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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.
OTHER AREAS OF COMPROMISE
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
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 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
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 occurrence.
CONCLUSION
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.
REFERENCES
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