CONTENTS
1 INTRODUCTION........................................... 1
1.1 UNIX Security.......................................... 1
1.2 The Internet Worm...................................... 2
1.3 Spies and Espionage.................................... 3
1.4 Other Break-Ins........................................ 4
1.5 Security is Important.................................. 4
2 IMPROVING SECURITY..................................... 5
2.1 Account Security....................................... 5
2.1.1 Passwords.............................................. 5
2.1.1.1 Selecting Passwords.................................... 6
2.1.1.2 Password Policies...................................... 8
2.1.1.3 Checking Password Security............................. 8
2.1.2 Expiration Dates....................................... 9
2.1.3 Guest Accounts......................................... 10
2.1.4 Accounts Without Passwords............................. 10
2.1.5 Group Accounts and Groups.............................. 10
2.1.6 Yellow Pages........................................... 11
2.2 Network Security....................................... 12
2.2.1 Trusted Hosts.......................................... 13
2.2.1.1 The hosts.equiv File................................... 13
2.2.1.2 The .rhosts File....................................... 14
2.2.2 Secure Terminals....................................... 15
2.2.3 The Network File System................................ 16
2.2.3.1 The exports File....................................... 16
2.2.3.2 The netgroup File...................................... 17
2.2.3.3 Restricting Super-User Access.......................... 18
2.2.4 FTP.................................................... 19
2.2.4.1 Trivial FTP............................................ 20
2.2.5 Mail................................................... 21
2.2.6 Finger................................................. 22
2.2.7 Modems and Terminal Servers............................ 23
2.2.8 Firewalls.............................................. 23
2.3 File System Security................................... 24
2.3.1 Setuid Shell Scripts................................... 25
2.3.2 The Sticky Bit on Directories.......................... 26
2.3.3 The Setgid Bit on Directories.......................... 26
2.3.4 The umask Value........................................ 27
2.3.5 Encrypting Files....................................... 27
2.3.6 Devices................................................ 28
2.4 Security Is Your Responsibility........................ 29
3 MONITORING SECURITY.................................... 31
3.1 Account Security....................................... 31
3.1.1 The lastlog File....................................... 31
3.1.2 The utmp and wtmp Files................................ 31
3.1.3 The acct File.......................................... 33
3.2 Network Security....................................... 34
iii
CONTENTS (concluded)
3.2.1 The syslog Facility.................................... 34
3.2.2 The showmount Command.................................. 35
3.3 File System Security................................... 35
3.3.1 The find Command....................................... 36
3.3.1.1 Finding Setuid and Setgid Files........................ 36
3.3.1.2 Finding World-Writable Files........................... 38
3.3.1.3 Finding Unowned Files.................................. 38
3.3.1.4 Finding .rhosts Files.................................. 39
3.3.2 Checklists............................................. 39
3.3.3 Backups................................................ 40
3.4 Know Your System....................................... 41
3.4.1 The ps Command......................................... 41
3.4.2 The who and w Commands................................. 42
3.4.3 The ls Command......................................... 42
3.5 Keep Your Eyes Open.................................... 42
4 SOFTWARE FOR IMPROVING SECURITY........................ 45
4.1 Obtaining Fixes and New Versions....................... 45
4.1.1 Sun Fixes on UUNET..................................... 45
4.1.2 Berkeley Fixes......................................... 46
4.1.3 Simtel-20 and UUNET.................................... 47
4.1.4 Vendors................................................ 47
4.2 The npasswd Command.................................... 48
4.3 The COPS Package....................................... 48
4.4 Sun C2 Security Features............................... 49
4.5 Kerberos............................................... 50
5 KEEPING ABREAST OF THE BUGS............................ 51
5.1 The Computer Emergency Response Team................... 51
5.2 DDN Management Bulletins............................... 51
5.3 Security-Related Mailing Lists......................... 52
5.3.1 Security............................................... 52
5.3.2 RISKS.................................................. 52
5.3.3 TCP-IP................................................. 53
5.3.4 SUN-SPOTS, SUN-NETS, SUN-MANAGERS...................... 53
5.3.5 VIRUS-L................................................ 53
6 SUGGESTED READING...................................... 55
7 CONCLUSIONS............................................ 57
REFERENCES..................................................... 59
APPENDIX A - SECURITY CHECKLIST................................ 63
iv
SECTION 1
INTRODUCTION
1.1 UNIX SECURITY
The UNIX operating system, although now in widespread use
in environments concerned about security, was not really
designed with security in mind [Ritc75]. This does not mean
that UNIX does not provide any security mechanisms; indeed,
several very good ones are available. However, most ``out of
the box'' installation procedures from companies such as Sun
Microsystems still install the operating system in much the
same way as it was installed 15 years ago: with little or no
security enabled.
The reasons for this state of affairs are largely histori-
cal. UNIX was originally designed by programmers for use by
other programmers. The environment in which it was used was
one of open cooperation, not one of privacy. Programmers typi-
cally collaborated with each other on projects, and hence pre-
ferred to be able to share their files with each other without
having to climb over security hurdles. Because the first sites
outside of Bell Laboratories to install UNIX were university
research laboratories, where a similar environment existed, no
real need for greater security was seen until some time later.
In the early 1980s, many universities began to move their
UNIX systems out of the research laboratories and into the com-
puter centers, allowing (or forcing) the user population as a
whole to use this new and wonderful system. Many businesses
and government sites began to install UNIX systems as well,
particularly as desktop workstations became more powerful and
affordable. Thus, the UNIX operating system is no longer being
used only in environments where open collaboration is the goal.
Universities require their students to use the system for class
assignments, yet they do not want the students to be able to
copy from each other. Businesses use their UNIX systems for
confidential tasks such as bookkeeping and payroll. And the
government uses UNIX systems for various unclassified yet sen-
sitive purposes.
To complicate matters, new features have been added to
UNIX over the years, making security even more difficult to
control. Perhaps the most problematic features are those
_________________________
UNIX is a registered trademark of AT&T. VAX is a trademark of
Digital Equipment Corporation. Sun-3 and NFS are trademarks of
Sun Microsystems. Annex is a trademark of Xylogics, Inc.
1
relating to networking: remote login, remote command execu-
tion, network file systems, diskless workstations, and elec-
tronic mail. All of these features have increased the utility
and usability of UNIX by untold amounts. However, these same
features, along with the widespread connection of UNIX systems
to the Internet and other networks, have opened up many new
areas of vulnerability to unauthorized abuse of the system.
1.2 THE INTERNET WORM
On the evening of November 2, 1988, a self-replicating
program, called a _w_o_r_m, was released on the Internet [Seel88,
Spaf88, Eich89]. Overnight, this program had copied itself
from machine to machine, causing the machines it infected to
labor under huge loads, and denying service to the users of
those machines. Although the program only infected two types
of computers,* it spread quickly, as did the concern, confu-
sion, and sometimes panic of system administrators whose
machines were affected. While many system administrators were
aware that something like this could theoretically happen - the
security holes exploited by the worm were well known - the
scope of the worm's break-ins came as a great surprise to most
people.
The worm itself did not destroy any files, steal any
information (other than account passwords), intercept private
mail, or plant other destructive software [Seel88]. However,
it did manage to severely disrupt the operation of the network.
Several sites, including parts of MIT, NASA's Ames Research
Center and Goddard Space Flight Center, the Jet Propulsion
Laboratory, and the U. S. Army Ballistic Research Laboratory,
disconnected themselves from the Internet to avoid recontamina-
tion. In addition, the Defense Communications Agency ordered
the connections between the MILNET and ARPANET shut down, and
kept them down for nearly 24 hours [Eich89, Elme88]. Ironi-
cally, this was perhaps the worst thing to do, since the first
fixes to combat the worm were distributed via the network
[Eich89].
This incident was perhaps the most widely described com-
puter security problem ever. The worm was covered in many
newspapers and magazines around the country including the _N_e_w
_Y_o_r_k _T_i_m_e_s, _W_a_l_l _S_t_r_e_e_t _J_o_u_r_n_a_l, _T_i_m_e and most computer-
oriented technical publications, as well as on all three major
_________________________
* Sun-3 systems from Sun Microsystems and VAX systems from
Digital Equipment Corp., both running variants of 4._x BSD UNIX
from the University of California at Berkeley.
2
television networks, the Cable News Network, and National Pub-
lic Radio. In January 1990, a United States District Court
jury found Robert Tappan Morris, the author of the worm, guilty
of charges brought against him under a 1986 federal computer
fraud and abuse law. Morris faces up to five years in prison
and a $250,000 fine [Schu90]. Sentencing is scheduled for May
4, 1990.
1.3 SPIES AND ESPIONAGE
In August 1986, the Lawrence Berkeley Laboratory, an
unclassified research laboratory at the University of Califor-
nia at Berkeley, was attacked by an unauthorized computer
intruder [Stol88, Stol89]. Instead of immediately closing the
holes the intruder was using, the system administrator, Clif-
ford Stoll, elected to watch the intruder and document the
weaknesses he exploited. Over the next 10 months, Stoll
watched the intruder attack over 400 computers around the
world, and successfully enter about 30. The computers broken
into were located at universities, military bases, and defense
contractors [Stol88].
Unlike many intruders seen on the Internet, who typically
enter systems and browse around to see what they can, this
intruder was looking for something specific. Files and data
dealing with the Strategic Defense Initiative, the space shut-
tle, and other military topics all seemed to be of special
interest. Although it is unlikely that the intruder would have
found any truly classified information (the Internet is an
unclassified network), it was highly probable that he could
find a wealth of sensitive material [Stol88].
After a year of tracking the intruder (eventually involv-
ing the FBI, CIA, National Security Agency, Air Force Intelli-
gence, and authorities in West Germany), five men in Hannover,
West Germany were arrested. In March 1989, the five were
charged with espionage: they had been selling the material
they found during their exploits to the KGB. One of the men,
Karl Koch (``Hagbard''), was later found burned to death in an
isolated forest outside Hannover. No suicide note was found
[Stol89]. In February 1990, three of the intruders (Markus
Hess, Dirk Bresinsky, and Peter Carl) were convicted of
espionage in a German court and sentenced to prison terms,
fines, and the loss of their rights to participate in elections
[Risk90]. The last of the intruders, Hans Hu"bner (``Pengo''),
still faces trial in Berlin.
3
1.4 OTHER BREAK-INS
Numerous other computer security problems have occurred in
recent years, with varying levels of publicity. Some of the
more widely known incidents include break-ins on NASA's SPAN
network [McLe87], the IBM ``Christmas Virus'' [Risk87], a virus
at Mitre Corp. that caused the MILNET to be temporarily iso-
lated from other networks [Risk88], a worm that penetrated DEC-
NET networks [Risk89a], break-ins on U. S. banking networks
[Risk89b], and a multitude of viruses, worms, and trojan horses
affecting personal computer users.
1.5 SECURITY IS IMPORTANT
As the previous stories demonstrate, computer security is
an important topic. This document describes the security
features provided by the UNIX operating system, and how they
should be used. The discussion centers around version 4._x of
SunOS, the version of UNIX sold by Sun Microsystems. Most of
the information presented applies equally well to other UNIX
systems. Although there is no way to make a computer com-
pletely secure against unauthorized use (other than to lock it
in a room and turn it off), by following the instructions in
this document you can make your system impregnable to the
``casual'' system cracker,* and make it more difficult for the
sophisticated cracker to penetrate.
_________________________
* The term ``hacker,'' as applied to computer users, originally
had an honorable connotation: ``a person who enjoys learning the
details of programming systems and how to stretch their
capabilities - as opposed to most users of computers, who prefer
to learn only the minimum amount necessary'' [Stee88].
Unfortunately, the media has distorted this definition and given
it a dishonorable meaning. In deference to the true hackers, we
will use the term ``cracker'' throughout this document.
4
SECTION 2
IMPROVING SECURITY
UNIX system security can be divided into three main areas
of concern. Two of these areas, account security and network
security, are primarily concerned with keeping unauthorized
users from gaining access to the system. The third area, file
system security, is concerned with preventing unauthorized
access, either by legitimate users or crackers, to the data
stored in the system. This section describes the UNIX security
tools provided to make each of these areas as secure as possi-
ble.
2.1 ACCOUNT SECURITY
One of the easiest ways for a cracker to get into a system
is by breaking into someone's account. This is usually easy to
do, since many systems have old accounts whose users have left
the organization, accounts with easy-to-guess passwords, and so
on. This section describes methods that can be used to avoid
these problems.
2.1.1 Passwords
The password is the most vital part of UNIX account secu-
rity. If a cracker can discover a user's password, he can then
log in to the system and operate with all the capabilities of
that user. If the password obtained is that of the super-user,
the problem is more serious: the cracker will have read and
write access to every file on the system. For this reason,
choosing secure passwords is extremely important.
The UNIX _p_a_s_s_w_d program [Sun88a, 379] places very few res-
trictions on what may be used as a password. Generally, it
requires that passwords contain five or more lowercase letters,
or four characters if a nonalphabetic or uppercase letter is
included. However, if the user ``insists'' that a shorter
password be used (by entering it three times), the program will
allow it. No checks for obviously insecure passwords (see
below) are performed. Thus, it is incumbent upon the system
administrator to ensure that the passwords in use on the system
are secure.
5
In [Morr78], the authors describe experiments conducted to
determine typical users' habits in the choice of passwords. In
a collection of 3,289 passwords, 16% of them contained three
characters or less, and an astonishing 86% were what could gen-
erally be described as insecure. Additional experiments in
[Gram84] show that by trying three simple guesses on each
account - the login name, the login name in reverse, and the
two concatenated together - a cracker can expect to obtain
access to between 8 and 30 percent of the accounts on a typical
system. A second experiment showed that by trying the 20 most
common female first names, followed by a single digit (a total
of 200 passwords), at least one password was valid on each of
several dozen machines surveyed. Further experimentation by
the author has found that by trying variations on the login
name, user's first and last names, and a list of nearly 1800
common first names, up to 50 percent of the passwords on any
given system can be cracked in a matter of two or three days.
2.1.1.1 Selecting Passwords
The object when choosing a password is to make it as dif-
ficult as possible for a cracker to make educated guesses about
what you've chosen. This leaves him no alternative but a
brute-force search, trying every possible combination of
letters, numbers, and punctuation. A search of this sort, even
conducted on a machine that could try one million passwords per
second (most machines can try less than one hundred per
second), would require, on the average, over one hundred years
to complete. With this as our goal, and by using the informa-
tion in the preceding text, a set of guidelines for password
selection can be constructed:
+o _D_o_n'_t use your login name in any form (as-is,
reversed, capitalized, doubled, etc.).
+o _D_o_n'_t use your first or last name in any form.
+o _D_o_n'_t use your spouse's or child's name.
+o _D_o_n'_t use other information easily obtained about
you. This includes license plate numbers, telephone
numbers, social security numbers, the brand of your
automobile, the name of the street you live on, etc.
+o _D_o_n'_t use a password of all digits, or all the same
letter. This significantly decreases the search time
for a cracker.
+o _D_o_n'_t use a word contained in (English or foreign
6
language) dictionaries, spelling lists, or other
lists of words.
+o _D_o_n'_t use a password shorter than six characters.
+o _D_o use a password with mixed-case alphabetics.
+o _D_o use a password with nonalphabetic characters,
e.g., digits or punctuation.
+o _D_o use a password that is easy to remember, so you
don't have to write it down.
+o _D_o use a password that you can type quickly, without
having to look at the keyboard. This makes it harder
for someone to steal your password by watching over
your shoulder.
Although this list may seem to restrict passwords to an
extreme, there are several methods for choosing secure, easy-
to-remember passwords that obey the above rules. Some of these
include the following:
+o Choose a line or two from a song or poem, and use the
first letter of each word. For example, ``In Xanadu
did Kubla Kahn a stately pleasure dome decree''
becomes ``IXdKKaspdd.''
+o Alternate between one consonant and one or two
vowels, up to eight characters. This provides non-
sense words that are usually pronounceable, and thus
easily remembered. Examples include ``routboo,''
``quadpop,'' and so on.
+o Choose two short words and concatenate them together
with a punctation character between them. For exam-
ple: ``dog;rain,'' ``book+mug,'' ``kid?goat.''
The importance of obeying these password selection rules
cannot be overemphasized. The Internet worm, as part of its
strategy for breaking into new machines, attempted to crack
user passwords. First, the worm tried simple choices such as
the login name, user's first and last names, and so on. Next,
the worm tried each word present in an internal dictionary of
432 words (presumably Morris considered these words to be
``good'' words to try). If all else failed, the worm tried
going through the system dictionary, /_u_s_r/_d_i_c_t/_w_o_r_d_s, trying
each word [Spaf88]. The password selection rules above suc-
cessfully guard against all three of these strategies.
7
2.1.1.2 Password Policies
Although asking users to select secure passwords will help
improve security, by itself it is not enough. It is also
important to form a set of password policies that all users
must obey, in order to keep the passwords secure.
First and foremost, it is important to impress on users
the need to keep their passwords in their minds only. Pass-
words should never be written down on desk blotters, calendars,
and the like. Further, storing passwords in files on the com-
puter must be prohibited. In either case, by writing the pass-
word down on a piece of paper or storing it in a file, the
security of the user's account is totally dependent on the
security of the paper or file, which is usually less than the
security offered by the password encryption software.
A second important policy is that users must never give
out their passwords to others. Many times, a user feels that
it is easier to give someone else his password in order to copy
a file, rather than to set up the permissions on the file so
that it can be copied. Unfortunately, by giving out the pass-
word to another person, the user is placing his trust in this
other person not to distribute the password further, write it
down, and so on.
Finally, it is important to establish a policy that users
must change their passwords from time to time, say twice a
year. This is difficult to enforce on UNIX, since in most
implementations, a password-expiration scheme is not available.
However, there are ways to implement this policy, either by
using third-party software or by sending a memo to the users
requesting that they change their passwords.
This set of policies should be printed and distributed to
all current users of the system. It should also be given to
all new users when they receive their accounts. The policy
usually carries more weight if you can get it signed by the
most ``impressive'' person in your organization (e.g., the
president of the company).
2.1.1.3 Checking Password Security
The procedures and policies described in the previous sec-
tions, when properly implemented, will greatly reduce the
chances of a cracker breaking into your system via a stolen
account. However, as with all security measures, you as the
8
system administrator must periodically check to be sure that
the policies and procedures are being adhered to. One of the
unfortunate truisms of password security is that, ``left to
their own ways, some people will still use cute doggie names as
passwords'' [Gram84].
The best way to check the security of the passwords on
your system is to use a password-cracking program much like a
real cracker would use. If you succeed in cracking any pass-
words, those passwords should be changed immediately. There
are a few freely available password cracking programs distri-
buted via various source archive sites; these are described in
more detail in Section 4. A fairly extensive cracking program
is also available from the author. Alternatively, you can
write your own cracking program, and tailor it to your own
site. For a list of things to check for, see the list of
guidelines above.
2.1.2 Expiration Dates
Many sites, particularly those with a large number of
users, typically have several old accounts lying around whose
owners have since left the organization. These accounts are a
major security hole: not only can they be broken into if the
password is insecure, but because nobody is using the account
anymore, it is unlikely that a break-in will be noticed.
The simplest way to prevent unused accounts from accumu-
lating is to place an expiration date on every account. These
expiration dates should be near enough in the future that old
accounts will be deleted in a timely manner, yet far enough
apart that the users will not become annoyed. A good figure is
usually one year from the date the account was installed. This
tends to spread the expirations out over the year, rather than
clustering them all at the beginning or end. The expiration
date can easily be stored in the password file (in the full
name field). A simple shell script can be used to periodically
check that all accounts have expiration dates, and that none of
the dates has passed.
On the first day of each month, any user whose account has
expired should be contacted to be sure he is still employed by
the organization, and that he is actively using the account.
Any user who cannot be contacted, or who has not used his
account recently, should be deleted from the system. If a user
is unavailable for some reason (e.g., on vacation) and cannot
be contacted, his account should be disabled by replacing the
encrypted password in the password file entry with an asterisk
(*). This makes it impossible to log in to the account, yet
9
leaves the account available to be re-enabled on the user's
return.
2.1.3 Guest Accounts
Guest accounts present still another security hole. By
their nature, these accounts are rarely used, and are always
used by people who should only have access to the machine for
the short period of time they are guests. The most secure way
to handle guest accounts is to install them on an as-needed
basis, and delete them as soon as the people using them leave.
Guest accounts should never be given simple passwords such as
``guest'' or ``visitor,'' and should never be allowed to remain
in the password file when they are not being used.
2.1.4 Accounts Without Passwords
Some sites have installed accounts with names such as
``who,'' ``date,'' ``lpq,'' and so on that execute simple com-
mands. These accounts are intended to allow users to execute
these commands without having to log in to the machine. Typi-
cally these accounts have no password associated with them, and
can thus be used by anyone. Many of the accounts are given a
user id of zero, so that they execute with super-user permis-
sions.
The problem with these accounts is that they open poten-
tial security holes. By not having passwords on them, and by
having super-user permissions, these accounts practically
invite crackers to try to penetrate them. Usually, if the
cracker can gain access to the system, penetrating these
accounts is simple, because each account executes a different
command. If the cracker can replace any one of these commands
with one of his own, he can then use the unprotected account to
execute his program with super-user permissions.
Simply put, accounts without passwords should not be
allowed on any UNIX system.
2.1.5 Group Accounts and Groups
Group accounts have become popular at many sites, but are
actually a break-in waiting to happen. A group account is a
10
single account shared by several people, e.g., by all the col-
laborators on a project. As mentioned in the section on pass-
word security, users should not share passwords - the group
account concept directly violates this policy. The proper way
to allow users to share information, rather than giving them a
group account to use, is to place these users into a group.
This is done by editing the group file, /_e_t_c/_g_r_o_u_p [Sun88a,
1390; Sun88b, 66], and creating a new group with the users who
wish to collaborate listed as members.
A line in the group file looks like
groupname:password:groupid:user1,user2,user3,...
The _g_r_o_u_p_n_a_m_e is the name assigned to the group, much like a
login name. It may be the same as someone's login name, or
different. The maximum length of a group name is eight charac-
ters. The password field is unused in BSD-derived versions of
UNIX, and should contain an asterisk (*). The _g_r_o_u_p_i_d is a
number from 0 to 65535 inclusive. Generally, numbers below 10
are reserved for special purposes, but you may choose any
unused number. The last field is a comma-separated (no spaces)
list of the login names of the users in the group. If no login
names are listed, then the group has no members. To create a
group called ``hackers'' with Huey, Duey, and Louie as members,
you would add a line such as this to the group file:
hackers:*:100:huey,duey,louie
After the group has been created, the files and direc-
tories the members wish to share can then be changed so that
they are owned by this group, and the group permission bits on
the files and directories can be set to allow sharing. Each
user retains his own account, with his own password, thus pro-
tecting the security of the system.
For example, to change Huey's ``programs'' directory to be
owned by the new group and properly set up the permissions so
that all members of the group may access it, the _c_h_g_r_p and
_c_h_m_o_d commands would be used as follows [Sun88a, 63-66]:
# chgrp hackers ~huey/programs
# chmod -R g+rw ~huey/programs
2.1.6 Yellow Pages
The Sun Yellow Pages system [Sun88b, 349-374] allows many
11
hosts to share password files, group files, and other files via
the network, while the files are stored on only a single host.
Unfortunately, Yellow Pages also contains a few potential secu-
rity holes.
The principal way Yellow Pages works is to have a special
line in the password or group file that begins with a ``+''.
In the password file, this line looks like
+::0:0:::
and in the group file, it looks like
+:
These lines should only be present in the files stored on Yel-
low Pages client machines. They should not be present in the
files on the Yellow Pages master machine(s). When a program
reads the password or group file and encounters one of these
lines, it goes through the network and requests the information
it wants from the Yellow Pages server instead of trying to find
it in the local file. In this way, the data does not have to
be maintained on every host. Since the master machine already
has all the information, there is no need for this special line
to be present there.
Generally speaking, the Yellow Pages service itself is
reasonably secure. There are a few openings that a sophisti-
cated (and dedicated) cracker could exploit, but Sun is rapidly
closing these. The biggest problem with Yellow Pages is the
``+'' line in the password file. If the ``+'' is deleted from
the front of the line, then this line loses its special Yellow
Pages meaning. It instead becomes a regular password file line
for an account with a null login name, no password, and user id
zero (super-user). Thus, if a careless system administrator
accidentally deletes the ``+''. the whole system is wide open
to any attack.*
Yellow Pages is too useful a service to suggest turning it
off, although turning it off would make your system more
secure. Instead, it is recommended that you read carefully the
information in the Sun manuals in order to be fully aware of
Yellow Pages' abilities and its limitations.
2.2 NETWORK SECURITY
_________________________
* Actually, a line like this without a ``+'' is dangerous in
any password file, regardless of whether Yellow Pages is in use.
12
As trends toward internetworking continue, most sites
will, if they haven't already, connect themselves to one of the
numerous regional networks springing up around the country.
Most of these regional networks are also interconnected, form-
ing the Internet [Hind83, Quar86]. This means that the users
of your machine can access other hosts and communicate with
other users around the world. Unfortunately, it also means
that other hosts and users from around the world can access
your machine, and attempt to break into it.
Before internetworking became commonplace, protecting a
system from unauthorized access simply meant locking the
machine in a room by itself. Now that machines are connected
by networks, however, security is much more complex. This sec-
tion describes the tools and methods available to make your
UNIX networks as secure as possible.
2.2.1 Trusted Hosts
One of the most convenient features of the Berkeley (and
Sun) UNIX networking software is the concept of ``trusted''
hosts. The software allows the specification of other hosts
(and possibly users) who are to be considered trusted - remote
logins and remote command executions from these hosts will be
permitted without requiring the user to enter a password. This
is very convenient, because users do not have to type their
password every time they use the network. Unfortunately, for
the same reason, the concept of a trusted host is also
extremely insecure.
The Internet worm made extensive use of the trusted host
concept to spread itself throughout the network [Seel88]. Many
sites that had already disallowed trusted hosts did fairly well
against the worm compared with those sites that did allow
trusted hosts. Even though it is a security hole, there are
some valid uses for the trusted host concept. This section
describes how to properly implement the trusted hosts facility
while preserving as much security as possible.
2.2.1.1 The hosts.equiv File
The file /_e_t_c/_h_o_s_t_s._e_q_u_i_v [Sun88a, 1397] can be used by
the system administrator to indicate trusted hosts. Each
trusted host is listed in the file, one host per line. If a
user attempts to log in (using _r_l_o_g_i_n) or execute a command
(using _r_s_h) remotely from one of the systems listed in
13
_h_o_s_t_s._e_q_u_i_v, and that user has an account on the local system
with the same login name, access is permitted without requiring
a password.
Provided adequate care is taken to allow only local hosts
in the _h_o_s_t_s._e_q_u_i_v file, a reasonable compromise between secu-
rity and convenience can be achieved. Nonlocal hosts (includ-
ing hosts at remote sites of the same organization) should
never be trusted. Also, if there are any machines at your
organization that are installed in ``public'' areas (e.g., ter-
minal rooms) as opposed to private offices, you should not
trust these hosts.
On Sun systems, _h_o_s_t_s._e_q_u_i_v is controlled with the Yellow
Pages software. As distributed, the default _h_o_s_t_s._e_q_u_i_v file
distributed by Sun contains a single line:
+
This indicates that _e_v_e_r_y _k_n_o_w_n _h_o_s_t (i.e., the complete con-
tents of the host file) should be considered a trusted host.
This is totally incorrect and a major security hole, since
hosts outside the local organization should never be trusted.
A correctly configured _h_o_s_t_s._e_q_u_i_v should never list any
``wildcard'' hosts (such as the ``+''); only specific host
names should be used. When installing a new system from Sun
distribution tapes, you should be sure to either replace the
Sun default _h_o_s_t_s._e_q_u_i_v with a correctly configured one, or
delete the file altogether.
2.2.1.2 The .rhosts File
The ._r_h_o_s_t_s file [Sun88a, 1397] is similar in concept and
format to the _h_o_s_t_s._e_q_u_i_v file, but allows trusted access only
to specific host-user combinations, rather than to hosts in
general.* Each user may create a ._r_h_o_s_t_s file in his home
directory, and allow access to her account without a password.
Most people use this mechanism to allow trusted access between
accounts they have on systems owned by different organizations
who do not trust each other's hosts in _h_o_s_t_s._e_q_u_i_v. Unfor-
tunately, this file presents a major security problem: While
_h_o_s_t_s._e_q_u_i_v is under the system administrator's control and can
be managed effectively, any user may create a ._r_h_o_s_t_s file
granting access to whomever he chooses, without the system
administrator's knowledge.
_________________________
Actually, _h_o_s_t_s._e_q_u_i_v may be used to specify host-user
combinations as well, but this is rarely done.
14
The only secure way to manage ._r_h_o_s_t_s files is to com-
pletely disallow them on the system. The system administrator
should check the system often for violations of this policy
(see Section 3.3.1.4). One possible exception to this rule is
the ``root'' account; a ._r_h_o_s_t_s file may be necessary to allow
network backups and the like to be completed.
2.2.2 Secure Terminals
Under newer versions of UNIX, the concept of a ``secure''
terminal has been introduced. Simply put, the super-user
(``root'') may not log in on a nonsecure terminal, even with a
password. (Authorized users may still use the _s_u command to
become super-user, however.) The file /_e_t_c/_t_t_y_t_a_b [Sun88a,
1478] is used to control which terminals are considered
secure.|- A short excerpt from this file is shown below.
console "/usr/etc/getty std.9600" sun off secure
ttya "/usr/etc/getty std.9600" unknown off secure
ttyb "/usr/etc/getty std.9600" unknown off secure
ttyp0 none network off secure
ttyp1 none network off secure
ttyp2 none network off secure
The keyword ``secure'' at the end of each line indicates that
the terminal is considered secure. To remove this designation,
simply edit the file and delete the ``secure'' keyword. After
saving the file, type the command (as super-user):
# kill -HUP 1
This tells the _i_n_i_t process to reread the _t_t_y_t_a_b file.
The Sun default configuration for _t_t_y_t_a_b is to consider
all terminals secure, including ``pseudo'' terminals used by
the remote login software. This means that ``root'' may log in
remotely from any host on the network. A more secure confi-
guration would consider as secure only directly connected ter-
minals, or perhaps only the console device. This is how file
servers and other machines with disks should be set up.
The most secure configuration is to remove the ``secure''
designation from all terminals, including the console device.
This requires that those users with super-user authority first
log in as themselves, and then become the super-user via the _s_u
_________________________
|- Under non-Sun versions of Berkeley UNIX, this file is called
/_e_t_c/_t_t_y_s.
15
command. It also requires the ``root'' password to be entered
when rebooting in single-user mode, in order to prevent users
from rebooting their desktop workstations and obtaining super-
user access. This is how all diskless client machines should
be set up.
2.2.3 The Network File System
The Network File System (NFS) [Sun88d] is designed to
allow several hosts to share files over the network. One of
the most common uses of NFS is to allow diskless workstations
to be installed in offices, while keeping all disk storage in a
central location. As distributed by Sun, NFS has no security
features enabled. This means that any host on the Internet may
access your files via NFS, regardless of whether you trust them
or not.
Fortunately, there are several easy ways to make NFS more
secure. The more commonly used methods are described in this
section, and these can be used to make your files quite secure
from unauthorized access via NFS. Secure NFS, introduced in
SunOS Release 4.0, takes security one step further, using
public-key encryption techniques to ensure authorized access.
Discussion of secure NFS is deferred until Section 4.
2.2.3.1 The exports File
The file /_e_t_c/_e_x_p_o_r_t_s [Sun88a, 1377] is perhaps one of the
most important parts of NFS configuration. This file lists
which file systems are exported (made available for mounting)
to other systems. A typical _e_x_p_o_r_t_s file as installed by the
Sun installation procedure looks something like this:
/usr
/home
/var/spool/mail
#
/export/root/client1 -access=client1,root=client1
/export/swap/client1 -access=client1,root=client1
#
/export/root/client2 -access=client2,root=client2
/export/swap/client2 -access=client2,root=client2
The _r_o_o_t= keyword specifies the list of hosts that are allowed
to have super-user access to the files in the named file
system. This keyword is discussed in detail in Section
16
2.2.3.3. The _a_c_c_e_s_s= keyword specifies the list of hosts
(separated by colons) that are allowed to mount the named file
system. If no _a_c_c_e_s_s= keyword is specified for a file system,
any host anywhere on the network may mount that file system via
NFS.
Obviously, this presents a major security problem, since
anyone who can mount your file systems via NFS can then peruse
them at her leisure. Thus, it is important that all file sys-
tems listed in _e_x_p_o_r_t_s have an _a_c_c_e_s_s= keyword associated with
them. If you have only a few hosts which must mount a file
system, you can list them individually in the file:
/usr -access=host1:host2:host3:host4:host5
However, because the maximum number of hosts that can be listed
this way is ten, the _a_c_c_e_s_s= keyword will also allow netgroups
to be specified. Netgroups are described in the next section.
After making any changes to the _e_x_p_o_r_t_s file, you should
run the command
# exportfs -a
in order to make the changes take effect.
2.2.3.2 The netgroup File
The file /_e_t_c/_n_e_t_g_r_o_u_p [Sun88a, 1407] is used to define
netgroups. This file is controlled by Yellow Pages, and must
be rebuilt in the Yellow Pages maps whenever it is modified.
Consider the following sample _n_e_t_g_r_o_u_p file:
A_Group (servera,,) (clienta1,,) (clienta2,,)
B_Group (serverb,,) (clientb1,,) (clientb2,,)
AdminStaff (clienta1,mary,) (clientb3,joan,)
AllSuns A_Group B_Group
This file defines four netgroups, called _A__G_r_o_u_p, _B__G_r_o_u_p,
_A_d_m_i_n_S_t_a_f_f, and _A_l_l_S_u_n_s. The _A_l_l_S_u_n_s netgroup is actually a
``super group'' containing all the members of the _A__G_r_o_u_p and
_B__G_r_o_u_p netgroups.
Each member of a netgroup is defined as a triple: (host,
user, domain). Typically, the _d_o_m_a_i_n field is never used, and
is simply left blank. If either the _h_o_s_t or _u_s_e_r field is left
17
blank, then any host or user is considered to match. Thus the
triple (host,,) matches any user on the named host, while the
triple (,user,) matches the named user on any host.
Netgroups are useful when restricting access to NFS file
systems via the _e_x_p_o_r_t_s file. For example, consider this modi-
fied version of the file from the previous section:
/usr -access=A_Group
/home -access=A_Group:B_Group
/var/spool/mail -access=AllSuns
#
/export/root/client1 -access=client1,root=client1
/export/swap/client1 -access=client1,root=client1
#
/export/root/client2 -access=client2,root=client2
/export/swap/client2 -access=client2,root=client2
The /_u_s_r file system may now only be mounted by the hosts in
the _A__G_r_o_u_p netgroup, that is, _s_e_r_v_e_r_a, _c_l_i_e_n_t_a_1, and _c_l_i_e_n_t_a_2.
Any other host that tries to mount this file system will
receive an ``access denied'' error. The /_h_o_m_e file system may
be mounted by any of the hosts in either the _A__G_r_o_u_p or _B__G_r_o_u_p
netgroups. The /_v_a_r/_s_p_o_o_l/_m_a_i_l file system is also restricted
to these hosts, but in this example we used the ``super group''
called _A_l_l_S_u_n_s.
Generally, the best way to configure the _n_e_t_g_r_o_u_p file is
to make a single netgroup for each file server and its clients,
and then to make other super groups, such as _A_l_l_S_u_n_s. This
allows you the flexibility to specify the smallest possible
group of hosts for each file system in /_e_t_c/_e_x_p_o_r_t_s.
Netgroups can also be used in the password file to allow
access to a given host to be restricted to the members of that
group, and they can be used in the _h_o_s_t_s._e_q_u_i_v file to central-
ize maintenance of the list of trusted hosts. The procedures
for doing this are defined in more detail in the Sun manual.
2.2.3.3 Restricting Super-User Access
Normally, NFS translates the super-user id to a special id
called ``nobody'' in order to prevent a user with ``root'' on a
remote workstation from accessing other people's files. This
is good for security, but sometimes a nuisance for system
administration, since you cannot make changes to files as
``root'' through NFS.
The _e_x_p_o_r_t_s file also allows you to grant super-user
18
access to certain file systems for certain hosts by using the
_r_o_o_t= keyword. Following this keyword a colon-separated list
of up to ten hosts may be specified; these hosts will be
allowed to access the file system as ``root'' without having
the user id converted to ``nobody.'' Netgroups may not be
specified to the _r_o_o_t= keyword.
Granting ``root'' access to a host should not be done
lightly. If a host has ``root'' access to a file system, then
the super-user on that host will have complete access to the
file system, just as if you had given him the ``root'' password
on the server. Untrusted hosts should never be given ``root''
access to NFS file systems.
2.2.4 FTP
The File Transfer Protocol, implemented by the _f_t_p and
_f_t_p_d programs [Sun88a, 195-201, 1632-1634], allows users to
connect to remote systems and transfer files back and forth.
Unfortunately, older versions of these programs also had
several bugs in them that allowed crackers to break into a sys-
tem. These bugs have been fixed by Berkeley, and new versions
are available. If your _f_t_p_d* was obtained before December
1988, you should get a newer version (see Section 4).
One of the more useful features of FTP is the
``anonymous'' login. This special login allows users who do
not have an account on your machine to have restricted access
in order to transfer files from a specific directory. This is
useful if you wish to distribute software to the public at
large without giving each person who wants the software an
account on your machine. In order to securely set up anonymous
FTP you should follow the specific instructions below:
1. Create an account called ``ftp.'' Disable the
account by placing an asterisk (*) in the password
field. Give the account a special home directory,
such as /_u_s_r/_f_t_p or /_u_s_r/_s_p_o_o_l/_f_t_p.
2. Make the home directory owned by ``ftp'' and unwrit-
able by anyone:
# chown ftp ~ftp
# chmod 555 ~ftp
_________________________
* On Sun systems, _f_t_p_d is stored in the file /_u_s_r/_e_t_c/_i_n._f_t_p_d.
On most other systems, it is called /_e_t_c/_f_t_p_d.
19
3. Make the directory ~_f_t_p/_b_i_n, owned by the super-user
and unwritable by anyone. Place a copy of the _l_s
program in this directory:
# mkdir ~ftp/bin
# chown root ~ftp/bin
# chmod 555 ~ftp/bin
# cp -p /bin/ls ~ftp/bin
# chmod 111 ~ftp/bin/ls
4. Make the directory ~_f_t_p/_e_t_c, owned by the super-user
and unwritable by anyone. Place copies of the pass-
word and group files in this directory, with all the
password fields changed to asterisks (*). You may
wish to delete all but a few of the accounts and
groups from these files; the only account that must
be present is ``ftp.''
# mkdir ~ftp/etc
# chown root ~ftp/etc
# chmod 555 ~ftp/etc
# cp -p /etc/passwd /etc/group ~ftp/etc
# chmod 444 ~ftp/etc/passwd ~ftp/etc/group
5. Make the directory ~_f_t_p/_p_u_b, owned by ``ftp'' and
world-writable. Users may then place files that are
to be accessible via anonymous FTP in this directory:
# mkdir ~ftp/pub
# chown ftp ~ftp/pub
# chmod 777 ~ftp/pub
Because the anonymous FTP feature allows anyone to access
your system (albeit in a very limited way), it should not be
made available on every host on the network. Instead, you
should choose one machine (preferably a server or standalone
host) on which to allow this service. This makes monitoring
for security violations much easier. If you allow people to
transfer files to your machine (using the world-writable _p_u_b
directory, described above), you should check often the con-
tents of the directories into which they are allowed to write.
Any suspicious files you find should be deleted.
2.2.4.1 Trivial FTP
The Trivial File Transfer Protocol, TFTP, is used on Sun
20
workstations (and others) to allow diskless hosts to boot from
the network. Basically, TFTP is a stripped-down version of FTP
- there is no user authentication, and the connection is based
on the User Datagram Protocol instead of the Transmission Con-
trol Protocol. Because they are so stripped-down, many imple-
mentations of TFTP have security holes. You should check your
hosts by executing the command sequence shown below.
% tftp
tftp> connect _y_o_u_r_h_o_s_t
tftp> get /etc/motd tmp
_E_r_r_o_r _c_o_d_e _1: _F_i_l_e _n_o_t _f_o_u_n_d
_t_f_t_p> _q_u_i_t
%
If your version does not respond with ``_F_i_l_e _n_o_t _f_o_u_n_d,'' and
instead transfers the file, you should replace your version of
_t_f_t_p_d* with a newer one. In particular, versions of SunOS
prior to release 4.0 are known to have this problem.
2.2.5 Mail
Electronic mail is one of the main reasons for connecting
to outside networks. On most versions of Berkeley-derived UNIX
systems, including those from Sun, the _s_e_n_d_m_a_i_l program
[Sun88a, 1758-1760; Sun88b, 441-488] is used to enable the
receipt and delivery of mail. As with the FTP software, older
versions of _s_e_n_d_m_a_i_l have several bugs that allow security vio-
lations. One of these bugs was used with great success by the
Internet worm [Seel88, Spaf88]. The current version of _s_e_n_d_-
_m_a_i_l from Berkeley is version 5.61, of January 1989. Sun is,
as of this writing, still shipping version 5.59, which has a
known security problem. They have, however, made a fixed ver-
sion available. Section 4 details how to obtain these newer
versions.
Generally, with the exception of the security holes men-
tioned above, _s_e_n_d_m_a_i_l is reasonably secure when installed by
most vendors' installation procedures. There are, however, a
few precautions that should be taken to ensure secure opera-
tion:
1. Remove the ``decode'' alias from the aliases file
(/_e_t_c/_a_l_i_a_s_e_s or /_u_s_r/_l_i_b/_a_l_i_a_s_e_s).
_________________________
* On Sun systems, _t_f_t_p_d is stored in the file
/_u_s_r/_e_t_c/_i_n._t_f_t_p_d. On most other systems, it is called
/_e_t_c/_t_f_t_p_d.
21
2. If you create aliases that allow messages to be sent
to programs, be absolutely sure that there is no way
to obtain a shell or send commands to a shell from
these programs.
3. Make sure the ``wizard'' password is disabled in the
configuration file, _s_e_n_d_m_a_i_l._c_f. (Unless you modify
the distributed configuration files, this shouldn't
be a problem.)
4. Make sure your _s_e_n_d_m_a_i_l does not support the
``debug'' command. This can be done with the follow-
ing commands:
% telnet localhost 25
220 yourhost Sendmail 5.61 ready at 9 Mar 90 10:57:36 PST
debug
500 Command unrecognized
quit
%
If your _s_e_n_d_m_a_i_l responds to the ``debug'' command
with ``_2_0_0 _D_e_b_u_g _s_e_t,'' then you are vulnerable to
attack and should replace your _s_e_n_d_m_a_i_l with a newer
version.
By following the procedures above, you can be sure that your
mail system is secure.
2.2.6 Finger
The ``finger'' service, provided by the _f_i_n_g_e_r program
[Sun88a, 186-187], allows you to obtain information about a
user such as her full name, home directory, last login time,
and in some cases when she last received mail and/or read her
mail. The _f_i_n_g_e_r_d program [Sun88a, 1625] allows users on
remote hosts to obtain this information.
A bug in _f_i_n_g_e_r_d was also exercised with success by the
Internet worm [Seel88, Spaf88]. If your version of _f_i_n_g_e_r_d* is
older than November 5, 1988, it should be replaced with a newer
version. New versions are available from several of the
sources described in Section 4.
_________________________
* On Sun systems, _f_i_n_g_e_r_d is stored in /_u_s_r/_e_t_c/_i_n._f_i_n_g_e_r_d. On
most other systems, it is called /_e_t_c/_f_i_n_g_e_r_d.
22
2.2.7 Modems and Terminal Servers
Modems and terminal servers (terminal switches, Annex
boxes, etc.) present still another potential security problem.
The main problem with these devices is one of configuration -
misconfigured hardware can allow security breaches. Explaining
how to configure every brand of modem and terminal server would
require volumes. However, the following items should be
checked for on any modems or terminal servers installed at your
site:
1. If a user dialed up to a modem hangs up the phone,
the system should log him out. If it doesn't, check
the hardware connections and the kernel configuration
of the serial ports.
2. If a user logs off, the system should force the modem
to hang up. Again, check the hardware connections if
this doesn't work.
3. If the connection from a terminal server to the sys-
tem is broken, the system should log the user off.
4. If the terminal server is connected to modems, and
the user hangs up, the terminal server should inform
the system that the user has hung up.
Most modem and terminal server manuals cover in detail how
to properly connect these devices to your system. In particu-
lar you should pay close attention to the ``Carrier Detect,''
``Clear to Send,'' and ``Request to Send'' connections.
2.2.8 Firewalls
One of the newer ideas in network security is that of a
_f_i_r_e_w_a_l_l. Basically, a firewall is a special host that sits
between your outside-world network connection(s) and your
internal network(s). This host does not send out routing
information about your internal network, and thus the internal
network is ``invisible'' from the outside. In order to config-
ure a firewall machine, the following considerations need to be
taken:
1. The firewall does not advertise routes. This means
that users on the internal network must log in to the
firewall in order to access hosts on remote networks.
Likewise, in order to log in to a host on the
23
internal network from the outside, a user must first
log in to the firewall machine. This is incon-
venient, but more secure.
2. All electronic mail sent by your users must be for-
warded to the firewall machine if it is to be
delivered outside your internal network. The
firewall must receive all incoming electronic mail,
and then redistribute it. This can be done either
with aliases for each user or by using name server MX
records.
3. The firewall machine should not mount any file sys-
tems via NFS, or make any of its file systems avail-
able to be mounted.
4. Password security on the firewall must be rigidly
enforced.
5. The firewall host should not trust any other hosts
regardless of where they are. Furthermore, the
firewall should not be trusted by any other host.
6. Anonymous FTP and other similar services should only
be provided by the firewall host, if they are pro-
vided at all.
The purpose of the firewall is to prevent crackers from
accessing other hosts on your network. This means, in general,
that you must maintain strict and rigidly enforced security on
the firewall, but the other hosts are less vulnerable, and
hence security may be somewhat lax. But it is important to
remember that the firewall is not a complete cure against
crackers - if a cracker can break into the firewall machine, he
can then try to break into any other host on your network.
2.3 FILE SYSTEM SECURITY
The last defense against system crackers are the permis-
sions offered by the file system. Each file or directory has
three sets of permission bits associated with it: one set for
the user who owns the file, one set for the users in the group
with which the file is associated, and one set for all other
users (the ``world'' permissions). Each set contains three
identical permission bits, which control the following:
_r_e_a_d If set, the file or directory may be read. In
the case of a directory, read access allows a
user to see the contents of a directory (the
24
names of the files contained therein), but not to
access them.
_w_r_i_t_e If set, the file or directory may be written
(modified). In the case of a directory, write
permission implies the ability to create, delete,
and rename files. Note that the ability to
remove a file is _n_o_t controlled by the permis-
sions on the file, but rather the permissions on
the directory containing the file.
_e_x_e_c_u_t_e If set, the file or directory may be executed
(searched). In the case of a directory, execute
permission implies the ability to access files
contained in that directory.
In addition, a fourth permission bit is available in each
set of permissions. This bit has a different meaning in each
set of permission bits:
_s_e_t_u_i_d If set in the owner permissions, this bit controls
the ``set user id'' (setuid) status of a file.
Setuid status means that when a program is exe-
cuted, it executes with the permissions of the
user owning the program, in addition to the per-
missions of the user executing the program. For
example, _s_e_n_d_m_a_i_l is setuid ``root,'' allowing it
to write files in the mail queue area, which nor-
mal users are not allowed to do. This bit is
meaningless on nonexecutable files.
_s_e_t_g_i_d If set in the group permissions, this bit controls
the ``set group id'' (setgid) status of a file.
This behaves in exactly the same way as the setuid
bit, except that the group id is affected instead.
This bit is meaningless on non-executable files
(but see below).
_s_t_i_c_k_y If set in the world permissions, the ``sticky''
bit tells the operating system to do special
things with the text image of an executable file.
It is mostly a holdover from older versions of
UNIX, and has little if any use today. This bit
is also meaningless on nonexecutable files (but
see below).
2.3.1 Setuid Shell Scripts
Shell scripts that have the setuid or setgid bits set on
25
them are _n_o_t secure, regardless of how many safeguards are taken
when writing them. There are numerous software packages avail-
able that claim to make shell scripts secure, but every one
released so far has not managed to solve all the problems.
Setuid and setgid shell scripts should never be allowed on
any UNIX system.
2.3.2 The Sticky Bit on Directories
Newer versions of UNIX have attached a new meaning to the
sticky bit. When this bit is set on a directory, it means that
users may not delete or rename other users' files in this direc-
tory. This is typically useful for the /_t_m_p directory. Nor-
mally, /_t_m_p is world-writable, enabling any user to delete
another user's files. By setting the sticky bit on /_t_m_p, users
may only delete their own files from this directory.
To set the sticky bit on a directory, use the command
# chmod o+t _d_i_r_e_c_t_o_r_y
2.3.3 The Setgid Bit on Directories
In SunOS 4.0, the setgid bit was also given a new meaning.
Two rules can be used for assigning group ownership to a file in
SunOS:
1. The System V mechanism, which says that a user's pri-
mary group id (the one listed in the password file) is
assigned to any file he creates.
2. The Berkeley mechanism, which says that the group id of
a file is set to the group id of the directory in which
it is created.
If the setgid bit is set on a directory, the Berkeley
mechanism is enabled. Otherwise, the System V mechanism is
enabled. Normally, the Berkeley mechanism is used; this mechan-
ism must be used if creating directories for use by more than one
member of a group (see Section 2.1.5).
To set the setgid bit on a directory, use the command
26
# chmod g+s _d_i_r_e_c_t_o_r_y
2.3.4 The umask Value
When a file is created by a program, say a text editor or a
compiler, it is typically created with all permissions enabled.
Since this is rarely desirable (you don't want other users to be
able to write your files), the _u_m_a_s_k value is used to modify the
set of permissions a file is created with. Simply put, while the
_c_h_m_o_d command [Sun88a, 65-66] specifies what bits should be
turned _o_n, the umask value specifies what bits should be turned
_o_f_f.
For example, the default umask on most systems is 022. This
means that write permission for the group and world should be
turned off whenever a file is created. If instead you wanted to
turn off all group and world permission bits, such that any file
you created would not be readable, writable, or executable by
anyone except yourself, you would set your umask to 077.
The umask value is specified in the ._c_s_h_r_c or ._p_r_o_f_i_l_e files
read by the shell using the _u_m_a_s_k command [Sun88a, 108, 459].
The ``root'' account should have the line
umask 022
in its /._c_s_h_r_c file, in order to prevent the accidental creation
of world-writable files owned by the super-user.
2.3.5 Encrypting Files
The standard UNIX _c_r_y_p_t command [Sun88a, 95] is not at all
secure. Although it is reasonable to expect that _c_r_y_p_t will keep
the casual ``browser'' from reading a file, it will present noth-
ing more than a minor inconvenience to a determined cracker.
_C_r_y_p_t implements a one-rotor machine along the lines of the Ger-
man Enigma (broken in World War II). The methods of attack on
such a machine are well known, and a sufficiently large file can
usually be decrypted in a few hours even without knowledge of
what the file contains [Reed84]. In fact, publicly available
packages of programs designed to ``break'' files encrypted with
_c_r_y_p_t have been around for several years.
There are software implementations of another algorithm, the
Data Encryption Standard (DES), available on some systems.
27
Although this algorithm is much more secure than _c_r_y_p_t, it has
never been proven that it is totally secure, and many doubts
about its security have been raised in recent years.
Perhaps the best thing to say about encrypting files on a
computer system is this: if you think you have a file whose con-
tents are important enough to encrypt, then that file should not
be stored on the computer in the first place. This is especially
true of systems with limited security, such as UNIX systems and
personal computers.
It is important to note that UNIX passwords are _n_o_t
encrypted with the _c_r_y_p_t program. Instead, they are encrypted
with a modified version of the DES that generates one-way encryp-
tions (that is, the password cannot be decrypted). When you log
in, the system does not decrypt your password. Instead, it
encrypts your attempted password, and if this comes out to the
same result as encrypting your real password, you are allowed to
log in.
2.3.6 Devices
The security of devices is an important issue in UNIX. Dev-
ice files (usually residing in /_d_e_v) are used by various programs
to access the data on the disk drives or in memory. If these
device files are not properly protected, your system is wide open
to a cracker. The entire list of devices is too long to go into
here, since it varies widely from system to system. However, the
following guidelines apply to all systems:
1. The files /_d_e_v/_k_m_e_m, /_d_e_v/_m_e_m, and /_d_e_v/_d_r_u_m should
never be readable by the world. If your system sup-
ports the notion of the ``kmem'' group (most newer sys-
tems do) and utilities such as _p_s are setgid ``kmem,''
then these files should be owned by user ``root'' and
group ``kmem,'' and should be mode 640. If your system
does not support the notion of the ``kmem'' group, and
utilities such as _p_s are setuid ``root,'' then these
files should be owned by user ``root'' and mode 600.
2. The disk devices, such as /_d_e_v/_s_d_0_a, /_d_e_v/_r_x_y_1_b, etc.,
should be owned by user ``root'' and group ``opera-
tor,'' and should be mode 640. Note that each disk has
eight partitions and two device files for each parti-
tion. Thus, the disk ``sd0'' would have the following
device files associated with it in /_d_e_v:
28
sd0a sd0e rsd0a rsd0e
sd0b sd0f rsd0b rsd0f
sd0c sd0g rsd0c rsd0g
sd0d sd0h rsd0d rsd0h
3. With very few exceptions, all other devices should be
owned by user ``root.'' One exception is terminals,
which are changed to be owned by the user currently
logged in on them. When the user logs out, the owner-
ship of the terminal is automatically changed back to
``root.''
2.4 SECURITY IS YOUR RESPONSIBILITY
This section has detailed numerous tools for improving secu-
rity provided by the UNIX operating system. The most important
thing to note about these tools is that although they are avail-
able, they are typically not put to use in most installations.
Therefore, it is incumbent on you, the system administrator, to
take the time and make the effort to enable these tools, and thus
to protect your system from unauthorized access.
29
30
SECTION 3
MONITORING SECURITY
One of the most important tasks in keeping any computer sys-
tem secure is monitoring the security of the system. This
involves examining system log files for unauthorized accesses of
the system, as well as monitoring the system itself for security
holes. This section describes the procedures for doing this. An
additional part of monitoring security involves keeping abreast
of security problems found by others; this is described in Sec-
tion 5.
3.1 ACCOUNT SECURITY
Account security should be monitored periodically in order
to check for two things: users logged in when they ``shouldn't''
be (e.g., late at night, when they're on vacation, etc.), and
users executing commands they wouldn't normally be expected to
use. The commands described in this section can be used to
obtain this information from the system.
3.1.1 The lastlog File
The file /_u_s_r/_a_d_m/_l_a_s_t_l_o_g [Sun88a, 1485] records the most
recent login time for each user of the system. The message
printed each time you log in, e.g.,
Last login: Sat Mar 10 10:50:48 from spam.itstd.sri.c
uses the time stored in the _l_a_s_t_l_o_g file. Additionally, the last
login time reported by the _f_i_n_g_e_r command uses this time. Users
should be told to carefully examine this time whenever they log
in, and to report unusual login times to the system administra-
tor. This is an easy way to detect account break-ins, since each
user should remember the last time she logged into the system.
3.1.2 The utmp and wtmp Files
The file /_e_t_c/_u_t_m_p [Sun88a, 1485] is used to record who is
31
currently logged into the system. This file can be displayed
using the _w_h_o command [Sun88a, 597]:
% who
hendra tty0c Mar 13 12:31
heidari tty14 Mar 13 13:54
welgem tty36 Mar 13 12:15
reagin ttyp0 Mar 13 08:54 (aaifs.itstd.sri.)
ghg ttyp1 Mar 9 07:03 (hydra.riacs.edu)
compion ttyp2 Mar 1 03:01 (ei.ecn.purdue.ed)
For each user, the login name, terminal being used, login time,
and remote host (if the user is logged in via the network) are
displayed.
The file /_u_s_r/_a_d_m/_w_t_m_p [Sun88a, 1485] records each login and
logout time for every user. This file can also be displayed
using the _w_h_o command:
% who /usr/adm/wtmp
davy ttyp4 Jan 7 12:42 (annex01.riacs.ed)
ttyp4 Jan 7 15:33
davy ttyp4 Jan 7 15:33 (annex01.riacs.ed)
ttyp4 Jan 7 15:35
hyder ttyp3 Jan 8 09:07 (triceratops.itst)
ttyp3 Jan 8 11:43
A line that contains a login name indicates the time the user
logged in; a line with no login name indicates the time that the
terminal was logged off. Unfortunately, the output from this
command is rarely as simple as in the example above; if several
users log in at once, the login and logout times are all mixed
together and must be matched up by hand using the terminal name.
The _w_t_m_p file may also be examined using the _l_a_s_t command
[Sun88a, 248]. This command sorts out the entries in the file,
matching up login and logout times. With no arguments, _l_a_s_t
displays all information in the file. By giving the name of a
user or terminal, the output can be restricted to the information
about the user or terminal in question. Sample output from the
_l_a_s_t command is shown below.
% last
davy ttyp3 intrepid.itstd.s Tue Mar 13 10:55 - 10:56 (00:00)
hyder ttyp3 clyde.itstd.sri. Mon Mar 12 15:31 - 15:36 (00:04)
reboot ~ Mon Mar 12 15:16
shutdown ~ Mon Mar 12 15:16
arms ttyp3 clyde0.itstd.sri Mon Mar 12 15:08 - 15:12 (00:04)
hyder ttyp3 spam.itstd.sri.c Sun Mar 11 21:08 - 21:13 (00:04)
reboot ~ Sat Mar 10 20:05
davy ftp hydra.riacs.edu Sat Mar 10 13:23 - 13:30 (00:07)
32
For each login session, the user name, terminal used, remote host
(if the user logged in via the network), login and logout times,
and session duration are shown. Additionally, the times of all
system shutdowns and reboots (generated by the _s_h_u_t_d_o_w_n and
_r_e_b_o_o_t commands [Sun88a, 1727, 1765]) are recorded. Unfor-
tunately, system crashes are not recorded. In newer versions of
the operating system, pseudo logins such as those via the _f_t_p
command are also recorded; an example of this is shown in the
last line of the sample output, above.
3.1.3 The acct File
The file /_u_s_r/_a_d_m/_a_c_c_t [Sun88a, 1344-1345] records each exe-
cution of a command on the system, who executed it, when, and how
long it took. This information is logged each time a command
completes, but only if your kernel was compiled with the SYSACCT
option enabled (the option is enabled in some GENERIC kernels,
but is usually disabled by default).
The _a_c_c_t file can be displayed using the _l_a_s_t_c_o_m_m command
[Sun88a, 249]. With no arguments, all the information in the
file is displayed. However, by giving a command name, user name,
or terminal name as an argument, the output can be restricted to
information about the given command, user, or terminal. Sample
output from _l_a_s_t_c_o_m_m is shown below.
% lastcomm
sh S root __ 0.67 secs Tue Mar 13 12:45
atrun root __ 0.23 secs Tue Mar 13 12:45
lpd F root __ 1.06 secs Tue Mar 13 12:44
lpr S burwell tty09 1.23 secs Tue Mar 13 12:44
troff burwell tty09 12.83 secs Tue Mar 13 12:44
eqn burwell tty09 1.44 secs Tue Mar 13 12:44
df kindred ttyq7 0.78 secs Tue Mar 13 12:44
ls kindred ttyq7 0.28 secs Tue Mar 13 12:44
cat kindred ttyq7 0.05 secs Tue Mar 13 12:44
stty kindred ttyq7 0.05 secs Tue Mar 13 12:44
tbl burwell tty09 1.08 secs Tue Mar 13 12:44
rlogin S jones ttyp3 5.66 secs Tue Mar 13 12:38
rlogin F jones ttyp3 2.53 secs Tue Mar 13 12:41
stty kindred ttyq7 0.05 secs Tue Mar 13 12:44
The first column indicates the name of the command. The next
column displays certain flags on the command: an ``F'' means the
process spawned a child process, ``S'' means the process ran with
the set-user-id bit set, ``D'' means the process exited with a
core dump, and ``X'' means the process was killed abnormally.
The remaining columns show the name of the user who ran the
program, the terminal he ran it from (if applicable), the amount
33
of CPU time used by the command (in seconds), and the date and
time the process started.
3.2 NETWORK SECURITY
Monitoring network security is more difficult, because there
are so many ways for a cracker to attempt to break in. However,
there are some programs available to aid you in this task. These
are described in this section.
3.2.1 The syslog Facility
The _s_y_s_l_o_g facility [Sun88a, 1773] is a mechanism that
enables any command to log error messages and informational mes-
sages to the system console, as well as to a log file. Typi-
cally, error messages are logged in the file /_u_s_r/_a_d_m/_m_e_s_s_a_g_e_s
along with the date, time, name of the program sending the mes-
sage, and (usually) the process id of the program. A sample seg-
ment of the _m_e_s_s_a_g_e_s file is shown below.
Mar 12 14:53:37 sparkyfs login: ROOT LOGIN ttyp3 FROM setekfs.itstd.sr
Mar 12 15:18:08 sparkyfs login: ROOT LOGIN ttyp3 FROM setekfs.itstd.sr
Mar 12 16:50:25 sparkyfs login: ROOT LOGIN ttyp4 FROM pongfs.itstd.sri
Mar 12 16:52:20 sparkyfs vmunix: sd2c: read failed, no retries
Mar 13 06:01:18 sparkyfs vmunix: /: file system full
Mar 13 08:02:03 sparkyfs login: ROOT LOGIN ttyp4 FROM triceratops.itst
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