Version 4 of the Kerberos protocol (krb4), as used in Heimdal and other packages, allows an attacker to impersonate any principal in a realm via a chosen-plaintext attack.
https://oval.cisecurity.org/repository/search/definition/oval%3Aorg.mitre.oval%3Adef%3A248
http://www.securityfocus.com/bid/7113
http://www.securityfocus.com/archive/1/316960/30/25250/threaded
http://www.redhat.com/support/errata/RHSA-2003-091.html
http://www.redhat.com/support/errata/RHSA-2003-052.html
http://www.redhat.com/support/errata/RHSA-2003-051.html
http://www.kb.cert.org/vuls/id/623217
http://www.debian.org/security/2003/dsa-273
http://www.debian.org/security/2003/dsa-269
http://www.debian.org/security/2003/dsa-266
http://web.mit.edu/kerberos/www/advisories/MITKRB5-SA-2003-004-krb4.txt