According to a \ security advisory from the heimdal project, heimdal, a suite of software implementing the Kerberos protocol, has "a cross-realm vulnerability allowing someone with control over a realm to impersonate anyone in the cross-realm trust path."
For the current stable distribution (woody) this problem has been fixed in version 0.4e-7.woody.8.1.
For the unstable distribution (sid), this problem has been fixed in version 0.6.1-1.
We recommend that you update your heimdal package.