As mentioned in Question 1.18, one weakness in Kerberos is the ability to do
an offline dictionary attack by requested a TGT for a user and just trying
different passwords until you find one that decrypts the TGT successfully.

One way of preventing this particular attack is to do what is known as
preauthentication. This means to simply require some additional
authentication before the KDC will issue you a TGT.

The simplest form of preauthentication is known as PA-ENC-TIMESTAMP. This is
simply the current timestamp encrypted with the user's key.

There are various other types of preauthentication, but not all versions of
Kerberos 5 support them all.