Auth.js: OAuth state, nonce, and PKCE check cookies are not bound to the provider that created them

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

Reported by @Nadav0077. Thank you for the responsible disclosure.