NextAuth.js before 4.10.3 and 3.29.10 sending verification requests (magic link) to unwanted emails
Impact
next-auth users who are using the EmailProvider either in versions before 4.10.3 or 3.29.10 are affected.
If an attacker could forge a request that sent a comma-separated list of emails (eg.: [email protected],[email protected]) to the sign-in endpoint, NextAuth.js would send emails to both the attacker and the victim's e-mail addresses. The attacker could then login as a newly created user with the email being [email protected],[email protected]. This means that basic authorization like email.endsWith("@victim.com") in the signIn callback would fail to communicate a threat to the developer and would let the attacker bypass authorization, even with an @attacker.com address.
Patches
We patched this vulnerability in v4.10.3 and v3.29.10 by normalizing the email value that is sent to the sign-in endpoint before accessing it anywhere else. We also added a normalizeIdentifier callback on the EmailProvider configuration, where you can further tweak your requirements for what your system considers a valid e-mail address. (E.g.: strict RFC2821 compliance)
To upgrade, run one of the following:
```sh
npm i next-auth@latest
```
```sh
yarn add next-auth@latest
```
```sh
pnpm add next-auth@latest
```
(This will update to the latest v4 version, but you can change latest to 3 if you want to stay on v3. This is not recommended. v3 is unmaintained.)
Workarounds
If for some reason you cannot upgrade, you can normalize the incoming request like the following, using Advanced Initialization:
```ts
// pages/api/auth/[...nextauth].ts
function normalize(identifier) {
// Get the first two elements only,
// separated by @ from user input.
let [local, domain] = identifier.toLowerCase().trim().split("@")
// The part before "@" can contain a ","
// but we remove it on the domain part
domain = domain.split(",")[0]
return ${local}@${domain}
}
export default async function handler(req, res) {
if (req.body.email) req.body.email = normalize(req.body.email)
return await NextAuth(req, res, {/* your options */ })
}
```
References
- EmailProvider: https://next-auth.js.org/providers/email
- Normalize the email address: https://next-auth.js.org/providers/email#normalizing-the-email-address
- Email syntax: https://en.wikipedia.org/wiki/Email_address#Local-part
signIn callback: https://next-auth.js.org/configuration/callbacks#sign-in-callback- Advanced Initialization: https://next-auth.js.org/configuration/initialization#advanced-initialization
nodemailer address: https://nodemailer.com/message/addresses
For more information
If you have any concerns, we request responsible disclosure, outlined here: https://next-auth.js.org/security#reporting-a-vulnerability
Timeline
The issue was reported 26th of July, a response was sent out in less than 1 hour and after identifying the issue a patch was published within 5 working days.
Acknowledgments
We would like to thank [Socket](https://socket.dev) for disclosing this vulnerability in a responsible manner and following up until it got published.
