Implement preliminary pentest remediations

backend/src/controllers/v1/signupController.ts

@@ -8,6 +8,7 @@ import {
 import { createToken } from '../../helpers/auth';
 import { BadRequestError } from '../../utils/errors';
 import { getInviteOnlySignup, getJwtSignupLifetime, getJwtSignupSecret, getSmtpConfigured } from '../../config';
+import { validateUserEmail } from '../../validation';
 
 /**
  * Signup step 1: Initialize account for user under email [email] and send a verification code
@@ -20,6 +21,9 @@ export const beginEmailSignup = async (req: Request, res: Response) => {
 	let email: string;
 	try {
 		email = req.body.email;
+		
+		// validate that email is not disposable
+		validateUserEmail(email);
 
 		const user = await User.findOne({ email }).select('+publicKey');
 		if (user && user?.publicKey) {