Middleware doesn't support basePath option

[lavcraft]

Environment

next-auth: 4.12.2
next: 12.3.1

Reproduction URL

https://github.com/lavcraft/next-auth-basepath-bug

Describe the issue

This bug trigger when you use basePath option in next.js and configure it via next-auth SessionProvider (basePath property)

After call handleMiddleware function in next-auth package, return value with target callbackUrl doesn't polite with basePath

In this line:

signInUrl.searchParams.append("callbackUrl", `${pathname}${search}`)

I this, this will be fixed with simple hotfix

signInUrl.searchParams.append("callbackUrl", `${req.nextUrl.basePath ?? ''}${pathname}${search}`)

How to reproduce

1. Clone repo https://github.com/lavcraft/next-auth-basepath-bug
2. run npm ci
3. run npm run dev
4. open http://localhost:3001/custom/me
5. see invalid callbackUrl in query param (without /custom basePath)

Expected behavior

Stored callbackUrl include basePath for handle it correctly after authentication process or in other cases

PS: May be default value for pages: { .. } in next-auth middleware withAuth args also support basePath?

import { withAuth } from "next-auth/middleware"

const basePath = process.env.NEXT_PATH_PREFIX ?? '';
// More on how NextAuth.js middleware works: https://next-auth.js.org/configuration/nextjs#middleware
export default withAuth({
  callbacks: {
    ....
  },
  pages: {
    signIn: `${basePath}/api/auth/signin` //<-- i mean this, avoid basePath for defaultValue
  },
})

export const config = { matcher: ["/admin", "/me"] }

[ThangHuuVu]

There is an ongoing PR to fix this 👀 #5109 It has a different approach than yours. I suggest we continue in #5109 and work together to find the best solution there! 🙌 Let's keep this issue open in the meantime.

[lavcraft]

@ThangHuuVu thanks. I have looked in details your PR. I this you follow very similar approach. Only one major difference -

1. i'm using basePath field from next.js object,
2. You trying construct basePath by you own util function

And both implementation has a problem with URL with templated args like /order/[orderId]/page i think :) But i advice to solve this in next issues - sorry, check my solution, its work correctly in this case :)