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.