UserfrontSession.ts

// FILE #1: app/utils/session.ts
import jwt from 'jsonwebtoken';
import { redirect } from 'remix';

export function requireValidSession(request: Request) {  

  // assuming key is stored as base64
  // if not, no need for this line, just use env var directly
  const publicKey = Buffer.from(process.env.JWT_PUBLIC_KEY || '', 'base64').toString('ascii');
  const tokenName = process.env.USERFRONT_TOKEN_NAME || '';

  if (!tokenName) { 
    throw redirect('/login', 401); 
  }

  const cookieObj = parseCookie(request.headers.get("Cookie"));
  const accessToken = cookieObj.hasOwnProperty(tokenName) && cookieObj[tokenName] || null;

  if (!accessToken || !publicKey) {
    throw redirect('/login', 401);
  }

  try {
    const verifiedPayload = jwt.verify(accessToken, publicKey, { algorithms: ['RS256'] });
    return verifiedPayload;
  } catch(err) {
    throw redirect('/login', 401);
  }
  
}

// https://www.30secondsofcode.org/js/s/parse-cookie
function parseCookie(str: string | null) {
  if (!str) return {};

  return str.split(';').map(v => v.split('='))
    .reduce((acc: any, v) => {
      acc[decodeURIComponent(v[0].trim())] = decodeURIComponent(v[1].trim());
      return acc;
    }, {});
}

// FILE #2: Protected Route
import { useLoaderData } from 'remix';
import { requireValidSession } from "../../utils/session";

export async function loader({ request }: { request: Request }) {
    export async function loader({ request }: { request: Request }) {
    return requireValidSession(request);
}

export default function Dashboard() {
    const userData = useLoaderData();
    return (
        <div>
            Welcome to the Dashboard!
            {JSON.stringify(userData)}
        </div>
    );
}

I spent a few days trying to figure out how to use Userfront with Remix Run. Caveat to this code: it works but I'm not confident it's the most secure or best way to do this. I think I would have preferred to use Remix built-in functions but I couldn't get it working. If you have a better way of doing it, please post!