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.

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&flowName=GeneralOAuthFlow
   to
   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&service=lso&o2v=2&flowName=GeneralOAuthFlow.
   Notice the parameters state, code_challenge and code_verifier are removed from the victim's address bar.

4. The victim attempts to log in using their OAuth account.

5. The Authorization Server logs the victim in and calls back to the next-auth api/auth/callback/:providerIdendpoint.
   5.1. The attacker intercepts and logs this callback URL for later use.
   5.2. next-auth checks the callback call from OAuth Authorization Server (doesn't have checks) and compares the checks with the cookies set (has checks) at step 2. This check will fail, resulting in the victim isn't logged in. However, at this step, the Authorization Server has already accepted the victim's request to log in and generated/sent a code in the URL.

6. The attacker now has an authorization URL with the code that the AS will exchange for valid access_token/id_token and can log in as the victim automatically. They can open a new browser window and paste in the URL logged at step 5.1 and log in as the victim.

Patches

We patched the vulnerability in next-auth v4.20.1
To upgrade, run one of the following:

npm i next-auth@latest

yarn add next-auth@latest

pnpm add next-auth@latest

Workarounds

Upgrading to latest is the recommended way to fix this issue. However, using Advanced Initialization, developers can manually check the callback request for state, pkce, and nonce against the provider configuration, and abort the sign-in process if there is a mismatch. Check out the source code for help.

References

• CSRF
• PKCE vs nonce
• OAuth provider options
• checks provider config