OAuth 2.1 Provider: Unprivileged users can register OAuth clients

Am I affected?

You're affected if all of the following are true:

• Using @better-auth/oauth-provider at version specified below
• You configured clientPrivileges in the plugin options expecting it to gate who can create OAuth clients
• The /oauth2/create-client or /admin/oauth2/create-client endpoints are reachable by authenticated users you don't fully trust

If clientPrivileges is not configured, this bug has no security consequence for your deployment

Summary

The clientPrivileges option documents a create action, but the OAuth client creation endpoints did not invoke the hook before persisting new clients. Deployments that configured clientPrivileges to restrict client registration were not actually restricted — any authenticated user could reach the create endpoints and register an OAuth client with attacker-chosen redirect URIs and metadata.

Non-create operations (read, list, update, delete, rotate) enforced the hook correctly. Only the create path was missing the check.

Impact

• Unauthorized registration of OAuth clients by any authenticated user, under deployments that expected clientPrivileges to block them.
• Attacker-controlled redirect_uris on those clients enable phishing flows that present as registered first-party applications.
• If the SERVER_ONLY admin creation endpoint is also exposed to low-privilege users (a separate deployment misconfiguration), additional sensitive fields including skip_consent become writable.

Patches

Fixed in @better-auth/oauth-provider@1.6.5 Both create endpoints now call the clientPrivileges hook with action "create" before persisting the client record.

Workarounds

If you cannot upgrade immediately:

• Block the /oauth2/create-client and /admin/oauth2/create-client routes at your reverse proxy or middleware layer for any user who should not be able to register clients.
• Do not expose the admin creation endpoint (it is SERVER_ONLY by design and should not be reachable by end-user sessions).