Implement server-side validation for the relevant fields.

Server-side validation is the process of checking data on the server to ensure correctness and security before saving or processing. For example, if you have a form with email and password fields, the server should verify that the email is valid and the password meets the requirements (length, characters).

An example in Node.js using Express and Joi for validation:

const express = require('express');
const Joi = require('joi');
const app = express();
app.use(express.json());

const schema = Joi.object({
  email: Joi.string().email().required(),
  password: Joi.string().min(8).required()
});

app.post('/register', (req, res) => {
  const { error } = schema.validate(req.body);
  if (error) {
    return res.status(400).json({ message: error.details[0].message });
  }
  // Continue processing if validation is successful
  res.send('User registered');
});

app.listen(3000);

Thus, the server verifies data independently of the client, preventing errors and attacks.