3f66f23f-3f60-46c4-af76-63a168b0d8d6

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 provider.
Exploitation requires luring the victim into starting a legitimate same-origin flow; it cannot be performed by cross-site request forgery alone, which reduces practical likelihood.

Patched version

The fix binds the OAuth check cookies to the provider/authorization flow that created them, so a callback cannot consume a check value minted for a different provider. Upgrade to the first releases containing this fix (pending; this advisory will be updated with exact patched versions before publication).

Workarounds

If you cannot upgrade immediately:

Enable PKCE (checks: ["pkce"], in addition to state/nonce) on every provider that supports it; PKCE blocks the practical code-swap variant because the attacker cannot observe the relying party's verifier.
Avoid offering logged-in account linking across multiple providers where one provider is lower-trust or attacker-observable.
Treat events.linkAccount as sensitive: add audit logging, user notification, or out-of-band confirmation so that any unexpected link is visible (defense-in-depth, not a root-cause fix).

Credit