krb5 -- client impersonation vulnerability

The MIT Kerberos team reports:

MIT krb5 KDC may issue tickets not requested
by a client, based on an attacker-chosen KrbFastArmoredReq.
An authenticated remote attacker that controls a legitimate service
principal could obtain a valid service ticket to itself containing
valid KDC-generated authorization data for a client whose TGS-REQ it
has intercepted. The attacker could then use this ticket for
S4U2Proxy to impersonate the targeted client even if the client
never authenticated to the subverted service. The vulnerable
configuration is believed to be rare.