Original issue date: July 14, 1993<BR>
Last revised: October 8, 1997<BR>
Attached copyright statement

<P>A complete revision history is at the end of this file.

<P>The CERT Coordination Center has been receiving a continuous stream of
reports from sites that are experiencing unwanted activities within their
anonymous FTP areas.  We recognize that this is not a new problem, and we
have been striving to handle requests for assistance on a one-to-one basis
with the reporting administrator. However, since this activity does not seem
to be diminishing, CERT believes that a broad distribution of information
concerning this problem and corresponding solution suggestions should help
to address the widespread nature of this activity.

<P>We are seeing three types of activity regarding anonymous FTP areas.

   <LI TYPE="A">Improper configurations leading to system compromise.

<P><LI>Excessive transfer of data causing deliberate over-filling of
      disk space thus leading to denial of service.

<P><LI>Use of writable areas to transfer copyrighted software and other
      sensitive information.

<P>This advisory provides an updated version of the anonymous FTP configuration
guidelines that is available from CERT.  The purpose of these guidelines is
to assist system administrators at sites that offer anonymous FTP services.
These guidelines are intended to aid a system administrator in configuring
anonymous FTP capabilities so as to minimize unintended use of services or
resources.  Systems administrators should be aware that anonymous FTP
capabilities should be configured and managed according to the policies
established for their site.

<P>You may obtain future copies of these guidelines through anonymous FTP from
cert.org in <A HREF="ftp://ftp.cert.org/pub/tech_tips">ftp://ftp.cert.org/pub/tech_tips</A>.



Anonymous FTP can be a valuable service if correctly configured and
administered. The first section of this document provides general guidance in
initial configuration of an anonymous FTP area.  The second section addresses
the issues and challenges involved when a site wants to provide writable
directories within their anonymous FTP areas. The third section provides
information about previous CERT advisories related to FTP services.

<P>The following guidelines are a set of suggested recommendations that have been
beneficial to many sites. CERT recognizes that there will be sites that have
unique requirements and needs, and that these sites may choose to implement
different configurations.

<H3><LI TYPE="I">Configuring anonymous FTP</H3>

<H4><LI TYPE="A">FTP daemon</H4>

<P>Sites should ensure that they are using the most recent version
       of their FTP daemon.

<H4><LI>Setting up the anonymous FTP directories</H4>

<P>The anonymous FTP root directory (~ftp) and its subdirectories
       should not be owned by the ftp account or be in the same group as
       the ftp account.  This is a common configuration problem.  If any of
       these directories are owned by ftp or are in the same group as the
       ftp account and are not write protected, an intruder will be able to
       add files (such as a .rhosts file) or modify other files.  Many sites
       find it acceptable to use the root account.  Making the ftp root
       directory and its subdirectories owned by root, part of the system
       group, and protected so that only root has write permission will help
       to keep your anonymous FTP service secure.

<P>Here is an example of an anonymous FTP directory setup:

    drwxr-xr-x  7   root    system  512 Mar 1       15:17 ./
    drwxr-xr-x 25   root    system  512 Jan 4       11:30 ../
    drwxr-xr-x  2   root    system  512 Dec 20      15:43 bin/
    drwxr-xr-x  2   root    system  512 Mar 12      16:23 etc/
    drwxr-xr-x 10   root    system  512 Jun 5       10:54 pub/

<P>Files and libraries, especially those used by the FTP daemon and
       those in ~ftp/bin and ~ftp/etc, should have the same protections
       as these directories.  They should not be owned by ftp or be in the
       same group as the ftp account; and they should be write protected.

<H4><LI>Using proper password and group files</H4>

<P>We strongly advise that sites not use the system's /etc/passwd file as
       the password file or the system's /etc/group as the group file in the
       ~ftp/etc directory.  Placing these system files in the ~ftp/etc
       directory will permit intruders to get a copy of these files.
       These files are optional and are not used for access control.

<P>We recommend that you use a dummy version of both the ~ftp/etc/passwd
       and ~ftp/etc/group files. These files should be owned by root. The
       dir command uses these dummy versions to show owner and group
       names of the files and directories instead of displaying arbitrary

<P>Sites should make sure that the ~/ftp/etc/passwd file contains no
       account names that are the same as those in the system's /etc/passwd
       file.  These files should include only those entries that are relevant
       to the FTP hierarchy or needed to show owner and group names. In
       addition, ensure that the password field has been cleared.  The
       examples below show the use of asterisks (*) to clear the password

<P>Below is an example of a passwd file from the anonymous FTP area on

    ssphwg:*:3144:20:Site Specific Policy Handbook Working Group::
    cops:*:3271:20:COPS Distribution::
    tools:*:9921:20:CERT Tools::
    ftp:*:9922:90:Anonymous FTP::
    nist:*:9923:90:NIST Files::

<P>Here is an example group file from the anonymous FTP area on cert.org:



<H3><LI>Providing writable directories in your anonymous FTP configuration</H3>

<P>There is a risk to operating an anonymous FTP service that permits
    users to store files.  CERT strongly recommends that sites do not
    automatically create a "drop off" directory unless thought has been
    given to the possible risks of having such a service.  CERT has received
    many reports where these directories have been used as "drop off"
    directories to distribute bootlegged versions of copyrighted software or
    to trade information on compromised accounts and password files.  CERT
    has also received numerous reports of files systems being maliciously
    filled causing denial of service problems.

<P>This section discusses three ways to address these problems. The first is
    to use a modified FTP daemon. The second method is to provide restricted
    write capability through the use of special directories. The third method
    involves the use of a separate directory.

<H4><LI TYPE="A">Modified FTP daemon</H4>

<P>If your site is planning to offer a "drop off" service, CERT suggests
       using a modified FTP daemon that will control access to the "drop off"
       directory.  This is the best way to prevent unwanted use of writable
       areas. Some suggested modifications are:

<OL><LI>Implement a policy where any file dropped off cannot
          be accessed until the system manager examines the file
          and moves it to a public directory.
<LI>Limit the amount of data transferred in one session.
<LI>Limit the overall amount of data transferred based on
          available disk space.
<LI>Increase logging to enable earlier detection of abuses.

<P>For those interested in modifying the FTP daemon, source code is
       usually available from your vendor. Public domain sources are
       available from:

    wuarchive.wustl.edu   ~ftp/packages/wuarchive-ftpd
    ftp.uu.net            ~ftp/systems/unix/bsd-sources/libexec/ftpd
    gatekeeper.dec.com    ~ftp/pub/DEC/gwtools/ftpd.tar.Z

<P>The CERT Coordination Center has not formally reviewed, evaluated,
       or endorsed the FTP daemons described.  The decision to use the FTP
       daemons described is the responsibility of each user or organization,
       and we encourage each organization to thoroughly evaluate these
       programs before installation or use.

<H4><LI>Using protected directories</H4>

<P>If your site is planning to offer a "drop off" service and is unable
       to modify the FTP daemon, it is possible to control access by using a
       maze of protected directories.  This method requires prior coordination
       and cannot guarantee protection from unwanted use of the writable FTP
       area, but has been used effectively by many sites.

<P>Protect the top level directory (~ftp/incoming) giving only execute
       permission to the anonymous user (chmod 751 ~ftp/incoming).  This will
       permit the anonymous user to change directory (cd), but will not allow
       the user to view the contents of the directory.

    drwxr-x--x  4   root    system  512 Jun 11      13:29 incoming/

<P>Create subdirectories in the ~ftp/incoming using names known only
       between your local users and the anonymous users that you want to
       have "drop off" permission.  The same care used in selecting passwords
       should be taken in selecting these subdirectory names because the
       object is to choose names that cannot be easily guessed.  Please do not
       use our example directory names of jAjwUth2 and MhaLL-iF.

    drwxr-x-wx 10   root    system  512 Jun 11      13:54 jAjwUth2/
    drwxr-x-wx 10   root    system  512 Jun 11      13:54 MhaLL-iF/

<P>This will prevent the casual anonymous FTP user from writing files in
       your anonymous FTP file system.  It is important to realize that this
       method does not protect a site against the result of intentional or
       accidental disclosure of the directory names.  Once a directory name
       becomes public knowledge, this method provides no protection at all
       from unwanted use of the area.  Should a name become public, a site
       may choose to either remove or rename the writable directory.

<H4><LI>Using a single disk drive</H4>

<P>If your site is planning to offer a "drop off" service and is
       unable to modify the FTP daemon, it may be desirable to limit
       the amount of data transferred to a single file system mounted
       as ~ftp/incoming.

<P>If possible, dedicate a disk drive and mount it as ~ftp/incoming.
       If this dedicated disk becomes full, it will not cause a denial
       of service problem.

<P>The system administrator should monitor this directory (~ftp/incoming)
       on a continuing basis to ensure that it is not being misused.

<H3><LI>Related CERT Advisories</H3>

<P>The following CERT Advisories directly relate to FTP daemons or impact
    on providing FTP service:

<P><A HREF="http://www.cert.org/advisories/CA-93.06.wuarchive.ftpd.vulnerability.html">CA-93.06.wuarchive.ftpd.vulnerability.</A><BR>
        <A HREF="http://www.cert.org/advisories/CA-92.09.AIX.anonymous.ftp.vulnerability.html">CA-92.09.AIX.anonymous.ftp.vulnerability.</A><BR>
       <A HREF="http://www.cert.org/advisories/CA-88.01.ftpd.hole.html">CA-88.01.ftpd.hole</A><Br>

<P>Past advisories are available for anonymous FTP from cert.org.


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<P>Copyright 1993 Carnegie Mellon University.</P>


Revision History
October 8, 1997  Attached copyright statement