For the old beast you 'll need at least this (read
ftp.cdrom.com/pub/Linux/distributions/Slackware/Slackware-3.1/ChangeLog.txt for things what I've missed):

       1.rm /usr/bin/sendmail and install something more secure in place
       2.rm /usr/bin/ncftp and install more appropriate version than 1.xx (in which rubbish pile
         Patrick found it?)
       3.or do same thing with regular ftp client - at least, it may be compiled with libreadline (you 'll need
         libreadline sources for this and also for ncftp)
       4.replace /bin/mount, /bin/smbmount, /bin/smbumount or remove setuid bit
       5.remove setuid from /usr/X11/bin/SuperProbe
       6.replace misconfigured /usr/sbin/wu.ftpd with patched version
       7.replace lpd stuff with LPRng (you may find it on ftp.cdrom.com in 'patches' for 3.1, if it not already
         included in your version)
       8.replace outdated proc-ps package by proc-101 suite
       9.if you are 'lucky' owner of the very early version, go to /dev, ls tty[0-9]* and rm all that curiosity
         'block' tty's. Create enough number of normal ttys with mknod /dev/ttyNN 4 NN
       10.modify permissions on /dev/null (to 666), /dev/zero (the same), /dev/tty (at least 662)
       11.find latest sources for ld.so, make and install - it contains security bug and misses dlfcn.h (Don't
         try to use anything except for this include file from 'official' ldso.tgs patch, or you 'll got also
         "massive filesystem corruption problems" as free bonus - seems like it compiled with wrong gcc
         version)
       12.find latest sources for gdbm library and install gdbm.h from it
       13.rm /usr/sbin/pppd-2.1 and /usr/sbin/pppd (it just wastes space), rename /usr/sbin/pppd-2.2 to
         pppd
       14.consider about removing /usr/sbin/ppp-on|off |on-dialer - this is the complete shit and this scripts
         are bad even as examples
         (you may look here for some better secured dialing scripts or look for diald daemon and forget it all)
       15.(optional) grab pppd-2.0.0f sources and remake pppd and chat. Slackware version had strange
         syslog facility - I can't figure it for chat (just not logged anything on my system) and don't like
         'daemon' facility for pppd - it runs too often and 'll quickly fill logfiles with unwanted messages.
         (And it logs to 'local2' on many commercial unices, why not to do same on linux?)
       16.go to /etc/securetty and remove all, except for first six virtual consoles
       17.rm /usr/sbin/in.telnetsnoopd - snooping is the bad thing, if you want to know. (May be you will
         prefer to know what it have security hole? Not bad for such kind of software :)
       18.enjoy...

It must be enough to work, but I  also like to
mv /etc/rc.d/rc.cdrom to /etc/rc.d/rc.nocdrom (it should stop this
boring CD mounting on system startup - newer versions of this
distributive already do this, but seems likely all
CD's created with much earlier version) into /etc/fstab something like

/dev/cdrom /cdrom  iso9660  noauto,user,ro,exec,nosuid,nodev      0      0