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. We recognize 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. We strongly recommend that sites do not
automatically create a "drop off" directory unless thought has been
given to the possible risks of having such a service. The CERT incident
response staff 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. The CERT staff has also received reports of file 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, we suggest
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.
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-95:16.wu-ftpd.vul
CA-93:06.wuarchive.ftpd.vulnerability
CA-92:09.AIX.anonymous.ftp.vulnerability
CA-88:01.ftpd.hole
Past advisories are available from
ftp://info.cert.org/pub/cert_advisories
http://www.sei.cmu.edu/technology/cert.cc.html
Copyright 1995 Carnegie Mellon University
This material may be reproduced and distributed without permission provided it
is used for noncommercial purposes and the copyright statement is included.
CERT is a service mark of Carnegie Mellon University.