<HR SIZE=2 NOSHADE ALIGN=LEFT>
<P>
<A HREF="#intro">Introduction</A>
<P>
A. <A HREF="#A">Commonly Exploited Configuration Problems</A>
<P>
<OL TYPE="1">
<LI><A HREF="#A.1.i">Weak Passwords</A></LI>
<LI><A HREF="#A.1.ii">Accounts with default passwords</A></LI>
<LI><A HREF="#A.1.iii">Reusable passwords</A></LI>
<LI><A HREF="#A4">Use of TFTP (Trivial File Transfer Protocol) to obtain password files</A></LI>
<LI><A HREF="#A5">Vulnerabilities in sendmail</A></LI>
<LI><A HREF="#A6">Misconfigured anonymous FTP</A></LI>
<LI><A HREF="#A7">Inapropriate network configuration file entries</A></LI>
<LI><A HREF="#A8">Inappropriate 'secure' settings in /etc/ttys and /etc/ttytab</A></LI>
<LI><A HREF="#A9">Inappropriate entries in /etc/aliases (or /usr/lib/aliases)</A></LI>
<LI><A HREF="#A10">Inappropriate file and directory protections</A></LI>
<LI><A HREF="#A11">Old versions of system software</A></LI>
<LI><A HREF="#A12">Use of setuid shell scripts</A></LI>
<LI><A HREF="#A13">Inappropriate export settings</A></LI>
<LI><A HREF="#A14">Vulnerable protocols and services</A></LI>
</OL>
B. <A HREF="#B">Other Suggestions</A>
<p>
<A HREF="#history">Revision History</A>
<P>
<HR SIZE=2 NOSHADE ALIGN=LEFT>
<P>
<A NAME="intro"></a>
This document describes commonly exploited UNIX system configuration
problems and recommends practices that can be used to help deter
several types of break-ins. We encourage system administrators to
review all sections of this document and modify their systems to fix
potential weaknesses.
<P>
In addition to the information in this document, we provide three
companion documents that may help you.
<P>
<UL>
<LI><A HREF="http://www.cert.org/tech_tips/intruder_detection_checklist.html">
http://www.cert.org/tech_tips/intruder_detection_checklist.html</A><BR>
contains suggestions for determining if your system may have
been compromised</LI>
<LI><A HREF="http://www.cert.org/tech_tips/win-UNIX-system_compromise.html">
http://www.cert.org/tech_tips/win-UNIX-system_compromise.html</A><BR>
contains suggested steps for recovering from a root compromise on
a UNIX and Windows NT systems</LI>
</UL>
<P>
Also, please see our CERT advisory page, our CERT incident notes page,
and our CERT vulnerability notes page which contain brief descriptions
of all past CERT advisories, incident notes, and vulnerability
notes. These files are available from
<P>
<UL>
<LI><A HREF="http://www.cert.org/advisories/">
http://www.cert.org/advisories/</A></LI>
<LI><A HREF="http://www.cert.org/incident_notes/">
http://www.cert.org/incident_notes/</A></LI>
<LI><A HREF="http://www.cert.org/vul_notes/">
http://www.cert.org/vul_notes/</A></LI>
</UL>
<P>
We encourage you to review the documents that pertain to your
system(s), and to consider taking the suggested steps to protect your
system(s) from attack. We also encourage you to check with your
vendor(s) regularly for any software updates or new software patches
that relate to your systems.
<P>
<HR SIZE=2 NOSHADE ALIGN=LEFT>
<P>
<H3>A. <A NAME="A"></a>Commonly Exploited Configuration Problems</H3>
<P>
<OL TYPE="1">
<B><LI><A NAME="A.1"></a>Poor Password Security</B>
<P>
The basic form of authentication used to control access to a UNIX
host is a username and password combination. Intruders have
established mechanisms and tools to compromise password information
by leveraging a variety of common problems.
<P>
<OL TYPE="i">
<B><LI><A NAME="A.1.i"></a>Weak passwords</B>
<P>
Encourage your users to choose passwords that are difficult to guess
(for example, words that are not in any dictionary of any language;
no proper nouns, including names of "famous" real or fictitious
characters; no acronyms that are commonly used by computer
professionals; no simple variations of first or last names.)
Furthermore, inform your users not to leave any cleartext
username/password information in files on any system.
<P>
A good heuristic for choosing a password is to choose an
easy-to-remember phrase, such as "By The Dawn's Early Light", and
use the first letters to form a password. Add some punctuation or
mix case letters as well. For the phrase above, one example password
might be: bt}DeL{. (DO NOT use this sample phrase for your
password.)
<P>
If intruders can get a password file, they usually move or copy it
to another machine and run password-guessing programs on it. These
programs involve large dictionary searches, and they run quickly
even on slow machines. Most systems that do not put any controls of
the type of passwords used probably have at least one password that
can be easily guessed. CERT Incident Note IN-98.03 describes
intruder activity that is based on a stolen password file.
<P>
<DL>
<DD><A HREF="http://www.cert.org/incident_notes/IN-98.03.html">
http://www.cert.org/incident_notes/IN-98.03.html</A></DD>
</DL>
<P>
If you believe that your password file may have been taken, change
all the passwords on the system. At the very least, you should
change all system passwords because an intruder may concentrate on
those and may be able to guess even a reasonably "good"
password. Intruders often use compromised accounts to attempt to
gain privelaged access on vulnerable systems, so we encourage you to
follow the steps in
<P>
<UL>
<LI><A HREF="http://www.cert.org/tech_tips/intruder_detection_checklist.html">
http://www.cert.org/tech_tips/intruder_detection_checklist.html</A></LI>
<LI><A HREF="http://www.cert.org/tech_tips/win-UNIX-system_compromise.html">
http://www.cert.org/tech_tips/win-UNIX-system_compromise.html</A></LI>
</UL>
<P>
For further information about protecting your system from
password-based attacks, see
<P>
<UL>
<LI><A HREF="http://www.cert.org/tech_tips/passwd_file_protection.html">
http://www.cert.org/tech_tips/passwd_file_protection.html</A></LI>
</UL>
<P>
</LI>
<B><LI><A NAME="A.1.ii"></a>Accounts with default passwords</B>
<P>
Intruders exploit system default passwords that have not been
changed since installation, including accounts with vendor-supplied
default passwords. In some cases, accounts do not have a password
assigned by default. CERT Incident Note IN-98.01 describes intruder
activity that is based on exploitations of accounts without
passwords.
<P>
<DL>
<DD><A HREF="http://www.cert.org/incident_notes/IN-98.01.irix.html">
http://www.cert.org/incident_notes/IN-98.01.irix.html</A></DD>
</DL>
<P>
Be sure to change all default passwords on computer systems and
networking equipment prior to deployment. Also, be aware that
product upgrades can quietly change account passowrds to a new
default. It is best to change the passwords of default accounts
after applying updates.
<P>
Scan your password file for extra UID 0 accounts, accounts with no
password, or new entries in the password file. Do not allow any
accounts without passwords. Remove entries for unused accounts from
the password file. To disable an account, change the password field
in the /etc/passwd file to an asterisk '*' and change the login
shell to /bin/false to ensure that an intruder cannot login to the
account from a trusted system on the network.
<P>
</LI>
<B><LI><A NAME="A.1.iii"></a>Reusable and shared passwords</B>
<P>
Even excellent passwords are not safe. They can be captured by
programs such as packet sniffers if the passwords are sent across
networks in cleartext (whether on a subnet, a local network, or
the Internet). It is common for intruders to use packet sniffers
on compromised systems to harvest passwords.
<P>
CERT Incident Note IN-99-06 describes widespread intruder activity
involving distributed sniffers used to harvest username and
password information from a network.
<P>
<DL>
<DD><A HREF="http://www.cert.org/incident_notes/IN-99-06.html">
http://www.cert.org/incident_notes/IN-99-06.html</A></DD>
</DL>
<P>
At the very least, a single password should not be used to protect
multiple accounts. If an intruder is able to compromise a shared
password just once, all of the accounts sharing the password are
compromised. Each account, or resource, protected by a password
should have it's own unique password.
<P>
To overcome the threat posed by packet sniffers, we recommend
using one-time passwords, especially for authenticated access from
external networks and for access to sensitive resources like name
servers and routers. For more information, see Appendix B of the
following advisory:
<P>
<DL>
<DD><A HREF="http://www.cert.org/advisories/CA-94.01.ongoing.network.monitoring.attacks.html">
http://www.cert.org/advisories/CA-94.01.ongoing.network.monitoring.attacks.html</A></DD>
</DL>
<P>
Another approach is to use a strong authentication mechanisms such
as secure shell, SSL, or kerberos. Secure shell, or ssh, is widely
available for many different platforms. For more information about
secure shell, see
<P>
<UL>
<LI><A HREF="http://www.ssh.com/index.html">http://www.ssh.com/index.html</A></LI>
<LI><A HREF="http://www.openssh.com/">http://www.openssh.com/</A></LI>
</UL>
<P>
</LI>
</OL>
<!-- TODO -->
<LI><A NAME="A4"></a>Use of TFTP (Trivial File Transfer Protocol) to obtain password files
<P>
To test your system for this vulnerability, connect to your
system using tftp and try
<PRE>
get /etc/motd
</PRE>
If you can do this, anyone else on the network can probably get
your password file. To avoid the problem, disable tftpd. If you
must have tftpd, ensure that it is configured with restricted
access. For further information, see
<P>
<DL>
<DD><A HREF="http://www.cert.org/advisories/CA-91.18.Active.Internet.tftp.Attacks.html">
http://www.cert.org/advisories/CA-91.18.Active.Internet.tftp.Attacks.html</A></DD>
</DL><P>
As mentioned in Section 1 above, if you believe your password
file may have been taken, the safest course is to change all
passwords in the system.<P>
</LI>
<LI><A NAME="A5"></a>Vulnerabilities in sendmail
<P>
There have been a number of vulnerabilities identified over
the years in sendmail(8). To the best of our knowledge, the
current version of sendmail addresses those known
vulnerabilities.
</P>
To determine which version of sendmail is running, use telnet
to connect to the SMTP port (25) on your system:
<PRE>
telnet <your hostname> 25
</PRE>
We encourage you to keep up to date with the latest version of
sendmail from your vendor, and ensure that it is up to date
with security patches or workarounds detailed in CERT advisories
advisories and bulletins.
In addition, we encourage you to use the following tools, both
of which are distributed with the latest versions of sendmail:
<OL TYPE="a">
<LI>smrsh, the sendmail restricted shell, controls the way o
that incoming mail messages can interact with your
operating system. For instance, when configured
correctly, smrsh can prevent an intruder from using
pipes to execute arbitrary commands on your system.
</LI>
<LI>mail.local can be used to control the way in which the
/bin/mail program is used on your system. This tool is
described in CERT advisory CA-95:02.
<BLOCKQUOTE>
<A HREF="http://www.cert.org/advisories/CA-1995-02.html">
http://www.cert.org/advisories/CA-1995-02.html</A>
</BLOCKQUOTE>
</LI>
</OL>
<LI><A NAME="A6"></a>Misconfigured anonymous FTP
<P>
In addition to making sure that you are running the most
recent version of ftpd, check your anonymous FTP
configuration. It is important to follow the instructions
provided with the operating system to properly configure
the files and directories available through anonymous FTP
(for example, file and directory permissions, ownership and
group). Note that you should not use your system's standard
password file or group file as the password file or group
file for FTP. The anonymous FTP root directory and its two
subdirectories, etc and bin, should not be owned by ftp.
For more information about configuring anonymous FTP, see
<BLOCKQUOTE>
<A HREF="http://www.cert.org/tech_tips/anonymous_ftp_config.html">
http://www.cert.org/tech_tips/anonymous_ftp_config.html</A>
</BLOCKQUOTE>
</LI>
<LI><A NAME="A7"></a>Inappropriate network configuration file entries
<P>
Several vendors supply /etc/hosts.equiv files with a '+'
(plus sign) entry. The '+' entry should be removed from
this file because it means that your system will trust all
other systems. Other files that should not contain a '+'
entry include all .rhosts files on the system. These files
should not be world-writable.
</P>
<P>
If your /usr/lib/X11/xdm/Xsession file includes an 'xhost'
command with a '+' entry, such as
<PRE>
/usr/bin/X11/xhost +
</PRE>
remove that line. (Note that the 'xhost' command may be in
a different directory tree on your system.) If such a line
remains intact, anyone on the network can talk to the X
server and potentially insert commands into windows or read
console keystrokes.
</P>
</LI>
<LI><A NAME="A8"></a>Inappropriate 'secure' settings in /etc/ttys and /etc/ttytab
<P>
Check the file /etc/ttys or /etc/ttytab (depending on the
release of UNIX being used). The ONLY terminal that should
be set to 'secure' should be the console.
</P>
</LI>
<LI><A NAME="A9"></a>Inappropriate entries in /etc/aliases (or /usr/lib/aliases)
<P>
Examine the /etc/aliases (or /usr/lib/aliases) mail alias
file for inappropriate entries. Some alias files include an
alias named 'uudecode' or just 'decode.' If this alias
exists on your system and you are not explicitly using it,
then you should remove it.
</P>
</LI>
<LI><A NAME="A10"></a>Inappropriate file and directory protections
<P>
Check your system documentation to establish the correct
file and directory protections and ownership for system
files and directories. In particular, check the '/' (root)
and '/etc' directories, and all system and network
configuration files. Examine file and directory protections
before and after installing software or running verification
utilities. These procedures can cause file and directory
protections to change.
</P>
<LI><A NAME="A11"></a>Old versions of system software
<P>
Older versions of operating systems often have security
vulnerabilities that are well known to intruders. To
minimize your vulnerability to attacks, keep the version of
your operating system up to date and apply security patches
appropriate to your system(s) as soon as they become
available.
</P>
</LI>
<LI><A NAME="A12"></a>Use of setuid shell scripts
<P>
Setuid shell scripts (especially setuid root) can pose
potential security problems, a fact that has been well
documented in many UNIX system administration texts. Do not
create or allow setuid shell scripts, especially setuid
root.
</P>
</LI>
<LI><A NAME="A13"></a>Inappropriate export settings
<P>
Use the showmount(8) utility to check that the
configuration of the /etc/exports files on your hosts
are correct.
<UL>
<LI>Wherever possible, file systems should be exported
read-only.
<LI>Do not self-reference an NFS server in its own
exports file. That is, the exports file should not
export an NFS server to itself nor to any netgroups
that include the NFS server.
<LI>Do not allow the exports file to contain a
"localhost" entry.
<LI>Export file systems only to hosts that require them.
<LI>Export only to fully qualified hostnames.
<LI>Ensure that export lists do not exceed 256
characters (after the aliases have been expanded)
or that all security patches relating to this
problem have been applied.
</UL>
The CERT Coordination Center is aware that intruders are
using tools that exploit a number of NFS vulnerabilities.
This can result in a root compromise, depending on the
vulnerability being exploited. We encourage you to limit
your exposure to these attacks by implementing the security
measures outlined in CERT advisory CA-94:15. For this and
other information about the NFS vulnerability, see
<BLOCKQUOTE>
<A HREF="http://www.cert.org/advisories/CA-1994-15.html">
http://www.cert.org/advisories/CA-1994-15.html</A>
</BLOCKQUOTE>
</LI>
<LI><A NAME="A14"></a>Vulnerable protocols and services
<P>
You may want to consider filtering certain TCP/IP services
at your firewall or router. For some related suggestions,
please refer to "Packet Filtering For Firewall Systems,"
available from
<BLOCKQUOTE>
<A HREF="http://www.cert.org/tech_tips/packet_filtering.html">
http://www.cert.org/tech_tips/packet_filtering.html</A>
</BLOCKQUOTE>
</LI>
</OL>
</P>
</LI>
For a list of some recommended books and articles on computer
security topics, see the CERT(sm) Coordination Center FAQ,
available from
<P>
<UL>
<LI><A HREF="http://www.cert.org/faq/cert_faq.html">
http://www.cert.org/faq/cert_faq.html</A></LI>
</UL>
</P>
</LI>
</UL>
<p><!--#include virtual="/include/footer_nocopyright.html" --> </p>
<p>Copyright 1997 Carnegie Mellon University.</p>
<HR SIZE=2 NOSHADE ALIGN=LEFT>
<TABLE>
<A NAME="history"></a>
<TR>
<TD>
<FONT SIZE=3 FACE="Helvetica, Geneva, Arial">
Revision History
</TD>
</TR>
<TR>
<TD VALIGN=TOP WIDTH=30%>
<FONT SIZE=2 FACE="Helvetica, Geneva, Arial">
Oct 02, 1997<BR>
Feb 12, 1999<BR>
Jun 04, 2003<BR>
Apr 24, 2006<BR>
</TD>
<TD VALIGN=TOP WIDTH=70%>
<FONT SIZE=2 FACE="Helvetica, Geneva, Arial">
Initial Release<BR>
Converted to new web format<BR>
Updated broken links<BR>
</TD>
</TR>
</SMALL>
</TABLE>
|