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.
https://exchange.xforce.ibmcloud.com/vulnerabilities/15701
http://www.pdc.kth.se/heimdal/advisory/2004-04-01/