Secure Invitation Links: Client-Side Generation

G5 Cyber Security

1 week ago

TL;DR

No, you can’t create truly secure invitation links client-side only. While you can generate a random token on the client and use it in a URL, this isn’t secure enough for sensitive applications. The server *must* be involved to verify uniqueness, prevent guessing, and associate the link with a user or resource.

Why Client-Side Alone Isn’t Secure

Generating a random string (like a UUID) in JavaScript is easy, but it doesn’t guarantee security. Here’s why:

Predictability: A predictable random number generator can be exploited to guess valid tokens.
No Uniqueness Guarantee: The client has no way of knowing if the generated token already exists in the system.
Manipulation: Users could potentially modify the link and create invalid or unwanted invitations.

How to Create Secure Invitation Links (Server-Side Required)

Here’s a step-by-step guide on how to generate secure invitation links, emphasizing the server’s role:

Generate a Unique Token Server-Side: Use a cryptographically secure random number generator on your server.

Example (Python with Flask):

from flask import Flask, jsonify
import secrets

app = Flask(__name__)

@app.route('/generate-token')
def generate_token():
  token = secrets.token_urlsafe(32) # Generates a 32-character random URL-safe token
  # Store the token in your database associated with the user/resource.
  return jsonify({'token': token})

Store the Token: Save the generated token in your database, linked to the relevant user or resource. Include an expiry date for security.

Create the Invitation Link: Construct the invitation link using the token.

Example: https://yourdomain.com/invite?token=generated_token

Verify the Token on Access (Server-Side): When a user clicks the link, your server *must* verify the following:

Token Existence: Check if the token exists in the database.
Token Validity: Ensure the token hasn’t expired.
Token Usage: Prevent reuse of the same token (mark it as used).

Redirection/Activation: If the token is valid, redirect the user to the appropriate activation page or grant access.

Example (Node.js with Express):

const express = require('express');
const app = express();

app.get('/invite', async (req, res) => {
  const token = req.query.token;
  // Check database for token...
  if (isValidToken(token)) {
    res.redirect('/activation?token=' + token);
  } else {
    res.status(400).send('Invalid or expired invitation link.');
  }
});

Client-Side Role (Limited)

The client-side can be used for:

Displaying the Link: Show the generated link to the user.
Copying the Link: Allow the user to copy the link to share it.

Important Considerations

HTTPS: Always use HTTPS to protect the invitation link during transmission.
Token Length: Use sufficiently long tokens (at least 32 characters) for adequate security.
Expiry Dates: Set reasonable expiry dates on tokens to limit their usefulness if compromised.
Rate Limiting: Implement rate limiting to prevent brute-force attacks.