Affected: npm: @auth/core, npm: next-auth

## Summary

Auth.js stores the OAuth/OIDC anti-CSRF checks (`state`, `nonce`, and the PKCE verifier) in global cookies that are not bound to the provider that created them. On callback, a check value minted during a sign-in started with one provider can satisfy the callback for a different provider, because the stored cookie is not verified against the callback provider's identity (provider id, issuer, client id, or redirect URI). In a multi-provider app that allows account linking while logged in, this provider-confusion / mix-up condition can let an attacker link their account at a second provider to a victim's user.

## Am I affected?

You may be affected if **all** of the following hold:

- You use `next-auth` `<= 4.24.14` or `>= 5.0.0-beta.1, <= 5.0.0-beta.31`, or `@auth/core` `<= 0.41.2`. - You configure multiple OAuth/OIDC providers. - You allow users to link additional providers while logged in. - At least one configured provider's authorization request is observable by an attacker, and at least one target provider's callback can be satisfied without a PKCE verifier (i.e. it relies only on `state` or only on `nonce`).

You are **not** affected if you use a single OAuth provider, do not allow logged-in account linking, or all providers enforce PKCE.

## Impact

- Account-linking confusion: an attacker can get their account at a target provider linked to the victim's Auth.js user, granting the attacker persistent sign-in to the victim's account through that linked prov…