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.