Network Working Group A. Gwinn
INTERNET-DRAFT Networld+Interop NOC Team
Obsoletes: None April 1997
Category: Informational
Network Security For Large Trade Shows
<draft-rfced-info-gwinn-00.txt>
Status of this Memo
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Abstract
This document is designed to assist vendors and other participants in
large trade shows, such as Networld+Interop, in designing effective
protection against network and system attacks by unauthorized
individuals. Generally, it has been observed that many system
administrators and trade show coordinators tend to overlook the
importance of system security at trade shows. In fact, systems at
trade shows are just as prone to attack as office-based platforms.
Trade show systems should be treated as seriously as an office
computer. A breach of security of a trade show system can render (and
has rendered) a vendor's demonstrations inoperable--quite possibly
for the entire show!
This dcoument is not intended to replace the multitudes of comprehensive
books on the subject of Internet security. Rather, its purpose is to
provide a checklist-style collection of frequently overlooked, simple
ways to minimize the chance of a costly attack. Vendors are
encouraged to pay special attention to this document and share it
with all associated representatives.
Physical Security
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Before addressing the technical, one of the most frequently
underrated (and overlooked) security breaches is the simple low-tech
attack. The common victim is the one who leaves a console logged in,
perhaps as root, and walks away. Other times, an anonymous "helpful
soul" might ask for a password in order to assist the user in
"identifying a problem." This type of method allows an intruder,
especially one logged in as "root", access to system files.
Tips:
* Educate sales and support staff as to the importance of keeping
an eye on logged-in systems--especially "root" or other
privileged accounts.
* Identify individuals who are not using exhibit systems for their
intended purpose (i.e. playing "Quake" or "Doom" on one of
your workstations).
* Request identification from anyone wishing to access systems
for maintenance purposes unless they are known personally.
System Security
This section discusses technical security procedures for workstations
on the vendor network. Although primarily aimed at Unix systems,
many general procedures can be applied to other platforms.
Password Security
Lack of passwords or easy to guess passwords are a relatively low-
tech door into systems, but are responsible for a significant number
of breakins. Good passwords are a cornerstone of system security.
Tips:
* Check /etc/passwd for lack of passwords. Some vendors ship systems
with null passwords, in some cases even in root accounts.
* Change passwords, especially system passwords.
* Mix case, numbers and punctuation especially on root passwords.
* Change system passwords on a regular basis.
Extra "root" Accounts
Some system vendors have been known to ship systems with accounts,
other than root, that have root privileges (UID=0). For example, some
vendors may include a separate system administration account that
places a user in a specific administrative program. If a system does
not need additional root accounts, these can be disabled by placing
"*" in the password field of /etc/passwd. Check all systems for extra
"root" accounts and either disable them or change their password as
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appropriate.
Use of Authentication Tokens
Authentication tokens such as SecureID, Cryptocard, DES Gold and
others, provide a method of producing "one-use" passwords for
specific access. The advantages are obvious. Remember that there are
many packet sniffers and other administration tools constantly
watching the network-especially at a large network-oriented trade
show. Typed passwords, by default, are sent clear text across the
network, allowing others to view them. Authentication tokens provide
a password that is only valid for that one instance, and are useless
after they are used. A logical extension of the use of authentication
tokens would be to use them for "return trips home" (show network
back to a home site) to minimize the chance of off-site security
problems.
Tips:
* Contact vendors of authentication tokens/cards for further
information as to how to integrate into specific environments, or
on to specific platforms.
* The public-domain utility "cryptosu" (csu), when used with a
Cryptocard, provides a replacement for Unix's "su" command,
employing a challenge/response style of authentication for root
access.
Anonymous FTP
Anonymous FTP accounts can easily turn into a security hole. The
simplest rule-of-thumb to follow is to disable this service if it is
not specifically needed. However, if anonymous FTP is to be used,
the following tips may provide assistance into securing it.
* When a user logs in as "anonymous", they should be locked into a
specific directory tree. Be sure that it properly chroots to the
appropriate directory. A "cd /" should put an anonymous user at the
top of a tree.
* Some systems may allow symbolic links to take a user outside the
allowed tree. Verify all links inside the anonymous hierarchy.
* Make sure that ftp's root directory is owned by someone other than
the 'ftp' account. Typically, it should be owned by "root".
* Examine the need for a world-writeable incoming directory. Many
sites use these as a way for outside users to transfer files into
the site. This, however, can turn into an archive (and frequently
does) for stolen software. Removing the "read" bit from the
directory permissions (chmod 733) prohibits an anonymous user from
being able to list the contents of a directory. Files can be
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deposited as usual, but not retrieved unless the exact name of the
file is known.
NFS Exports
Exporting an writeable NFS filesystem to the world grants anyone the
ability to read and write any file in the exported mount point. If
this is done, for example, with a system directory such as "/" or
"/etc", it is a simple matter to edit password files to create one-
self access to a system. Therefore, /etc/exports should be closely
examined to be certain that nothing of a sensitive nature is exported
to anyone but another trusted host. Anything exported to the general
public should be exported "read-only".
Trusted Hosts
Trusted host entries are a method for allowing other hosts
"equivalent" security access to another host computer. Some vendors
ship systems with open trusted host files. This should be addressed.
Tips:
* Check for a '+' entry (all systems trusted) in /etc/hosts.equiv and
all ".rhosts" files (there may be multiple .rhosts files) and
remove it.
* Check for an "xhost +" entry in the "...X11/xdm/Xsession" file.
Most often, an "xhost" entry will appear with a pathname such as
"/usr/local/lib/xhost +". This should be disabled.
SetUID and SetGID binaries
The "suid" bit on a system executable program allows the program to
execute as the owner. A program that is setUID to "root" will allow
the program to execute with root privileges. There are multiple
legitimate reasons for a program to have root privileges, and many
do. However, it may be unusual to have suid programs in user
directories, or other non-system places. A scan of the filesystems
can turn up any program with its suid or sgid bit set. Before
disabling any programs, however, it is strongly suggested that the
legitimacy be confirmed.
Tips:
* "find / -user root -perm -4000 -print" will find any occurrence of
a setuid file anywhere in the system, including those on NFS
mounted partitions.
* "find / -group kmem -perm -2000 -print" will do the same for kmem
group permissions.
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System Directory Ownership and Write Permissions
Check ownership of all system directories and permissions needed to
write or modify files. A directory with permissions such as
"drwxrwxrwx" (such as /tmp) is world-writeable and anyone can create
or modify files in such area. Pay special attention to "/" and
"/etc". These should be owned by some system account-not by an
individual user. If in doubt, contact the vendor of the system
software for confirmation of these settings.
Network Services in /etc/inetd
Any servers not needed should be disabled. The notorious "R services"
(rexec, rsh, and rlogin) are particularly prone to security problems
and should be disabled unless specifically needed. If "R services"
are required, pay particular attention to trusted hosts, and be aware
of the risk of IP spoofing attacks from machines "pretending" to be
trusted hosts.
Tips:
* Comment out "R services" (rexec, rsh, rlogin) in /etc/inetd.
* Check for unknown or unneeded services.
Trivial File Transfer Protocol (TFTP)
TFTP can be an easy way for an intruder to access system files. A
good practice is to disable TFTP if it is not needed. If it is
needed, check to see that sensitive files are not accessible.
Attempting to tftp files such as /etc/passwd or /etc/motd will verify
accessiblity of a system from the outside.
TCP Connection Montoring
Public domain software (TCP Wrappers or "tcpd") allows TCP
connections to be restricted and monitored on a host by host basis.
This software can be configured to notify an administrator (as well
as syslog) attempts to access the host by unauthorized parties. This
software is available from:
ftp://info.cert.org/pub/tools/tcp_wrappers/
BIND (Berkeley Internet Name Daemon)
Earlier versions of BIND have been prone to various attacks. If a
host is going to be acting as DNS, the latest version of BIND should
be used. It can be downloaded from:
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ftp://ftp.isc.org/isc/bind
Sendmail and Mailer Security
A great number of previous versions of Sendmail have known security
holes. All Sendmail versions should be checked for the most recent
version. Alternatively, operating system vendors should be consulted
for their most recent release.
Web Server cgi-bin Security
All server cgi-bin scripts and binaries should be checked (especially
the "...httpd/cgi-bin" directory) for those that allow shell commands
to be executed. Many attacks, of recent months, have centered around
the use of utilities such as "phf" for accessing /etc/passwd on a
target system. Any cgi that is not needed in the course of operation
of a web server should be removed.
Other Suggestions
* Check with the vendor of operating systems for known security
issues. Make certain that all systems have the latest version of
the software as well as any security patches to fix specific
problems.
* Examine log files on the host frequently. The "last" command will
furnish information on recent logins and where they came from. The
"syslogs" will contain more specific information on system events.
Web server logfiles (...httpd/log/access_log and
...httpd/log/error_log) will contain information on who has been
accessing a WWW server, what has been accessed, and what has
failed.
* Good backups are the best defense against system damage. A
rule-of-thumb is to "back information up when it can't afford to be
lost".
General Network Security
As would be expected at trade shows (large or otherwise), there are
many entities running packet sniffers. Most are vendors who have a
legitimate need to run them during the course of product
demonstrations. A caveat to this is that there are many "listening
ears" on network segments-any of whom can "hear" or "see" information
as it crosses the net. Particularly prone to eavesdropping are telnet
sessions. A good rule of thumb is to assume that "when you type your
password, the only one that doesn't see it is you!"
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It is a good practice to not log in (or "su") to an account with root
privileges, across the network if at all possible. As mentioned
previously, authentication tokens are a simple way to add security to
system account access.
Packet Filtering
Many routers support basic packet filtering. Below is listed a good
"general" packet filter approach. The approach itself is ordered into
categories:
* General global denials/acceptance.
* Specific global service denials.
* Specific service acceptance.
* Final denial of all other TCP/UDP services.
Based on this theory, a good approach to a filter ruleset, in order
of execution priority, might be:
General Global Denials/Acceptance
1 Filter spoofed source addresses by interface. Match source
addresses to routing information available for the interface.
Discard packets with source addresses arriving on one interface
(from the "outside" for example) claiming a source address on
another interface (the "inside").
2 Filter all source routed packets unless source routing is
specifically needed.
3 Allow outbound connections from "inside" hosts.
4 Allow established TCP connections (protocol field contains 6 and
the TCP flags field either contains ACK or does NOT contain SYN
bit). Only filter requests for 'new' connections.
5 Filter 'new' connections with source port of 25. Prevents people
from pretending to be a remote mail server.
6 Filter loopback address (source address 127.0.0.1). Prevents
packets resulting from misconfigured DNS resolver.
Specific Global Service Denials
1 If not required, specifically block all "R-command" ports
(destination ports 512-515).
2 Block telnet (destination port 23) from any host not requiring
telnet access from the outside.
3 Add specific filters to deny other specific protocols to the
network, as needed.
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Specific Host/Service Acceptance
1 Add general open access, if desired, to specific "public" hosts
(unsecure FTP or WWW servers).
2 Allow SMTP (source and destination port 25) for electronic mail.
3 Allow inbound FTP connections (source port 20).
4 Allow DNS (source and destination port 53, UDP & TCP). If zone
transfers are not needed, block the TCP ports.
5 Allow RIP packets in (source and destination port 520, UDP).
6 Add specific filters to allow other desired specific protocols
and/or open certain ports to specific machines.
Final Service Denial
1 Deny all other UDP and TCP services not addressed in the previous
filters.
Author's Address
R. Allen Gwinn, Jr.
Associate Director, Computing
Business Information Center
Southern Methodist University
Dallas, TX 75275
Phone: 214/768-3186
EMail: allen@mail.cox.smu.edu or allen@radio.net
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