Improper handling of email input in next-auth

Vulnerability Description

NextAuth.js is a complete open source authentication solution for Next.js applications. An attacker can pass a compromised input to the e-mail [signin endpoint](https://next-auth.js.org/getting-started/rest-api#post-apiauthsigninprovider) that contains some malicious HTML, tricking the e-mail server to send it to the user, so they can perform a phishing attack. Eg.: `balazs@email.com, <a href="http://attacker.com">Before signing in, claim your money!</a>`. This was previously sent to `balazs@email.com`, and the content of the email containing a link to the attacker's site was rendered in the HTML. This has been remedied in the following releases, by simply not rendering that e-mail in the HTML, since it should be obvious to the receiver what e-mail they used: next-auth v3 users before version 3.29.8 are impacted. (We recommend upgrading to v4, as v3 is considered unmaintained. next-auth v4 users before version 4.9.0 are impacted. If for some reason you cannot upgrade, the workaround requires you to sanitize the `email` parameter that is passed to `sendVerificationRequest` and rendered in the HTML. If you haven't created a custom `sendVerificationRequest`, you only need to upgrade. Otherwise, make sure to either exclude `email` from the HTML body or efficiently sanitize it.

References

• https://next-auth.js.org/getting-started/upgrade-v4
• https://github.com/nextauthjs/next-auth/security/advisories/GHSA-pgjx-7f9g-9463
• https://github.com/nextauthjs/next-auth/commit/ae834f1e08a4a9915665eecb9479c74c6b039c9c
• https://github.com/nextauthjs/next-auth/releases/tag/next-auth%40v4.9.0
• https://next-auth.js.org/providers/email#customizing-emails