Creating a CAPTCHA (Completely Automated Public Turing test to tell Computers and Humans Apart) in Node.js involves generating and validating a challenge-response system to prevent automated bots from abusing web forms or accessing certain resources. Below, I'll outline the steps to create a basic CAPTCHA system using Node.js.

1. Set up a Node.js project:

Create a new folder for your project and initialize a new Node.js project by running the following command in your terminal:

```bash

npm init -y

```

2. Install required packages:

You'll need some packages to create a CAPTCHA. Install them using npm:

```bash

npm install express ejs express-session body-parser svg-captcha

```

3. Create the server file (app.js):

Now, create an app.js file in the root of your project folder.

```javascript

// app.js

const express = require('express');

const session = require('express-session');

const bodyParser = require('body-parser');

const svgCaptcha = require('svg-captcha');

const app = express();

// Middleware setup

app.use(bodyParser.urlencoded({ extended: false }));

app.use(session({

secret: 'your_secret_key_here',

resave: false,

saveUninitialized: true,

}));

// Route to serve the CAPTCHA image

app.get('/captcha', (req, res) => {

const captcha = svgCaptcha.create();

req.session.captcha = captcha.text;

res.type('svg').send(captcha.data);

});

// Route to handle form submission and validate CAPTCHA

app.post('/submit', (req, res) => {

const userCaptcha = req.body.captcha;

const correctCaptcha = req.session.captcha;

if (userCaptcha === correctCaptcha) {

// CAPTCHA is correct, process the form submission

res.send('Form submitted successfully.');

} else {

// CAPTCHA is incorrect, show an error message

res.send('Invalid CAPTCHA. Please try again.');

}

});

const port = 3000;

app.listen(port, () => {

console.log(`Server running on http://localhost:${port}`);

});

```

4. Create an HTML form (index.ejs):

Create an "views" folder in your project root and create an index.ejs file inside it with the following content:

```html

CAPTCHA Example

```

5. Start the server:

Run the following command in your terminal to start the Node.js server:

```bash

node app.js

```

Now, open your browser and navigate to http://localhost:3000. You should see a form with a CAPTCHA image. When the user submits the form, the server will check if the CAPTCHA entered by the user matches the one stored in the session. If they match, the form submission is considered successful; otherwise, an error message will be displayed.

This is a basic implementation of CAPTCHA in Node.js. In a real-world scenario, you might want to enhance the CAPTCHA generation and validation to make it more secure and robust against potential attacks.