Heimdal releases prior to 0.6.1 and 0.5.3 have a cross-realm vulnerability allowing someone with control over a realm to impersonate anyone in the cross-realm trust path.
You can download this advisory from the following link: https://packetstormsecurity.com/files/download/33018/heimdal.html
Source: https://packetstormsecurity.com/files/33018/heimdal.html.html