Heimdal 0.6.x before 0.6.1 and 0.5.x before 0.5.3 does not properly perform certain consistency checks for cross-realm requests, which allows remote attackers with control of a realm to impersonate others in the cross-realm trust path.
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.