Better Auth: OAuth callback accepts mismatched `state` when cookie-backed state storage is used without PKCE

Am I affected?

Users are affected if all of the following are true:

• The application uses better-auth at a version below 1.6.2 (or @better-auth/sso paired with such a version).
• betterAuth({ account: { storeStateStrategy } }) is set to "cookie". The default "database" is not affected.
• The application wires at least one OAuth provider through genericOAuth({ config }) with pkce: false, or it supplies a custom getToken or tokenUrl that does not require the stored codeVerifier. Stock social providers with PKCE on are not affected.
• The provider returns arbitrary code values to the configured callback URL.

If users are on [email protected] or later, they are not affected.

Fix:

1. Upgrade to [email protected] or later (current stable is 1.6.10).
2. If users cannot upgrade, see workarounds below.

Summary

In parseGenericState, the cookie branch decrypted the oauth_state cookie and validated expiry, but did not compare the incoming OAuth state query parameter to the nonce that generateGenericState issued at sign-in. Any callback to /api/auth/oauth2/callback/<providerId> that arrived with a forged state and any code was therefore accepted as long as the browser still held a live oauth_state cookie. With pkce: false (or any getToken path that does not enforce a code-verifier round-trip), an attacker who forced the victim to deliver an attacker-controlled authorization code to the callback would mint a session bound to the attacker's external identity in the victim's browser. Account-linking flows behaved the same way, binding the attacker's external account to an authenticated victim row.

Details

The cookie branch of parseGenericState did not compare the cookie's stored nonce to the incoming parameter. The database branch (the default) was not affected because the verification row is keyed by and the lookup itself enforces equality.