Nextjs middleware is not able to read cookies, when deployed on vercel

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04-10-2024

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Next.js Middleware Cookie Conundrum: Why Your Cookies Disappear on Vercel

Problem: You've diligently set up Next.js middleware to authenticate users using cookies, but everything works flawlessly in development. Then, you deploy to Vercel, and suddenly your middleware can't read the cookies anymore.

Rephrased: Imagine you're building a secure website where users log in using cookies. It all works great locally, but when you launch it on Vercel, your login system breaks.

Scenario and Code:

Let's say your middleware looks something like this:

// pages/api/middleware.js
import { NextResponse } from 'next/server';

export async function middleware(req) {
  const token = req.cookies.token; // Assuming you have a cookie named 'token'

  if (!token) {
    return NextResponse.redirect('/login');
  }

  // Perform authentication logic here
  // ...
}

This middleware aims to check if the token cookie exists. If not, it redirects the user to the login page. However, on Vercel, the req.cookies object remains empty, leading to a redirect loop.

Analysis and Explanation:

The root cause lies in how Vercel handles cookies and the req.cookies object in middleware:

1. Cookies as Headers: Vercel treats cookies as headers, meaning they are not accessible through req.cookies in middleware. Instead, they are accessed as part of the req.headers object.

2. Cookie Header Structure: Cookies are stored in the Cookie header of HTTP requests. The req.headers object contains the raw header information, so you need to parse it manually.

Solution and Example:

Here's a revised middleware function that correctly extracts the token cookie from the req.headers object:

// pages/api/middleware.js
import { NextResponse } from 'next/server';

export async function middleware(req) {
  const cookieHeader = req.headers.get('Cookie'); // Get the Cookie header

  if (!cookieHeader) {
    return NextResponse.redirect('/login');
  }

  // Parse the Cookie header to extract the token value
  const token = cookieHeader.split(';').find(cookie => cookie.trim().startsWith('token=')).split('=')[1];

  if (!token) {
    return NextResponse.redirect('/login');
  }

  // Perform authentication logic here
  // ...
}

Key Takeaways:

• Middleware and Cookies: Understand that in Vercel, cookies are accessible through the req.headers object.
• Parsing Cookies: You need to manually parse the Cookie header to extract individual cookie values.
• Vercel Deployment Considerations: Be aware of Vercel's unique handling of cookies when deploying Next.js applications.

Additional Value:

• Cookie Security: Always secure your cookies with the HttpOnly and Secure flags to prevent cross-site scripting (XSS) attacks.
• Alternative Authentication: Consider using alternatives to cookies, such as JWTs (JSON Web Tokens), for more robust and secure authentication.

References:

• Next.js API Routes and Middleware
• Vercel Deployment
• Cookie Security Best Practices