity and by session where sessions exist.
• Apply stricter limits to mutation-capable and high-cost tools; when limits are exceeded, fail closed with HTTP 429 and Retry-After and do not execute the tool.

What to check

• Rate-limit middleware or equivalent is present on discovery and invocation endpoints in server code, not just in ingress or proxy config.
• Limits are keyed by identity and session, with tighter budgets for write/high-cost operations.
• Exceeded requests stop before backend action and return 429 with Retry-After.

Starting thresholds (tune to actual load, downstream limits, and cost):

Key pitfall: Gateway-only throttling or one flat bucket leaves bypasses and under-protects expensive tools.

MCP-04 — Schema validation

Scope: MCP servers exposing tools with structured arguments

Condition

• Validate all tool arguments against explicit schemas before execution.
• Schemas define types, required fields, enums, and bounds, and reject unspecified properties by default (additionalProperties: false or equivalent).
• Validation runs server-side on every invocation; invalid input fails closed with a 400/MCP error and no backend action.

What to check

• Each tool descriptor has a schema covering types, required fields, enums, bounds, and property restrictions.
• Validation occurs at the server boundary on every call, not only in clients, gateways, or downstream services.
• Negative tests reject malformed input, extra properties, and bounds violations.

Key pitfall: Allowing extra properties or client-only validation creates hidden attack surface and scope creep.

MCP-05 — SDK-first

Scope: Remote MCP servers

Condition

• Build remote MCP servers on an official MCP SDK for your server's language:
  • Tier 1 (fully supported): TypeScript (modelcontextprotocol/typescript-sdk), Python (modelcontextprotocol/python-sdk), C#/.NET (modelcontextprotocol/csharp-sdk), Go (modelcontextprotocol/go-sdk)
  • Tier 2/3 (developing): Java (modelcontextprotocol/java-sdk), Kotlin (modelcontextprotocol/kotlin-sdk), Rust (modelcontextprotocol/rust-sdk), Swift (modelcontextprotocol/swift-sdk), PHP (modelcontextprotocol/php-sdk), Ruby (modelcontextprotocol/ruby-sdk)
• If not using an official SDK, mark MCP-05 as NEEDS INVESTIGATION.
• Keep the SDK current and patched, and verify which controls are automatic versus manual.

What to check

• Dependencies reference an official MCP SDK rather than a hand-rolled HTTP/SSE stack.
• If no SDK is used, the repo contains direct evidence for auth/authz, sessions, rate limits, and schema validation.
• Dependency pinning and update hygiene show the SDK is maintained.

Key pitfall: Hand-rolled servers often miss one "small" primitive—per-request auth, throttling, or validation—and the gaps compound.

RCE Vectors

OWASP MCP Top 10

MCP01:2025 — Token Mismanagement & Secret Exposure Test: Search for hardcoded secrets and token logging; verify secrets come from env vars or a secrets manager; verify short-lived/rotated tokens. Pass: No hardcoded secrets, sensitive fields redacted, short-lived/rotated tokens. Fail: Hardcoded secrets, token logging, or long-lived tokens without rotation.

MCP02:2025 — Privilege Escalation via Scope Creep Test: Review scopes/roles; confirm least privilege and per-request authorization; reject wildcard admin scopes unless justified; check for runtime capability expansion. Pass: Least-privilege scopes, per-request authorization, no runtime capability expansion. Fail: Broad scopes, one-time auth only, or self-escalating tools.

MCP03:2025 — Tool Poisoning Test: Check whether tool definitions are static and server-controlled, whether tools can alter metadata, and whether outputs contain LLM-parseable instructions. Pass: Static server-controlled definitions and data-only outputs. Fail: External metadata sources or outputs with embedded instructions.

MCP04:2025 — Supply Chain Attacks & Dependency Tampering Test: Check for lock files, exact pinning, suspicious postinstall scripts, dependency audit results, and trusted registries. Pass: Pinned deps, committed lock file, no known vulnerabilities, no suspicious post-install scripts. Fail: Unpinned deps, no lock file, unpatched CVEs, or untrusted registries.

MCP05:2025 — Command Injection & Execution Test: Search for shell execution APIs and string-built commands; trace whether tool input reaches shell execution; test ; ls, $(whoami), | cat /etc/passwd. Pass: No shell execution from untrusted input, or only parameterized allowlisted execution. Fail: User input reaches shell commands, shell=True with formatted strings, or unsafe concatenation.

MCP06:2025 — Prompt Injection via Contextual Payloads Test: Check whether tool output goes back to the LLM, whether external content is sanitized/truncated/sandboxed, and whether chained tool calls are guarded; test adversarial instruction-bearing output. Pass: Tool outputs are data, untrusted content is sanitized/truncated/sandboxed, and chaining has guardrails. Fail: Raw external content returns to the model and there are no chaining limits.

MCP07:2025 — Insufficient Authentication & Authorization Test: Send requests without auth and with expired/invalid tokens; verify per-tool authorization; confirm auth is enforced in the server, not only at the gateway. Pass: All endpoints require valid auth, per-tool authorization exists, and enforcement happens server-side. Fail: Any unauthenticated access, missing per-tool auth, or gateway-only enforcement.

MCP08:2025 — Lack of Audit and Telemetry Test: Invoke a tool and confirm logs capture caller identity, tool name, and timestamp; trigger an error and confirm useful context; verify centralized logging and alerting. Pass: Tool invocations are logged with identity, logs are centralized, and alerts exist. Fail: Missing logs, no caller identity, local-only logging, or no alerting.

MCP09:2025 — Shadow MCP Servers Test: Verify the server exists in service inventory; inspect for undocumented MCP endpoints or exposed non-standard ports; check dev/staging isolation; verify an owner and review trail. Pass: All servers are inventoried, isolated appropriately, and owned. Fail: Undocumented servers, dev/test exposure into production networks, or no ownership.

MCP10:2025 — Context Injection & Over-Sharing Test: Inspect tool responses for data minimization; check for PII or full objects when only subsets are needed; verify context isolation. Pass: Minimal data is returned, sensitive fields are masked/excluded, and context is isolated. Fail: Full objects are returned unnecessarily, PII is exposed, or context is shared across users.

Compliance Output Format

In every summary table below, the Justification cell must cite specific file/line evidence for the status.

Control summary

Use PASS only when the code clearly satisfies the control. Use FAIL when the violation is observable. Use NEEDS INVESTIGATION when compliance depends on deployment config, identity-provider state, logs, or other evidence not visible in source.

RCE summary

OWASP summary

Manual follow-ups

List every check that could not be fully resolved from source code, specifying what artifact or access is needed to verify it.

Exception process

• Document the gap: Identify the unmet control, the exact deviation, residual risk, and any compensating controls.
• Get explicit approval: Route the exception through security/release approval with an owner and an expiration or review date.
• Track and re-evaluate: Record the approved exception with compliance results and revisit it on expiry or whenever the server, tools, traffic profile, or exposure changes.