When pretix is used as the identity provider in a Single-Sign-On (SSO) setup for customer accounts, the system did not properly validate the `response_type` requested in an authorization request against the configured grant type of the SSO client. This means that if an SSO client was configured to only allow the `code` response type, which requires knowledge of the client secret to use and would wrongfully send a response type like `id_token`, it would receive back a valid token. Exploiting this requires the attacker to run JavaScript on the page specified by `redirect_uri`, which must be from a list specified in the client configuration.