A flaw was found in Keycloak 3.4.3.Final, 4.0.0.Beta2, 4.3.0.Final. When using 'response_mode=form_post' it is possible to inject arbitrary Javascript-Code via the 'state'-parameter in the authentication URL. This allows an XSS-Attack upon succesfully login.
https://bugzilla.redhat.com/show_bug.cgi?id=CVE-2018-14655
https://access.redhat.com/errata/RHSA-2018:3595