Validate session cookies in Express

CSRF protection must be implemented when using cookies and forms. This can be easily done by comparing the Origin and Host header.

We recommend creating 2 middleware for CSRF protection and validating requests. You can get the cookie with Lucia.readSessionCookie() and validate the session cookie with Lucia.validateSession(). Make sure to delete the session cookie if it's invalid and create a new session cookie when the expiration gets extended, which is indicated by Session.fresh.

// src/middleware.ts
import { lucia } from "./auth.js";
import { verifyRequestOrigin } from "lucia";

import type { User } from "lucia";

app.use((req, res, next) => {
	if (req.method === "GET") {
		return next();
	}
	const originHeader = req.headers.origin ?? null;
	// NOTE: You may need to use `X-Forwarded-Host` instead
	const hostHeader = req.headers.host ?? null;
	if (!originHeader || !hostHeader || !verifyRequestOrigin(originHeader, [hostHeader])) {
		return res.status(403).end();
	}
});

app.use((req, res, next) => {
	const sessionId = lucia.readSessionCookie(req.headers.cookie ?? "");
	if (!sessionId) {
		res.locals.user = null;
		res.locals.session = null;
		return next();
	}

	const { session, user } = await lucia.validateSession(sessionId);
	if (session && session.fresh) {
		res.appendHeader("Set-Cookie", lucia.createSessionCookie(session.id).serialize());
	}
	if (!session) {
		res.appendHeader("Set-Cookie", lucia.createBlankSessionCookie().serialize());
	}
	res.locals.user = user;
	res.locals.session = session;
	return next();
});

declare global {
	namespace Express {
		interface Locals {
			user: User | null;
			session: Session | null;
		}
	}
}

This will allow you to access the current user with res.locals.

app.get("/", (req, res) => {
	if (!res.locals.user) {
		return res.status(403).end();
	}
	// ...
});