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CA-93:10 CERT Advisory
July 14, 1993
Anonymous FTP Activity
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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.
We are seeing three types of activity regarding anonymous FTP areas.
A. Improper configurations leading to system compromise.
B. Excessive transfer of data causing deliberate over-filling of
disk space thus leading to denial of service.
C. Use of writable areas to transfer copyrighted software and other
sensitive information.
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.
You may obtain future copies of these guidelines through anonymous FTP from
cert.org in /pub/tech_tips/anonymous_ftp.
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ANONYMOUS FTP CONFIGURATION GUIDELINES
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.
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.
I. Configuring anonymous FTP
A. FTP daemon
Sites should ensure that they are using the most recent version
of their FTP daemon.
B. Setting up the anonymous FTP directories
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.
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/
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.
C. Using proper password and group files
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.
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
numbers.
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
field.
Below is an example of a passwd file from the anonymous FTP area on
cert.org:
ssphwg:*:3144:20:Site Specific Policy Handbook Working Group::
cops:*:3271:20:COPS Distribution::
cert:*:9920:20:CERT::
tools:*:9921:20:CERT Tools::
ftp:*:9922:90:Anonymous FTP::
nist:*:9923:90:NIST Files::
Here is an example group file from the anonymous FTP area on cert.org:
cert:*:20:
ftp:*:90:
II. Providing writable directories in your anonymous FTP configuration
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.
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.
A. Modified FTP daemon
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:
1. 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.
2. Limit the amount of data transferred in one session.
3. Limit the overall amount of data transferred based on
available disk space.
4. Increase logging to enable earlier detection of abuses.
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
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.
B. Using protected directories
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.
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/
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/
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.
C. Using a single disk drive
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.
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.
The system administrator should monitor this directory (~ftp/incoming)
on a continuing basis to ensure that it is not being misused.
III. Related CERT Advisories
The following CERT Advisories directly relate to FTP daemons or impact
on providing FTP service:
CA-93:06.wuarchive.ftpd.vulnerability
CA-92:09.AIX.anonymous.ftp.vulnerability
CA-88:01.ftpd.hole
Past advisories are available for anonymous FTP from cert.org.
Copyright (c) Carnegie Mellon University 1993
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If you believe that your system has been compromised, contact the CERT
Coordination Center or your representative in FIRST (Forum of Incident
Response and Security Teams).
Internet E-mail: cert@cert.org
Telephone: 412-268-7090 (24-hour hotline)
CERT personnel answer 8:30 a.m.-5:00 p.m. EST(GMT-5)/EDT(GMT-4),
and are on call for emergencies during other hours.
CERT Coordination Center
Software Engineering Institute
Carnegie Mellon University
Pittsburgh, PA 15213-3890
Past advisories, information about FIRST representatives, and other information
related to computer security are available for anonymous FTP from cert.org
(192.88.209.5).
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