> ## Documentation Index
> Fetch the complete documentation index at: https://docs.fingerprint.com/llms.txt
> Use this file to discover all available pages before exploring further.

# Account Sharing

> Learn how to detect and prevent unauthorized account sharing

## Overview

This tutorial walks through implementing Fingerprint to prevent account sharing, where multiple people use the same account across different devices.

You'll begin with a starter app that includes a mock login page and a basic login flow. From there, you'll add the JavaScript agent to identify each visitor and use server-side logic with Fingerprint data to detect when an account is accessed from a new device. For simplicity, the sample will block logins from new devices for an account, but in practice, you would likely choose to add friction, log out other devices, or flag for review instead.

By the end, you'll have a sample app that limits an account to a single device and can be customized to fit your use case and business rules.

This tutorial uses just plain JavaScript and a Node server with SQLite on the backend. For language- or framework-specific setups, see the quickstarts.

> Estimated time: \< 15 minutes

<iframe className="w-full aspect-video rounded-md" src="https://www.youtube.com/embed/xOiI-a-hn3k" title="YouTube video player" frameborder="0" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture; web-share" referrerpolicy="strict-origin-when-cross-origin" allowfullscreen />

## Prerequisites

Before you begin, make sure you have the following:

* A copy of the [starter repository](https://github.com/fingerprintjs/use-case-tutorials) (clone with Git or download as a ZIP)
* [Node.js](https://nodejs.org/) (v20 or later) and npm installed
* Your favorite code editor
* Basic knowledge of JavaScript

## 1. Create a Fingerprint account and get your API keys

1. [Sign up](https://dashboard.fingerprint.com/signup) for a free Fingerprint trial, or log in if you already have an account.
2. After signing in, go to the [**API keys**](https://dashboard.fingerprint.com/api-keys) page in the dashboard.
3. Save your **public API key**, which you'll use to initialize the JavaScript agent.
4. Create and securely store a **secret API key** for your server. Never expose it on the client side. You'll use this key on the backend to retrieve full visitor information through the Fingerprint Server API.

## 2. Set up your project

1. Clone or download the [starter repository](https://github.com/fingerprintjs/use-case-tutorials) and open it in your editor:

```bash Terminal theme={"theme":"github-dark-dimmed"}
git clone https://github.com/fingerprintjs/use-case-tutorials.git
```

2. This tutorial will be using the `account-sharing` folder. The project is organized as follows:

<Tree>
  <Tree.Folder name="public" defaultOpen>
    <Tree.File name="index.html - Login page" />

    <Tree.File name="index.js - Front-end logic to handle login" />
  </Tree.Folder>

  <Tree.Folder name="server" defaultOpen>
    <Tree.File name="server.js - Serves static files and login endpoint" />

    <Tree.File name="db.js - SQLite database connection" />

    <Tree.File name="accounts.js - Account sharing logic" />
  </Tree.Folder>

  <Tree.File name=".env.example - Example environment variables" />
</Tree>

3. Install dependencies:

```bash Terminal theme={"theme":"github-dark-dimmed"}
npm install
```

4. Copy or rename `.env.example` to `.env`, then add your Fingerprint API keys:

```bash Terminal theme={"theme":"github-dark-dimmed"}
FP_PUBLIC_API_KEY=your-public-key
FP_SECRET_API_KEY=your-secret-key
```

5. Start the server:

```bash Terminal theme={"theme":"github-dark-dimmed"}
npm run dev
```

6. Visit [http://localhost:3000](http://localhost:3000) to view the mock login page from the starter app. You can test the basic login form using the included test account (`demo@example.com` / `password123`) and clicking **Log in**. Then open the page in a completely different browser and log in with the same account. By default, the same account can be logged into from any device.

## 3. Add Fingerprint to the frontend

In this step, you'll load the JavaScript agent when the page loads and trigger identification when the user clicks **Log in**. The JavaScript agent returns both a `visitorId` and a `requestId`. Instead of relying on the `visitorId` from the browser, you'll send the `requestId` to your server along with the login payload. The server will then call the [Fingerprint Events API](/reference/v3/server-api-get-event) to securely retrieve the full identification details, including the verified `visitorId` and risk signals such as browser tampering or bot activity.

1. At the top of `public/index.js`, load the JavaScript agent:

```javascript public/index.js theme={"theme":"github-dark-dimmed"}
const fpPromise = import(`https://fpjscdn.net/v3/${window.FP_PUBLIC_API_KEY}`).then(
  (FingerprintJS) => FingerprintJS.load({ region: "us" }),
);
```

2. Make sure to change `region` to match your workspace region (e.g., `eu` for Europe, `ap` for Asia, `us` for Global (default)).
3. Near the bottom of `public/index.js`, the **Log in** button already has an event handler for submitting the credentials. Inside this handler, request visitor identification from Fingerprint using the `get()` method and include the returned `requestId` when sending the login request to the server:

```javascript public/index.js theme={"theme":"github-dark-dimmed"}
loginBtn.addEventListener("click", async () => {
  // ...

  const fp = await fpPromise;
  const { requestId } = await fp.get();

  try {
    const res = await fetch("/api/login", {
      method: "POST",
      headers: { "Content-Type": "application/json" },
	    body: JSON.stringify({ email, password, requestId }),
    });
    const data = await res.json();

    // ...
  }
});
```

The `get()` method sends signals collected from the browser to Fingerprint servers, where they are analyzed to identify the visitor. The returned `requestId` acts as a reference to this specific identification event, which your server can later use to fetch the full visitor details.

For lower latency in production, use [Sealed Client Results](/docs/v3/sealed-client-results) to return full identification details as an encrypted payload from the `get()` method.

## 4. Receive and use the request ID to get visitor insights

Next, pass the `requestId` through to your login logic, initialize the [Fingerprint Node Server SDK](/reference/node-server-sdk), and fetch the full visitor identification event so you can access the trusted `visitorId` and [Smart Signals](https://fingerprint.com/products/smart-signals/).

1. In the backend, the `server/server.js` file defines the API routes for the app. Update the `/api/login` route there to also extract `requestId` from the request body and pass it into the `attemptLogin` function:

```javascript server/server.js theme={"theme":"github-dark-dimmed"}
app.post("/api/login", async (req, reply) => {
  const { email, password, requestId } = req.body || {};
  const result = await attemptLogin({ email, password, requestId });
  return reply.send(result);
});
```

2. The `server/accounts.js` file contains the logic for handling logins. Start by importing and initializing the Fingerprint Node Server SDK there, and load your environment variables with `dotenv`:

```javascript server/accounts.js theme={"theme":"github-dark-dimmed"}
import { db } from "./db.js";
import { config } from "dotenv";
import { FingerprintJsServerApiClient, Region } from "@fingerprintjs/fingerprintjs-pro-server-api";

config();

const fpServerApiClient = new FingerprintJsServerApiClient({
  apiKey: process.env.FP_SECRET_API_KEY,
  region: Region.Global,
});
```

3. Make sure to change `region` to match your workspace region (e.g., `EU` for Europe, `AP` for Asia, `Global` for Global (default)).
4. Update the `attemptLogin` function to accept `requestId` and use it to fetch the full identification event details from Fingerprint:

```javascript server/accounts.js theme={"theme":"github-dark-dimmed"}
export async function attemptLogin({ email, password, requestId }) {
  if (!email || !password) {
    console.error("Missing credentials.");
    return { success: false, error: "Login failed." };
  }

  if (!requestId) {
    console.error("Missing requestId.");
    return { success: false, error: "Login failed." };
  }

  const user = findAccountByEmail(email);
  if (!user || user.password !== password) {
    console.error("Invalid credentials");
    return { success: false, error: "Login failed." };
  }

  const event = await fpServerApiClient.getEvent(requestId);

  // ...
}
```

Using the `requestId`, the getEvent will retrieve the full data for the visitor identification request. The returned object will contain the visitor ID, IP address, device, and browser details, and Smart Signals like bot detection, browser tampering detection, VPN detection, and more.

You can see a full example of the event structure and test it with your own device in the [demo playground](https://demo.fingerprint.com/playground).

For additional checks to ensure the validity of the data coming from your frontend, view [how to protect from client-side tampering and replay attacks](/docs/v3/protecting-from-client-side-tampering).

## 5. Block bots and suspicious devices

<Note>
  This optional step uses the Bot Detection and Suspect Score Smart Signals, which are only
  available on paid plans.
</Note>

A simple but powerful way to prevent automated abuse is to block bot logins. The `event` object includes the [Bot Detection Smart Signal](https://fingerprint.com/products/bot-detection/) that flags automated activity, making it easy to reject bot traffic.

This signal returns `good` for known bots like search engines, `bad` for automation tools, headless browsers, or other signs of automation, and `notDetected` when no bot activity is found.

1. Continuing in the `attemptLogin` function in `server/accounts.js`, check the bot signal returned in the `event` object:

```javascript server/accounts.js theme={"theme":"github-dark-dimmed"}
export async function attemptLogin({ email, password, requestId }) {
  // ...

  const event = await fpServerApiClient.getEvent(requestId);

  const botDetected = event.products?.botd?.data?.bot?.result !== "notDetected";

  if (botDetected) {
    console.error("Bot detected.");
    return { success: false, error: "Login failed." };
  }

  // ...
}
```

You can also use Fingerprint's [Suspect Score](/docs/v3/suspect-score) to flag high-risk logins. The Suspect Score is a weighted representation of all Smart Signals present in the identification payload, helping to identify suspicious activity. While it's not typical to block actions based solely on a high risk score, this example shows how you might incorporate it. In a real application, a better approach would be to flag the attempt for review or add some additional step-up authentication.

2. Below the bot detection check, add a condition that reads the Suspect Score from the `event` object and blocks the login if it exceeds a chosen threshold (for example, 20):

```javascript server/accounts.js theme={"theme":"github-dark-dimmed"}
export async function attemptLogin({ email, password, requestId }) {
  // ...

  const suspectScore = event.products?.suspectScore?.data?.result || 0;

  if (suspectScore > 20) {
    console.error(`High Suspect Score detected: ${suspectScore}`);
    return { success: false, error: "Login failed." };
  }

  // ...
}
```

## 6. Prevent account sharing

Next, use the trusted `visitorId` from the `event` object to enforce a one-device-per-account rule. If the same account logs in from a different device (`visitorId`), reject the login. In production you may choose to allow a certain number of devices, add step-up verification, or alert the user. *(This example oversimplifies login logic for demonstration purposes.)*

Note: The starter app includes a SQLite database with these tables already created for you:

```text SQLite database tables theme={"theme":"github-dark-dimmed"}
accounts - Stores login credentials for test accounts
  email TEXT PRIMARY KEY
  password TEXT NOT NULL

account_devices - Logs which device (visitorId) is associated with each account
  email TEXT PRIMARY KEY
  visitorId TEXT NOT NULL
  createdAt INTEGER NOT NULL
```

1. Add some helper functions to the bottom of the `server/accounts.js` file to read and write device details for an account:

```javascript server/accounts.js theme={"theme":"github-dark-dimmed"}
// Get the last seen device for the user
function getLastDeviceFor(email) {
  return db.prepare(`SELECT visitorId FROM account_devices WHERE email = ?`).get(email);
}

// Record the visitor's device on logging in
function saveDeviceFor(email, visitorId) {
  db.prepare(
    `INSERT OR IGNORE INTO account_devices (email, visitorId, createdAt)
     VALUES (?, ?, ?)`,
  ).run(email, visitorId, Date.now());
}
```

2. Update `attemptLogin` to retrieve the `visitorId` and use it to enforce the one device per account rule and record successful logins:

```javascript server/accounts.js theme={"theme":"github-dark-dimmed"}
export async function attemptLogin({ email, password, requestId }) {
  // ...

  if (botDetected) {
    console.error("Bot detected.");
    return { success: false, error: "Login failed." };
  }

  const visitorId = event.products.identification.data.visitorId;

  const last = getLastDeviceFor(email);
  if (last && last.visitorId !== visitorId) {
    console.error("Access from new device detected.");
    return {
      success: false,
      error:
        "You can only access this account from one device. (Reset the demo to simulate logging out on the other device.)",
    };
  }
  if (!last) saveDeviceFor(email, visitorId);

  return { success: true };
}
```

This gives you a basic system to detect and block account sharing. You can extend it by allowing a small number of devices per account, expiring devices after some time, adding step-up verification on change, notifying the account owner, etc.

<Info>
  This is a minimal example to show how to implement Fingerprint. In a real application, make sure
  to implement proper security practices, error checking, and password handling that align with your
  production standards.
</Info>

## 7. Test your implementation

Now that everything is wired up, you can test the full protected login flow.

1. Start your server if it isn't already running and open [http://localhost:3000](http://localhost:3000):

```bash Terminal theme={"theme":"github-dark-dimmed"}
npm run dev
```

2. Log in (`demo@example.com` / `password123`) in one browser. You should see a success response.
3. In a different browser or device, try logging in with the same account. The second attempt will be rejected because the account is tied to a different device.
4. Bonus: Test the flow using a headless browser or automation tool to see bot detection in action. A sample script is available in `test-bot.js`. While your app is running, run the script with `node test-bot.js` in your terminal and observe that the automated logins are blocked.

## Next steps

You now have a working login flow secured with Fingerprint. From here, you can expand the logic with more [Smart Signals](/docs/v3/smart-signals-reference), fine-tune rules based on your business policies, or layer in additional checks for suspicious visitors.

To dive deeper, explore the other use case tutorials for more step-by-step examples.

Check out these related resources:

* [Node SDK Reference](https://github.com/fingerprintjs/fingerprintjs-pro-server-api-node-sdk)
* [Vue frontend quickstart](/docs/v3/vue-quickstart)
* [React frontend quickstart](/docs/v3/react-quickstart)
* [API reference for the Events endpoint](/reference/v3/server-api-get-event)
* [Use case tutorial: Detecting new account fraud](/docs/v3/new-account-fraud-use-case-tutorial)
* [Low-latency identification with Sealed Client Results](/docs/v3/sealed-client-results)