Handling Static Asset Interception in Authentication Middleware

When implementing authentication middleware in a Next.js application, you might encounter a situation where your website appears broken or without styling. This typically happens when the middleware intercepts requests for static assets like CSS files and redirects them to the login page. Here’s how to diagnose and solve this issue.

Problem

Title: Static Assets Intercepted by Authentication Middleware

When using the auth function from @/auth to create an authentication middleware, all incoming requests, including those for static assets (CSS, JS, images), might be intercepted. This causes the middleware to apply redirect logic indiscriminately, leading to the CSS and other static resources not being loaded correctly. As a result, the website appears broken or without styling.

Example Problem Code

import { auth } from "@/auth";
 
export default auth((req) => {
  const allowedPaths = ["/", "/login"];
  const { pathname, origin } = req.nextUrl;
 
  if (!req.auth && !allowedPaths.includes(pathname)) {
    const newUrl = new URL("/login", origin);
    return Response.redirect(newUrl);
  }
});

In this example, the middleware redirects unauthenticated users to the login page for any request that is not explicitly allowed. However, it doesn't differentiate between page requests and static asset requests, causing the issue.

Solution

To fix this, modify the middleware to bypass authentication checks for static asset requests. Here’s how you can do it:

Example Solution Code

import { auth as middleware } from "@/auth";
 
export default middleware((req) => {
  const allowedPaths = ["/", "/login"];
  const { pathname, origin } = req.nextUrl;
 
  // Check if the request is for a static file (e.g., CSS, JS, images, etc.)
  const isStaticFile = pathname.startsWith("/_next/") || pathname.startsWith("/static/") || pathname.includes(".");
 
  if (!req.auth && !allowedPaths.includes(pathname) && !isStaticFile) {
    const newUrl = new URL("/login", origin);
    return Response.redirect(newUrl);
  }
 
  // Proceed with the request if it's for static files or an allowed path
});

Explanation

1. Identify Static Asset Requests:

   • The condition pathname.startsWith("/_next/") || pathname.startsWith("/static/") || pathname.includes(".") is used to identify requests for static files.

2. Adjust Middleware Logic:

   • The middleware now includes a check to bypass the authentication logic for these static asset requests. This ensures that static assets are served correctly without redirection, allowing the website to display properly.

Summary

By correctly configuring the middleware to differentiate between page requests and static asset requests, you can prevent unauthenticated users from being redirected when they request necessary static files. This ensures that your website loads correctly and maintains its intended styling and functionality.

This approach allows you to use the auth function from @/auth effectively without disrupting the user experience on your Next.js application.