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Posted Sep 23, 1999


systems | unix
MD5 | 98666b2068b980cdfa3978f0c38c487d


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Security Holes in UNIX Systems

The DOE Computer Incident Advisory Capability (CIAC) at LLNL has learned
that there are two security holes which leave certain systems running the
UNIX Operating System vulnerable to intrusion. The holes, when used together
in a very specific scenario, permit an intruder to attack UNIX systems
over the Internet and other pathways, and to assume superuser privileges.
Therefore, these vulnerabilities constitute a highly severe threat. Sun
systems running SunOS 3.x and 4.0 are susceptible; the hole may exist in
other versions of the UNIX operating system as well. A brief description
of the holes follow:

1. There is a long-known hole in tftp, the user interface to the
Internet TFTP (Trivial File Transfer Protocol). This hole allows
any user, without first logging in, to read any readable file and
to write any writable file on a remote system via Internet. This
hole exists in SunOS 3.x, but has been fixed in SunOS 4.0.

To determine whether this hole affects a system, do the following:

Enter tftp local_testing_system at the UNIX prompt and follow
with a carriage return. For example,

% tftp styx <RET>

The system should answer with the tftp prompt:


Enter the following at the prompt:

tftp> get /etc/passwd stolen_pw

If "Error code 1: File not found" appears, the tested system is
safe from remote attacks. However, it may still be susceptible to
local attacks.

2. There are holes in rwalld (the network rwall server daemon) and
wall (write to all users). The command "wall" writes to filenames
encoded within /etc/utmp. You are vulnerable to this hole if
/etc/utmp is world and/or group writable. In essence, an intruder
can use this hole to write on any file on the computer system.

Any UNIX machine which runs a windowing system is likely to have the
privileges which create the described vulnerabilities. The presence of
an intruder who exploits these holes is difficult to detect. It is our
understanding that knowledge of these holes has been distributed widely
within the hacker community. It is advisable, based on your vulnerability,
to take appropriate action for your site.

CIAC is currently working with Sun Microsystems and DARPA's Computer
Emergency Response Team (CERT) to eliminate these vulnerabilities as
quickly as possible. You will be notified when a patch is available. For
further information, contact CIAC at (415) 422-8193 or (FTS) 532-8193, or
send e-mail to


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