CVE-2017-11103: Orpheus' Lyre KDC-REP service name validation

In _krb5_extract_ticket() the KDC-REP service name must be obtained from
encrypted version stored in 'enc_part' instead of the unencrypted version
stored in 'ticket'.  Use of the unecrypted version provides an
opportunity for successful server impersonation and other attacks.

Identified by Jeffrey Altman, Viktor Duchovni and Nico Williams.

Change-Id: I45ef61e8a46e0f6588d64b5bd572a24c7432547c

Author: jaltman

lib/krb5/ticket.c

@@ -705,8 +705,8 @@ _krb5_extract_ticket(krb5_context context,
     /* check server referral and save principal */
     ret = _krb5_principalname2krb5_principal (context,
 					      &tmp_principal,
-					      rep->kdc_rep.ticket.sname,
-					      rep->kdc_rep.ticket.realm);
+					      rep->enc_part.sname,
+					      rep->enc_part.srealm);
     if (ret)
 	goto out;
     if((flags & EXTRACT_TICKET_ALLOW_SERVER_MISMATCH) == 0){