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CSL Bulletin - Connecting to the INTERNET: Security Considerations

                         CSL BULLETIN
                           July 1993


This bulletin focuses on security considerations for organizations
considering Internet connections.  Spurred by developments in high-
speed networking technology and the National Research and Education
Network (NREN), many organizations and individuals are looking at
the Internet as a means for expanding their research interests and
communications.  Consequently, the Internet is now growing faster
than any telecommunications system thus far, including the
telephone system.

New users of the Internet may fail to realize, however, that their
sites could be at risk to threats such as intruders who use the
Internet as a means for attacking systems and causing various forms
of computer security incidents.  Consequently, new Internet sites
are often prime targets for malicious activity, including break-
ins, file tampering, and service disruptions.  Such activity may be
difficult to discover and correct, may be highly embarrassing to
the organization, and can be very costly in terms of lost
productivity and damage to data.

New and existing Internet users need to be aware of the high
potential for computer security incidents from the Internet and the
steps they should take to secure their sites.  Many tools and
techniques now exist to provide sites with a higher level of
assurance and protection.

The Internet
The Internet is a world-wide "network of networks" that use the
TCP/IP protocol suite for communications.  The component networks
are interconnected at various points to provide multiple routes to
destinations and a high level of overall service to users.  Many
academic, business, and government organizations are connected to
the Internet.  In 1993, over five million users were connected,
with roughly half being business users.

The Internet provides several types of services, including terminal
emulation and remote system access (telnet), file exchange (ftp),
electronic mail (smtp), and a number of other services for
information exchange.

As outlined in CSL Bulletin TCP/IP or OSI?  Choosing a Strategy for
Open Systems, NIST advises that agencies procure Open Systems
Interconnect (OSI) networking products for new network
implementations and for multivendor information exchange.  At the
same time, it is practical for agencies to consider connections to
the Internet for the purpose of exchanging e-mail and files with
the large number of existing Internet sites.

Security Problems on the Internet
In recent years, a number of security problems with the Internet
have become apparent.  Newspapers have carried stories of high-
profile "cracker" attacks via the Internet against government,
business, and academic sites.  Crackers often roam the Internet
with impunity, covering their tracks by moving from system to
system.  Intruders have been known to use systems illegally to
exchange copyrighted software, to obtain sensitive information such
as business secrets, and in general to cause mischief.  System
administrators are often unaware of break-ins and unauthorized
users until they learn by accident.  A number of factors have
contributed to this state of affairs.

Complexity of Configuration:  Many sites connect systems to the
Internet with little thought to the complexity of system
administration and the increased potential for abuse from the
Internet.  New systems often arrive "out of the box" with network
access controls configured for maximum, i.e., least secure, access. 
The access controls are usually complex to configure and monitor. 
As a result, controls that are accidentally misconfigured can
result in unauthorized access.

Ease of Spying and Spoofing:  The vast majority of Internet traffic
is unencrypted and therefore easily readable.  As a result, e-mail,
passwords, and file transfers can be monitored and captured using
readily available software.  Intruders have been known to monitor
connections to well-known Internet sites for the purpose of gaining
information that would allow them to crack security or to steal
valuable information.  This information sometimes permits intruders
to spoof legitimate connections, i.e., trick system security into
permitting normally disallowed network connections.

Inherent Problems with TCP/IP Protocols:  The TCP/IP protocol
suite, as implemented in the Internet, does not contain provisions
for network security.  A number of the TCP/IP services, e.g.,
rlogin, rsh, etc., rely on mutually trusting systems and are
inherently vulnerable to misuse and spoofing.  Ironically, some of
these services, e.g., nis and nfs, are widely used to coordinate
local area network security and to distribute system resources
among other local systems.  If the vulnerabilities in these
services are exploited, security on the local area network could be
badly compromised.

Wide-Open Network Policies:  Many sites are configured
unintentionally for wide-open Internet access without regard to the
potential for abuse from the Internet.  Some systems still employ
password-less guest accounts or anonymous ftp accounts that can be
written to without restriction.  Others keep sensitive information
on network-accessible systems where it can be easily read.  The
vast majority of sites permit more TCP/IP services than they
require for their operations and do not attempt to limit access to
information about their computers that could prove valuable to

Recommendations for New and Existing Internet Connections
New and existing Internet sites need to take strong and specific
measures to improve computer security.  These measures include
creating a TCP/IP service access policy, using strong
authentication, and using a secure Internet gateway that can imple-
ment network access policies.

  浜様様様様様璃Service Access Policy様様様様様様
        Local                     敖陳陳陳陳朕 
        Area            strong      Secure  団彡 INTERNET
        Network     authentication  Gateway 団彡 OSI WANs
        Systems                   青陳陳陳陳潰 

Network Service Policy:  The first step is to create a policy that
details what types of connectivity will be permitted.  If, for
example, e-mail is the only service required, then other forms of
access such as telnet and ftp can be restricted and overall risks
reduced.  Eliminating the TCP/IP services that are not needed will
help to provide a simpler, more manageable network environment. 
The following figure is a partial list of TCP/IP services that
should be restricted or blocked from passing through a site's
Internet gateway:

                    High-Risk TCP/IP Services
 DNS zone transfers leaks names of internal
systems tftp intruders can read
password file RPC (eg., NIS,
NFS) intruders can read/write
files rlogin, rsh, etc. relies on mutually
trusting systems X windows,        
OpenWindows intruders can monitor
users' sessions telnet, ftp, smtp can be abused, access
should be re-
 stricted to selected
Strong Authentication:  Systems that can be accessed from the
Internet or via modem should require strong authentication such as
provided by smart cards and authentication tokens.  These systems
use one-time passwords that cannot be spoofed, regardless of
whether the passwords are monitored.  CSL Bulletin Advanced
Authentication Technology contains more information on strong
authentication techniques.

Secure Gateways:  Secure gateways, or firewalls, are highly
effective for improving site network security.  A secure gateway is
a collection of systems and routers placed at a site's central
connection to a network whose main purpose is to restrict access to
internal systems.  A secure gateway forces all network connections
to pass through the gateway where they can be examined and
evaluated.  The secure gateway may then restrict access to or from
selected systems or block certain TCP/IP services or provide other
security features.  A simple network usage policy that can be
implemented by a secure gateway is to provide access from internal
to external systems, but little or no access from external to
internal systems (except perhaps for e-mail).

For small sites, a simple router with packet-filtering capability
may serve as an effective gateway.  This type of router can
typically restrict access to selected systems and services by
examining each packet according to a sequence of filtering rules. 
A more flexible and robust approach is to combine the router with
other systems capable of logging network access and restricting
access and services on a finer basis.  Some secure gateways
implement proxy services that require all ftp or telnet connections
to be first authenticated at the gateway before being allowed to

Without a secure gateway, a site's security depends entirely on the
collective security of its individual systems.  As the number of
systems increases, it becomes more difficult to ensure that network
security policies are enforced.  Errors and simple mistakes in one
system's configuration can cause problems for other interconnected

Securing Modem Pools:  Unrestricted incoming and outgoing modem
pools (including PABX systems) can create backdoors that would let
intruders get around the access controls of secure gateways.  Modem
pools need to be configured to deny access to unauthorized users. 
Systems that can be accessed from modem pools should require strong
authentication such as one-time passwords.  Modem pools should not
be configured for outgoing connections unless access can be
carefully controlled.

Securing Public Access Systems:  Public access systems such as
anonymous ftp archives are often prime targets for abuse.  Such
systems, if misconfigured to allow writing, can permit intruders to
destroy or alter data or software, which can prove highly
embarrassing to the organization.  CSL Bulletin Security Issues in
Public Access Systems provides guidance on securing public access

System Security Tools:  The existence of a secure gateway does not
negate the need for stronger system security.  Many tools are
available for system administrators to enhance system security and
provide additional audit capability.  Such tools can check for
strong passwords, log connection information, detect changes in
system files, and provide other features that will help
administrators to detect signs of intruders and break-ins.

Keeping Up-to-Date
Sites need to be aware of other resources and information that will
permit them to update site security as new vulnerabilities are
discovered and as new tools and techniques to improve security
become available.  In particular, sites need to know who to contact
when trouble arises.

Vendor Support:  Several system vendors now regularly distribute
software update notifications and related security information via
e-mail.  Customers need to contact vendors and determine whether
they can receive such information.

Incident Handling Teams:  A number of vendors, other businesses,
and government-affiliated organizations have created computer
security incident handling teams.  These groups typically provide
assistance in determining whether an incident has occurred and how
to correct any vulnerabilities that were exploited.  NIST helps to
coordinate the Forum of Incident Response and Security Teams
(FIRST).  FIRST provides guidance and overall coordination of teams
and incident handling information.  For more information about
incident response teams and FIRST, contact NIST (see below).

For More Information
NIST will be issuing more guidance on open systems security and on
secure gateways.  For more information on Internet security and
other computer security issues, contact the National Institute of
Standards and Technology at the following address:  NIST, Building
225, Room A-216, Gaithersburg, MD 20899-0001; telephone (301) 975-
3359; fax (301) 948-0279.

NIST also maintains a computer security bulletin board system (BBS)
and Internet-accessible site for computer security information open
to the public at all times.  These resources provide information on
computer security publications, CSL Bulletins, alert notices,
information about viruses and anti-virus tools, a security events
calendar, and sources for more information.

To access the BBS, you need a computer with communications
capability and a modem.  For modems at 2400 bits per second (BPS)
or less, dial (301) 948-5717.  For 9600 BPS, dial (301) 948-5140. 
Modem settings for all speeds are 8 data bits, no parity, 1 stop

Internet users with telnet or ftp capability may telnet to the BBS
at cs-bbs.nist.gov (  To download files, users need to
use ftp as follows:  ftp to csrc.nist.gov (, log into
account anonymous, use your Internet address as the password, and
locate files in directory pub; an index of all files is available
for download.  For users with Internet-accessible e-mail
capability, send e-mail to docserver@csrc.nist.gov with the
following message:  send filename, where filename is the name of
the file you wish to retrieve.  send index will return an index of
available files.

The sources used to develop this bulletin provide excellent
resources for further information on Internet security and related
topics.  Ordering information is provided when appropriate.  All
references except [1] and [6] are available from the NIST BBS or
via ftp.

[1]    Cerf, Vinton, "A National Information Infrastructure," 
Connexions, June 1993.

[2]    "TCP/IP or OSI? Choosing a Strategy for Open Systems," CSL
       Bulletin, National Institute of Standards and Technology, June

[3]    Bellovin, Steve, "Security Problems in the TCP/IP Protocol
       Suite," Computer Communication Review, April 1989.

[4]    Holbrook, Paul, and Joyce Reynolds, "Site Security Handbook,"
       RFC 1244 prepared for the Internet Engineering Task Force,

[5]    "Advanced Authentication Technology," CSL Bulletin, National
       Institute of Standards and Technology, November 1991.

[6]    Curry, David, Unix System Security, Addison Wesley, 1992.

[7]    Ranum, Marcus, "Thinking About Firewalls," Proceedings of
       Second International Conference on System and Network
       Security, April 1993.

[8]    "Security Issues in Public Access Systems," CSL Bulletin,
       National Institute of Standards and Technology, May 1993.

[9]    Polk, W. Timothy, Automated Tools for Testing Computer System
       Vulnerability, NIST Special Publication 800-6, National
       Institute of Standards and Technology, December 1992.  Order
       from GPO, 202-783-3238, SN003-003-03189-9, or NTIS, 703-487-
       4650, PB93-146025.

[10]   Wack, John, Establishing A Computer Security Incident Response
       Capability, NIST Special Publication 800-3, National Institute
       of Standards and Technology, November 1991.  Order from NTIS,
       703-487-4650, PB92-123140.

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