Authentication#

Nansen account API calls accept a selected nansen:api browser session or conventional API key with the same permissions. NANSEN_API_KEY takes precedence; optional primaryEnv preserves configured-key injection. Run nansen auth status for offline selection. Cached access expiry alone permits automatic renewal during an authorized task. Stop on anonymous selection, invalid state, blocked/uncertain renewal or actual auth failure; never drop a credential or fall back to anonymous x402 payment. Browser login does not grant wallet signing, privileged service identity or a persistent MCP integration key. Preserve all confirmation, signing, sanctions and geographic checks below. Browser login is available in CLI 2.0.0 within the macOS arm64 preview scope.

Alert Webhook Listener#

Set up a local HTTP server to receive Nansen smart alert webhook payloads in real-time.

How It Works#

Nansen smart alerts support a webhook channel type. When an alert fires, Nansen sends an HTTP POST with a JSON payload to your webhook URL. This skill sets up:

1. A local HTTP server (Node.js, zero external dependencies) that receives and displays alert payloads
2. HMAC-SHA256 signature verification so only authentic Nansen payloads are accepted
3. A public tunnel so Nansen's servers can reach your local machine

This skill does NOT create or modify alerts. It sets up the listener infrastructure and then provides a summary of what the user needs to do to start receiving alerts.

OpenClaw users: If OpenClaw is running locally on the same machine, the webhook server can forward verified alert payloads to OpenClaw's Gateway ( /hooks/agent), triggering an agent turn for each alert. Set the OPENCLAW_GATEWAY_URL env var to enable this. See the OpenClaw Integration section below.

Security Warning#

Before proceeding, inform the user:

This skill starts an HTTP server on your machine and exposes it to the internet via a tunnel (ngrok or localtunnel). While the server only binds to localhost ( 127.0.0.1) — meaning no one on your local network can access it directly — the tunnel creates a public URL that anyone on the internet can send requests to.

Mitigations in place:

• HMAC-SHA256 signature verification rejects all requests not signed by Nansen
• 1 MB body size limit prevents memory abuse
• Only POST /webhook and GET /health are accepted; everything else returns 404

You should be aware that:

• The tunnel URL is publicly discoverable (ngrok URLs can be enumerated)
• Unsigned requests still reach your machine — they're rejected, but the connection is made
• Stop the tunnel when you're done to close the public endpoint

Wait for the user to confirm they want to proceed before continuing.

Execution Plan#

Follow these steps in order. Do not skip signature verification — it is mandatory.

Step 0: Choose a tunnel provider#

Before starting, ask the user which tunnel provider they want to use:

ngrok (recommended)localtunnelStabilityStable — persistent connections with keepaliveFlaky — free relay drops idle connections without warning, tunnels die randomlyInstallbrew install ngrok + free account at ngrok.comZero install ( npx localtunnel)HTTPSYesYesAuth requiredYes (free authtoken from ngrok.com)No

Recommend ngrok. localtunnel is convenient but unreliable — in testing, tunnels silently exit after minutes, causing alerts to fail with "503 Tunnel Unavailable". ngrok maintains stable connections.

Check if ngrok is available:

bash

which ngrok && ngrok version

If not installed, tell the user:

1. brew install ngrok (or download from ngrok.com)
2. Create a free account at ngrok.com and copy the authtoken
3. ngrok config add-authtoken <token>

If the user prefers localtunnel or can't install ngrok, proceed with localtunnel but warn them that the tunnel may drop and they'll need to restart it and update their alert's webhook URL.

Step 1: Generate a webhook secret#

bash

node -e "console.log(require('crypto').randomBytes(32).toString('hex'))"

Store the output — you need it for both the server and the alert configuration. Never log or echo the secret after this point.

Step 2: Write the webhook receiver script#

Create nansen-webhook-server.mjs in the current working directory. Use only Node.js built-in modules ( node:http, node:crypto). No npm install required.

Requirements — do not deviate:

RequirementDetailBind address127.0.0.1 only — never 0.0.0.0Default port9477 (override via PORT env var)Webhook pathPOST /webhook — reject all other method/path combos with 404Health checkGET /health → 200 {"status":"ok"}Signature verificationVerify x-nansen-signature header using HMAC-SHA256 with timing-safe comparison. Reject 401 on mismatch.Secret validationExit on startup if WEBHOOK_SECRET env var is missing or < 16 charsPayload loggingPretty-print valid JSON payloads to stdout with ISO timestampRequest size limitReject bodies > 1 MB (413) to prevent memory abuseGraceful shutdownHandle SIGINT and SIGTERM — close server, then exitOpenClaw forwardingIf OPENCLAW_GATEWAY_URL env var is set, forward verified payloads to <url>/hooks/agent via POST. Include OPENCLAW_AUTH_TOKEN as Bearer token if set. Log forward success/failure.No dependenciesOnly node:http, node:https, and node:crypto — nothing from npm

Signature verification — use timing-safe comparison:

javascript

import { createHmac, timingSafeEqual } from 'node:crypto';

function verifySignature(rawBody, signatureHeader, secret) {
  if (!signatureHeader || !secret) return false;
  // Nansen sends "sha256=<hex>" — strip the prefix before comparing
  const sig = signatureHeader.startsWith('sha256=') ? signatureHeader.slice(7) : signatureHeader;
  const expected = createHmac('sha256', secret).update(rawBody).digest('hex');
  try {
    return timingSafeEqual(Buffer.from(sig, 'utf8'), Buffer.from(expected, 'utf8'));
  } catch {
    return false; // length mismatch
  }
}

Full server template:

javascript

import { createServer } from 'node:http';
import { createHmac, timingSafeEqual } from 'node:crypto';

const PORT = parseInt(process.env.PORT || '9477', 10);
const SECRET = process.env.WEBHOOK_SECRET;
const MAX_BODY = 1_048_576; // 1 MB

// Optional: forward verified payloads to a local OpenClaw Gateway
const OPENCLAW_URL = process.env.OPENCLAW_GATEWAY_URL; // e.g. http://localhost:3000
const OPENCLAW_TOKEN = process.env.OPENCLAW_AUTH_TOKEN;

if (!SECRET || SECRET.length < 16) {
  console.error('WEBHOOK_SECRET env var required (minimum 16 characters).');
  console.error('Generate one: node -e "console.log(require(\'crypto\').randomBytes(32).toString(\'hex\'))"');
  process.exit(1);
}

function verifySignature(rawBody, signatureHeader) {
  if (!signatureHeader) return false;
  // Nansen sends "sha256=<hex>" — strip the prefix before comparing
  const sig = signatureHeader.startsWith('sha256=') ? signatureHeader.slice(7) : signatureHeader;
  const expected = createHmac('sha256', SECRET).update(rawBody).digest('hex');
  try {
    return timingSafeEqual(Buffer.from(sig, 'utf8'), Buffer.from(expected, 'utf8'));
  } catch {
    return false;
  }
}

async function forwardToOpenClaw(payload) {
  if (!OPENCLAW_URL) return;
  const url = `${OPENCLAW_URL.replace(/\/+$/, '')}/hooks/agent`;
  const headers = { 'Content-Type': 'application/json' };
  if (OPENCLAW_TOKEN) headers['Authorization'] = `Bearer ${OPENCLAW_TOKEN}`;
  try {
    const res = await fetch(url, {
      method: 'POST',
      headers,
      body: JSON.stringify(payload),
    });
    if (res.ok) {
      console.log(`[${ts()}] Forwarded to OpenClaw (${res.status})`);
    } else {
      console.error(`[${ts()}] OpenClaw forward failed (${res.status})`);
    }
  } catch (err) {
    console.error(`[${ts()}] OpenClaw forward error: ${err.message}`);
  }
}

function ts() { return new Date().toISOString(); }

const server = createServer((req, res) => {
  if (req.method === 'GET' && req.url === '/health') {
    res.writeHead(200, { 'Content-Type': 'application/json' });
    return res.end('{"status":"ok"}');
  }

  if (req.method !== 'POST' || req.url !== '/webhook') {
    res.writeHead(404);
    return res.end();
  }

  let size = 0;
  const chunks = [];

  req.on('data', (chunk) => {
    size += chunk.length;
    if (size > MAX_BODY) {
      res.writeHead(413);
      res.end('{"error":"Payload too large"}');
      req.destroy();
      return;
    }
    chunks.push(chunk);
  });

  req.on('end', () => {
    if (res.writableEnded) return;

    const rawBody = Buffer.concat(chunks).toString('utf8');
    const signature = req.headers['x-nansen-signature'];

    if (!verifySignature(rawBody, signature)) {
      console.error(`[${ts()}] REJECTED — invalid signature`);
      res.writeHead(401, { 'Content-Type': 'application/json' });
      return res.end('{"error":"Invalid signature"}');
    }

    let payload;
    try {
      payload = JSON.parse(rawBody);
      console.log(`\n[${ts()}] Alert received:`);
      console.log(JSON.stringify(payload, null, 2));
    } catch {
      console.error(`[${ts()}] WARNING — valid signature but malformed JSON`);
    }

    // Forward to OpenClaw if configured (fire-and-forget — don't block response)
    if (payload) forwardToOpenClaw(payload);

    res.writeHead(200, { 'Content-Type': 'application/json' });
    res.end('{"received":true}');
  });
});

for (const sig of ['SIGINT', 'SIGTERM']) {
  process.on(sig, () => {
    console.log(`\n${sig} — shutting down`);
    server.close(() => process.exit(0));
  });
}

server.listen(PORT, '127.0.0.1', () => {
  console.log(`Webhook listener ready — http://127.0.0.1:${PORT}/webhook`);
  if (OPENCLAW_URL) console.log(`OpenClaw forwarding → ${OPENCLAW_URL}/hooks/agent`);
  console.log('Waiting for alerts… (Ctrl+C to stop)\n');
});

Step 3: Start the server and tunnel#

Start the server:

bash

WEBHOOK_SECRET='<secret>' node nansen-webhook-server.mjs

Then start a public tunnel so Nansen's servers can reach it.

ngrok (recommended):

bash

ngrok http 9477

Get the public URL from ngrok's output or its local API:

bash

curl -s http://127.0.0.1:4040/api/tunnels | node -e "process.stdin.on('data',d=>console.log(JSON.parse(d).tunnels[0]?.public_url))"

The webhook URL is https://<subdomain>.ngrok-free.dev/webhook.

localtunnel (fallback — unreliable):

bash

npx localtunnel --port 9477

Prints a URL like https://xxx.loca.lt. The webhook URL is https://xxx.loca.lt/webhook.

Warning: localtunnel's free relay silently drops connections after minutes. When this happens, all alerts fail with "503 Tunnel Unavailable" until you restart the tunnel and update the alert webhook URL. Use ngrok unless you have a reason not to.

Note: Tunnel URLs are ephemeral — they change every restart. For permanent setups, deploy the server to a host with a static URL.

Step 4: Provide a next-steps summary#

Do NOT create or modify any alerts. Instead, print a clear summary for the user explaining what was set up and what they need to do next.

The summary MUST include:

1. Confirmation of what was created (the server script path and the generated secret)
2. The commands to start the server and tunnel (with the actual secret filled in)
3. The exact nansen alerts create or nansen alerts update command they should run, with the --webhook and --webhook-secret flags filled in with the tunnel URL and secret — but leave the alert-specific flags ( --name, --type, --chains, etc.) as placeholders for the user to fill in
4. A reminder that the server and tunnel must be running before the alert is created (Nansen validates the webhook endpoint on creation)
5. A note that tunnel URLs are ephemeral and will change on restart

Example summary format:

text

## Webhook listener ready

**Server script:** ./nansen-webhook-server.mjs
**Port:** 9477

### To start receiving alerts:

1. Start the server (keep this terminal open):
   WEBHOOK_SECRET='<actual-secret>' node nansen-webhook-server.mjs

2. In a new terminal, start the tunnel:
   ngrok http 9477          # recommended
   # or: npx localtunnel --port 9477  (unreliable — tunnel drops silently)

3. Create an alert pointing to your webhook (fill in your alert details):
   nansen alerts create \
     --name '<your alert name>' \
     --type <sm-token-flows|common-token-transfer|smart-contract-call> \
     --chains <chains> \
     --webhook 'https://<your-tunnel-url>/webhook' \
     --webhook-secret '<actual-secret>' \
     [type-specific flags...]

   Or add the webhook to an existing alert:
   nansen alerts update <alert-id> \
     --webhook 'https://<your-tunnel-url>/webhook' \
     --webhook-secret '<actual-secret>'

Note: The tunnel URL changes each time you restart. Update the alert
webhook URL if you restart the tunnel.

See `nansen alerts create --help` for full flag reference per alert type.

Security Checklist#

• Always use a webhook secret — the server refuses to start without one
• Always verify signatures — never accept unverified payloads
• Bind to localhost only — the tunnel handles public exposure; direct 0.0.0.0 binding exposes you to unauthenticated traffic
• Use HTTPS — both localtunnel and ngrok tunnel via HTTPS by default
• Body size limit — the 1 MB cap prevents memory exhaustion from oversized requests
• Timing-safe comparison — prevents timing side-channel attacks on the signature

Troubleshooting#

SymptomFix"Invalid signature" on every requestEnsure the exact same secret is in WEBHOOK_SECRET and --webhook-secret"Failed to send welcome message" on alert createStart the server and tunnel before creating the alertNo alerts arrivingCheck nansen alerts list --table — is the alert enabled? Is the webhook URL correct (includes /webhook)?Tunnel URL expired / tunnel diedRestart the tunnel, get the new URL, then nansen alerts update <id> --webhook '<new-url>/webhook'. If this keeps happening, switch from localtunnel to ngrok.Port already in useSet a different port: PORT=9478 WEBHOOK_SECRET='...' node nansen-webhook-server.mjs and update the tunnel accordingly

OpenClaw Integration#

If the user is running OpenClaw locally on the same machine, the webhook server can forward verified alert payloads to OpenClaw's Gateway, triggering an agent turn for each alert.

Flow: Nansen → ngrok → webhook server (signature check) → OpenClaw /hooks/agent

Additional env vars#

VarRequiredPurposeOPENCLAW_GATEWAY_URLYesOpenClaw Gateway base URL (e.g. http://localhost:3000)OPENCLAW_AUTH_TOKENIf auth enabledBearer token for OpenClaw webhook endpoints

Start command (with OpenClaw forwarding)#

bash

WEBHOOK_SECRET='<secret>' \
OPENCLAW_GATEWAY_URL='http://localhost:3000' \
OPENCLAW_AUTH_TOKEN='<token>' \
node nansen-webhook-server.mjs

The server logs both the alert payload and the OpenClaw forward status. If OpenClaw is unreachable, the forward fails silently (the alert is still logged to stdout).

Ask the user#

Before enabling OpenClaw forwarding, ask:

1. Is OpenClaw running locally? What port?
2. Does their Gateway require auth? If so, what's the Bearer token?

If they don't know or aren't running OpenClaw, skip — the server works fine standalone.

Notes#

• The server uses zero npm dependencies — only Node.js built-ins
• One server can receive alerts from multiple Nansen alerts (as long as they share the same webhook secret)
• For production use, deploy to a cloud host with a static URL and run behind a reverse proxy with TLS
• The x-nansen-signature header format is sha256=<HMAC-SHA256(secret, rawBody)> — strip the sha256= prefix before comparing