excalidraw-mcp-sentinel/SECURITY.md

Security

Threat Model

This server is designed for local and self-hosted use — it runs on the same machine as your AI agent and browser. The primary threat surface is:

1. A malicious website making cross-origin requests to the canvas API (CSRF / drive-by reads or writes).
2. A compromised or untrusted network exposing the canvas port to other hosts.
3. Malicious input (oversized payloads, prototype pollution, injection) reaching route handlers.

The threat model does not cover:

• An attacker with local OS access (they can read the SQLite file directly).
• Server-side request forgery from within the canvas server itself.

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Mitigations

CORS — corsMiddleware (src/security.ts)

Restricts cross-origin requests to an explicit allowlist (ALLOWED_ORIGINS env var, defaults to localhost:3000 / 127.0.0.1:3000). Requests with no Origin header (MCP stdio, curl, same-origin) are always allowed.

WebSocket origin check — verifyWsClient (src/security.ts)

WebSocket upgrades are verified against the same allowlist before the connection is established. Rejects browser-originated connections from unlisted origins.

WebSocket auth challenge-response

When EXCALIDRAW_API_KEY is set, the server immediately sends { type: "auth_required" } after each new WebSocket connection. The client must respond with a hello message containing { type: "hello", apiKey: "<key>", ... } within 5 seconds. If the key is missing, wrong, or the timeout fires, the server closes the connection with close code 4001. All other message types are silently dropped until auth succeeds. When auth is disabled, the hello handshake proceeds without key validation.

Auth bootstrap — GET / key injection

When EXCALIDRAW_API_KEY is set, GET / injects <script>window.__EXCALIDRAW_API_KEY__=…</script> into the served HTML before </head>. The browser canvas reads this value at startup and includes it in the WebSocket hello message automatically, so users don't need to configure the key in the browser separately. The value is JSON-encoded with < escaped to \u003c to prevent script injection.

API key auth — apiKeyAuth (src/security.ts)

When EXCALIDRAW_API_KEY is set, all /api/* routes require the header X-API-Key: <key>. Disabled by default for backward compatibility and zero-config local use. The /health endpoint is always exempt.

Security headers — helmetMiddleware (src/security.ts)

Sets X-Content-Type-Options: nosniff, X-Frame-Options, X-DNS-Prefetch-Control, and removes X-Powered-By. CSP and COEP are intentionally disabled to allow Excalidraw's React bundle (inline scripts/styles).

Rate limiting — generalRateLimit / destructiveRateLimit / writeBurstLimit (src/security.ts)

Three limiters apply, all returning RateLimit-* headers (draft-7) so clients can self-throttle:

Limiter	Applied to	Default	Override env var
generalRateLimit	All /api/* routes	100 req / 15 min	EXCALIDRAW_RATE_LIMIT_GENERAL_MAX
destructiveRateLimit	DELETE /api/elements/clear	10 req / 1 min	EXCALIDRAW_RATE_LIMIT_DESTRUCTIVE_MAX
writeBurstLimit	POST /api/elements/sync, POST /api/elements/sync/v2	10 req / 1 min	EXCALIDRAW_RATE_LIMIT_WRITE_BURST_MAX

Ceilings are read from env vars at server start. The E2E test harness sets them to high values via playwright.config.ts so tests are not self-throttled.

Confirmation guard — requireConfirm (src/security.ts)

The DELETE /api/elements/clear endpoint requires ?confirm=true. Prevents accidental or CSRF-triggered canvas wipes.

Body size limits (src/server.ts)

• Default body limit: 100 KB (standard API requests).
• Batch/sync endpoints: 5 MB (element arrays and sync payloads).
• Oversized payloads return 413 Payload Too Large.

Prototype pollution guard — sanitizeBody (src/security.ts)

Rejects any request body containing __proto__, constructor, or prototype as object keys. Returns 400 Bad Request before any route handler sees the data.

Search query sanitization — sanitizeSearchQuery (src/security.ts)

GET /api/elements/search?q=… passes the query through sanitizeSearchQuery before handing it to the SQLite FTS5 engine. The function rejects queries that contain FTS5 operators (AND, OR, NOT, NEAR/N), double-quote quoting constructs, or special characters (*, (, ), {, }, ^). This prevents malformed FTS5 syntax from bubbling up as SQLite parse errors and closes a narrow injection surface into the FTS virtual table.

Mermaid input validation — validateMermaidInput (src/security.ts)

• Diagram string: max 50 KB (prevents DoS via large Mermaid parse).
• Config object: max 10 keys (prevents unbounded config expansion).

Error handling (src/server.ts)

The global error handler never exposes stack traces, file paths, or node_modules references in responses. 500 errors return the generic message "Internal server error". Non-500 errors surface the error message only.

Docker host binding (Dockerfile.canvas, docker-compose.yml)

HOST=0.0.0.0 inside Docker is intentional: the container binds all interfaces, but the port is only reachable via the published port mapping. For local non-Docker use, the server defaults to 127.0.0.1 (loopback only).

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Pinned Dependencies

Security-critical packages are pinned to exact versions (no ^ range) to prevent silent upgrades introducing regressions:

Package	Reason
helmet	Security headers — pin to known-good config
express-rate-limit	Rate limiter — header format changes between major versions
cors	CORS policy enforcement
express	HTTP server — patch releases may change middleware behavior
ws	WebSocket server — security patches applied selectively
better-sqlite3	Native module — ABI compatibility with pinned Node.js
zod	Input validation — schema breaking changes between minors
@modelcontextprotocol/sdk	Protocol — pin to tested version

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Reporting Vulnerabilities

Open an issue in the project repository. For sensitive disclosures, contact the maintainer directly via GitHub.

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Known Limitations

• No HTTPS: The canvas server speaks plain HTTP. Use a reverse proxy (nginx, Caddy) with TLS for any non-localhost deployment.
• Single shared API key: There is no per-user or per-tenant auth. The key protects the entire API surface equally.
• Rate limits are in-memory: They reset on process restart and are not shared across multiple server instances.
• SQLite is not encrypted: The database file is stored in plaintext. Apply OS-level encryption if needed.