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This FAQ deals with some suggestions for securing your Unix machine after ithas already been compromised. Even if your machines have not been compromised, there are many helpful tips on securing a unix machine:

Try to trace/follow the intruder back to his origin via looking at:

1. who
2. w
3. last
4. lastcomm
5. netstat
6. snmpnetstat
7. router information.
8. /var/adm/messages (many crackers send e-mail to their "home" accounts)
9. syslog (sends logs to other hosts as well)
10. wrapper logs
11. do a 'finger' to all local users(and check where they last logged in from)
12. history files from shells, such as .history, .rchist, and similiar files.

Foot note: 'who', 'w', 'last', and 'lastcomm' are commands that rely on
/var/adm/pacct, /usr/adm/wtmp, and /etc/utmp to report the information
to you. Most backdoors will keep the intruder from being shown in these
logs. Even if the intruder has not installed any backdoors yet, it is
trivial to remove any detection in these logs. But they may just forget
about one or two of them. Especially if you have some additional,
non-standard ones.

Suggestion: Install xinetd or tcp_wrapper that will log all connections
to your machine to see if someone is knocking on its doors. Forward
syslogs to another machine so intruder will not easily detect the logs
and modify.

It might be wise to monitor the intruder via some ethernet sniffer to
see how he is exploiting his systems before taking corrective measures.

2. Close the machine from outside access. Remove from network to stop
further access via intruder. If the intruder finds out that the administrator is unto him, he may try to hide his tracks by rm -rf /.

3. Check the binaries with the originals. Especially check the following
binaries because they are commonly replaced backdoors for regaining
access:

1. /bin/login
2. all the /usr/etc/in.* files (ie. in.telnetd)
3. and /lib/libc.so.* (on Suns).
4. anything called from inetd

Other commonly replaced backdoor binaries:

1. netstat - allows hiding connections
2. ps - allows hiding processes (ie Crack)
3. ls - allows hiding directories
4. ifconfig - hides the fact that promiscuity mode is on the ethernet
5. sum - fools the checksum for binaries, not necessarily replaced
anymore because its possible to change the checksum of the
binaries to the correct value without modifying sum. *EMPHASIZE*
Do NOT Rely on sum.

Use 'ls -lac' to find the real modification time of files. Check /etc/wtmp (if you still have one) for any system time adjustments.

Check the files with the distribution media (CD or tape) or calculate MD5 checksums and compare them with the originals kept offline (you did calculate them sometime ago, didn't you?) Suggestion: cmp the files with copies that are known to be good.

Another popular backdoor is suid'ing a common command (ie. /bin/time) to allow root access with regular accounts.

To find all suid programs you may use:

find / -type f -perm -4000 -ls

To be thorough you may need to re-load the entire OS to make sure there
are no backdoors. Tripwire helps prevent modifying binaries and system
files (ie. inetd.conf) on the system, without the administrator knowing.

4. Implement some password scheme for your users to verify that they
change their passwords often. Install anlpasswd, npasswd, or passwd+ in
place of passwd (or yppasswd) so that your users are forced to set
reasonable passwords. Then run Crack, which is available on
ftp://ftp.cert.org/pub/tools/ to make sure that your users aren't
bypassing the password check. Crack ensures that users are picking
difficult passwords. With the network, clear text passwords are a
problem. Other possible choices: smart hubs (stops ethernet sniffing of
the whole LAN) and one-time password technology.

5. Check all the users' .rhosts and .forward files to make sure none of
them are weird or out of the ordinary. If .rhosts file contains '+ +',
the account can be accessed anywhere by anyone without a password. COPS
has a scripts for checking .rhosts.

find / -name .rhosts -ls -o -name .forward -ls

Look also for all the files created/modified in the time you are
suspecting the break-in has taken place, eg:

find / -ctime -2 -ctime +1 -ls

To find all the files modified not less than one day ago, but not more
than 2.

All .login, .logout, .profile, .cshrc files are also worth looking at
(at least for the modification date/time). Make sure there are no
'.rhosts' for the locked or special accounts (like 'news', 'sundiag',
'sync', etc.) The shell for such accounts should be something like
'/bin/false' anyway (and not '/bin/sh') to make them more secure. Also
search for directories that have like ". ", ".. " as names. They are
usually found in /tmp , /var/tmp, /usr/spool/* and any other publicly
writeable directory.

6. Check to make sure your NFS exports are not world writable to everyone.
NFSwatch available on ftp://harbor.ecn.purdue.edu/pub/ , a program by
David Curry, will log any NFS transactions that are taking place. Try
'showmount -e' to see whether system agrees with your opinion of what
are you exporting and where. There are bugs in some nfsd
implementations which ignore the access lists when they exceed some
limit (256 bytes). Check also what are you IMPORTING!!! Use 'nosuid'
flag whenever possible. You do not want to be cracked by a sysadm from
another host (or a cracker there) running suid programs mounted via
NFS, do you?

7. Make sure you have implemented the newest sendmail daemon. Old sendmail
daemons allowed remote execution of commands on any Unix machine. See
the computer-security/security-patch FAQ.

8. Try to install all the security patches available from the vendor on
your machine. See the computer-security/security-patch FAQ.

9. Here is a check list of common ways that a machine is vulnerable:

1. Do an rpcinfo -p on your machine to make sure it is not running
any processes that are not needed. (ie. rexd).

2. Check for '+' in /etc/hosts.equiv.

3. Check whether tftp is disabled on your system. If not - disable
it, or at least use '-s' flag to chroot it to some safe area, if
you really can't live without it (it is mostly used for booting up
Xterminals, but sometimes can be avoided by NFS-mounting
appropriate disks). Under no circumstances you should run it as
root. Change the line describing it in /etc/inetd.conf to
something like:

tftp dgram udp wait nobody /usr/etc/in.tftpd in.tftpd -s
/tftpboot

or better yet, use tcpd wrapper program to protect it from
addresses which should not get access to tftp and log all other
connections:

tftp dgram udp wait nobody /usr/etc/tcpd in.tftpd -s
/tftpboot

and edit appropriately /etc/hosts.allow to restrict access to
in.tftpd to only those addresses that really need it.

4. Check crontabs and at-jobs. Make sure there are no delayed bombs
which will explode after you think you have got rid of all the
nasty things left by a intruder.

5. Check /etc/rc.boot /etc/rc.local (SYSV: /etc/rc?.d/* ) and other
files cruicial for the system startup. (The best would be if you
could compare them with the copies kept off-line). Check all other
files containing system configuration (sendmail.cf, sendmail.fc,
hosts.allow, at.allow, at.deny, cron.allow, hosts, hosts.lpd,
etc.) In 'aliases' look for aliases expanding to some unusual
programs (uudecode is one but example).

6. Check your inetd.conf and /etc/services files to find if there are
no additional services set up by an intruder.

7. Copy all the log files you still have (pacct, wtmp, lastlog,
sulog, syslog, authlog, any additional logs you have set up
earlier) to some safe place (offline) so you may examine them
later. Otherwise, do not be surprised if they disappear the next
day when the cracker realises he forgot to remove one of them. Use
your own imagination to find what other traces he could have left
in your system (What about /tmp/* files? Check them BEFORE you
reboot).

8. Make backup copy of /etc/passwd (best offline) then change all
root passwords (after verifying that 'su' and 'passwd' are not the
trojan versions left by an intruder). It may sound like a horrible
thing to do (especially if you have something like 2000 users) but
*do* lock them all by putting '*' in the password field. If the
intruder has a copy of your passwords file he may possibly sooner
or later guess all the passwords contained there (It is all the
matter of proper dictionaries). In fact he could have inserted few
passwords that he only knows for some users who for example have
not logged in for a long time.

On the NIS servers check not only the real /etc/passwd /etc/groups
etc files but also those used for building NIS maps (if they are
different).

9. Check if your anonymous ftp (and other services) are configured
properly (if you have any of course) See the
computer-security/anonymous-ftp FAQ.

10. If you want to make your life easier next time (or if you still
cannot get rid of an intruder) consider installing 'ident' daemon.
Together with tcpd on a set of hosts it can be used to find what
accounts the intruder is using.

11. Make sure the only 'secure' terminal is console (if at all). This
way you prevent root logins just from the net. Maybe it is not a
big deal as if somebody knows the root password he may already
know other peoples' passwords too, but maybe not?

12. Check hosts.equiv, .rhosts, and hosts.lpd for having # as comments
within those files. If an intruder changes his hostname to #, it
will be considered a trusted host and allow him to access your
machines.

13. And remember... There are so many ways that somebody could have
modified your system, that you really have to have your eyes and
ears wide open for a loooooong long time. Above, are the pointers
just to the most obvious things to check.

10. Mail all the sites that you were able to find out that the intruder was
going through and warn them. Also, CC: This e-mail address is being protected from spambots. You need JavaScript enabled to view it . Check all the
sites in your near-by, ie. in your domain/institution/whatever. It's
usually trivial for a hacker to get to another system by a simple
'rlogin' if the two systems have a common subset of users (and using
.rhosts to make the access easier).

11. A preventive from stopping many intruders from even trying your network
is to install a firewall.