NextAuth.js is an open source authentication solution for Next.js applications. `next-auth` applications using OAuth provider versions before `v4.20.1` have been found to be subject to an authentication vulnerability. A bad actor who can read traffic on the victim's network or who is able to social engineer the victim to click a manipulated login link could intercept and tamper with the authorization URL to **log in as the victim**, bypassing the CSRF protection. This is due to a partial failure during a compromised OAuth session where a session code is erroneously generated. This issue has been addressed in version 4.20.1. Users are advised to upgrade. Users unable to upgrade may using Advanced Initialization, manually check the callback request for state, pkce, and nonce against the provider configuration to prevent this issue. See the linked GHSA for details.

Affected: npm: next-auth

### Impact `next-auth` applications using OAuth provider versions before `v4.20.1` are affected.

A bad actor who can spy on the victim's network or able to social engineer the victim to click a manipulated login link could intercept and tamper with the authorization URL to **log in as the victim**, bypassing the CSRF protection.

As an example, an attack can happen in the following scenario.

> TL;DR: The attacker steals the victim's authenticated callback by intercepting and tampering with the authorization URL created by `next-auth`.

1. The victim attempts to log in to the `next-auth` site. For example https://next-auth-example.vercel.app/ 2. `next-auth` sets the `checks` cookies according to how the OAuth provider is configured. In this case, `state` and `pkce` are set by default for the Google Provider. <img width="1971" alt="Screen Shot 2023-03-03 at 09 54 26" src="https://user-images.githubusercontent.com/31528554/222619750-a2062bb8-99eb-4985-a75c-d75acd3da67e.png">

3. The attacker intercepts the returned authorization URL, strips away the OAuth check (nonce, state, pkce), and returns the URL _without_ the check to the victim's browser. For example: From ```https://accounts.google.com/o/oauth2/v2/auth/oauthchooseaccount?client_id=client_id&scope=openid%20email%20profile&response_type=code&redirect_uri=https%3A%2F%2Fnext-auth-example.vercel.app%2Fapi%2Fauth%2Fcallback%2Fgoogle&state=state&code_challenge=code_challenge&code_challenge_method=S256&service=lso&o2v=2&fl…