CERT Advisory CA-97.27 - FTP_bounce

Aleph One (aleph1@DFW.NET)
Wed, 10 Dec 1997 16:47:35 -0600


CERT* Advisory CA-97.27
Original issue date: Dec. 10, 1997
Last revised: --

Topic: FTP Bounce
- -----------------------------------------------------------------------------

In some implementations of FTP daemons, the PORT command can be misused to
open a connection to a port of the attacker's choosing on a machine that the
attacker could not have accessed directly. There have been ongoing discussions
about this problem (called "FTP bounce") for several years, and some vendors
have developed solutions for this problem.

The CERT/CC staff urges you to install a comprehensive patch if one is
available. Until then, we recommend the wu-ftpd package identified in Section
III.B. as a workaround.

We will update this advisory as we receive additional information. Please
check our advisory files regularly for updates that relate to your site.

- -----------------------------------------------------------------------------

I. Description

In the past few years there have been ongoing discussions about a
problem known as "FTP bounce." In its simplest terms, the problem is
based on the misuse of the PORT command in the FTP protocol.

To understand the FTP bounce attack, please see the tech tip at


The core component of the problem is that by using the PORT command in
active FTP mode, an attacker may be able to establish connections to
arbitrary ports on machines other than the originating client. This
behavior is RFC compliant, but it is also potentially a source of
security problems for some sites. The example attacks described in the
tech tip demonstrate the potential of this vulnerability.

II. Impact

An attacker may be able to establish a connection between the FTP server
machine and an arbitrary port on another system. This connection may be
used to bypass access controls that would otherwise apply.

III. Solution

Because the core element of the attack (the FTP server can establish
connections to arbitrary machines and arbitrary ports) is also a required
component for RFC compliance, there is no clear-cut solution. With this
in mind, we urge you to carefully consider the type of service that your
site offers.

The best solution solely from a security perspective is to ensure that
your FTP server software cannot establish connections to arbitrary
machines. However, sites that rely on the RFC-compliant behavior may
find that implementing this solution will affect applications that they
use. (We have not received any first-hand reports of such cases.)
Consequently, many vendors offer solutions that allow sites offering the
FTP service to make the choice that best suits them. You should check to
see what type of behavior your vendor's FTP daemon adopts (Section A).

If you wish to implement an FTP service that does not allow this attack
and your vendor does not offer a daemon with this functionality, consider
using the wu-ftpd package described in Section B. Other steps you can
take are described in Section C.

A. Vendor Information

It is our experience that vendor implementations fall into one of
these groups:

(1) strict conformance with RFC functionality: The PORT command
may be used to connect directly to a third-party machine, and
this is the only functionality allowed. Some vendors who
choose to maintain strict conformance have addressed this
problem by modifying all other network services to reject
connections originating from the FTP data port (port 20).

(2) strict suppression of the PORT command: The PORT command may
be used to connect to the originating client, and this is the
only functionality allowed.

(3) variable PORT command behavior: The PORT command may be used
in either of the above two ways, with one way being the
default. Switching between them is usually achieved with a
command line parameter. You should be careful to verify which
is the default.

Appendix A contains a list of vendors who have provided
information about this problem. We will update the appendix as we
receive more information. If you do not see your vendor's name,
the CERT/CC did not hear from that vendor. Please contact your
vendor directly.

B. Use the wu-ftpd package as a workaround.

The wu-ftpd package addresses the FTP bounce problem by ensuring that
the PORT command cannot be used to establish connections to machines
other than the originating client. Please read the wu-ftpd README
file "FIXES-2.4-HOBBIT" before installing the package.

The latest version of wu-ftpd, which we recommend, is available from


DFN-CERT mirrors this software at


MD5 (wu-ftpd-2.4.2-beta-15.tar.Z) = 6c8172b83ab2545a5b91a9aba4840630

If you use a previous version (whether a beta version or full
release), do not assume that your site is immune from these problems
or other problems discussed in previous advisories.

C. FTP Configuration

Some attacks rely on an intermediate file being uploaded to one or
more server machines via (usually anonymous) FTP. This file is
used in a later phase of the attack.

Your site should offer anonymous upload facilities only if it is
absolutely necessary. Even then, you must carefully configure the
incoming area. For further details, see "Anonymous FTP Configuration
Guidelines" at


Note that these steps only repel attacks that rely on intermediate
uploads. The steps are not effective against other attacks.

If your site allows file uploads, we urge your to ensure that the
FTP service restricts the PORT command so that it can only be used
to connect to the originating client.


Appendix A - Vendor Information

Below is a list of the vendors who have provided information for this
advisory. We will update this appendix as we receive additional information.
If you do not see your vendor's name, the CERT/CC did not hear from that
vendor. Please contact the vendor directly.

Cray Research - A Silicon Graphics Company
- ------------------------------------------

The ftpd supplied with Unicos and Unicos/mk is currently in category 1.
We are working to make it category 3.

The FreeBSD Project
- -------------------

FreeBSD 2.2.0 and all later releases do not allow the FTP bounce attack
(unless explicitly allowed by the -R option). FreeBSD 2.1.7 and earlier
releases can be abused by the bounce attack.

Hewlett-Packard Company
- -----------------------

This problem is addressed HP Security Bulletin 028. This bulletin can
be found at one of these URLs:

(for US, Canada, Asia-Pacific, & Latin-America)

(for Europe)

Current patches for SB#28 as of 11/5/97 from security patch matrix

Security Bulletin 028: Security Vulnerability in FTP

Current Original
-------------------- --------------------
s300 8.00: None s300 8.00: None
s300 9.00: PHNE_6146 s300 9.00: PHNE_6146
s300 9.03: PHNE_6146 s300 9.03: PHNE_6146
s300 9.10: PHNE_6146 s300 9.10: PHNE_6146
s700 8.05: None s700 8.05: None
s700 8.07: None s700 8.07: None
s700 9.01: PHNE_10008 s700 9.01: PHNE_6013
s700 9.03: PHNE_10008 s700 9.03: PHNE_6013
s700 9.05: PHNE_10008 s700 9.05: PHNE_6013
s700 9.07: PHNE_10008 s700 9.07: PHNE_6013
s700 9.09: PHNE_6169 s700 9.09: PHNE_6169
PHNE_6170 PHNE_6170
s700 10.00: PHNE_10009 s700 10.00: PHNE_6014
s700 10.01: PHNE_10009 s700 10.01: PHNE_6014
s700 10.09: PHNE_5965 s700 10.09: PHNE_5965
s700 10.10: PHNE_10009 s700 10.10: None
s700 10.16: None s700 10.16: None
s700 10.20: None s700 10.20: None
s700 10.24: None s700 10.24: None
s700 10.30: None s700 10.30: None
s800 8.00: None s800 8.00: None
s800 8.02: None s800 8.02: None
s800 8.06: None s800 8.06: None
s800 9.00: PHNE_10008 s800 9.00: PHNE_6013
s800 9.04: PHNE_10008 s800 9.04: PHNE_6013
s800 9.08: PHNE_6171 s800 9.08: PHNE_6171
s800 10.00: PHNE_10009 s800 10.00: PHNE_6014
s800 10.01: PHNE_10009 s800 10.01: PHNE_6014
s800 10.09: None s800 10.09: None
s800 10.10: PHNE_10009 s800 10.10: None
s800 10.16: None s800 10.16: None
s800 10.20: None s800 10.20: None
s800 10.24: None s800 10.24: None
s800 10.30: None s800 10.30: None

Accessing the HP ESC
Hewlett Packard's HP-UX patches/Security Bulletins/Security
patches are available via email and/or WWW (via the browser
of your choice) on HP Supportline (HPSL).
To subscribe to automatically receive future NEW HP Security Bulletins from
the HP SupportLine Digest service via electronic mail, do the following:

1) From your Web browser, access the URL:

http://us-support.external.hp.com (US,Canada,Asia-Pacific,
and Latin-America)

http://europe-support.external.hp.com (Europe)

Login with your user ID and password, or register for one (remember
to save the User ID assigned to you, and your password). Once you are
on the Main Menu, Click on the Technical Knowledge Database, and it
will connect to a HP Search Technical Knowledge DB page. Near the
bottom is a hyperlink to our Security Bulletin archive. Once in the
archive there is another link to our current security patch matrix.
Updated daily, this matrix is categorized by platform/OS release,
and by bulletin topic.

IBM Corporation
- ---------------

All AIX ftp servers are vulnerable to the FTP bounce attack. The
following fixes are in progress:

AIX 3.2: upgrade to v4
AIX 4.1: IX73075
AIX 4.2: IX73076
AIX 4.3: IX73077

To Order
APARs may be ordered using Electronic Fix Distribution (via FixDist)
or from the IBM Support Center. For more information on FixDist,
reference URL:


or send e-mail to aixserv@austin.ibm.com with a subject of "FixDist".

- -------

This problem is fixed in MGFTP V2.2-2, which was released several months
ago. That version restricts the port numbers to ports above 1024.
However, it does not block access to third-party machines. V2.2-4,
scheduled for release next week, will do that as well.

Microsoft Corporation
- ---------------------

We prevent this attack by disallowing "third party" transfers. This is
done via a modification to our implementation of the PORT command. When
the FTP server receives a PORT command, the specified IP address *must*
match the client's source IP address for the control channel.

In other words, then the client sends a PORT command to the FTP server,
giving the server an IP address & port number to connect back to the
client for the data transfer, the IP address *must* be the client's
original IP address.

We have one other fix in which we disallow the PORT command from
specifying reserved ports (those less than 1024) except port 20 (the
default data port). By default, any client attempt to issue a port
command with (port < 1024 && port != 20) will cause the PORT command to
fail. This check can be disabled setting the EnablePortAttack registry

NCR Corporation
- ---------------

NCR is delivering a set of operating system dependent patches which
contain an update for this problem. Accompanying each patch is a
README file which discusses the general purpose of the patch and
describes how to apply it to your system.

Recommended solution: Apply one of the following patches depending on
the revision of the inet package installed on your system. To check its
version execute:

pkginfo -x inet

For inet 5.01.xx.xx: - PINET501 (Version later than
For inet 6.01.xx..xx: - PINET601 (Version later than
For inet 6.02.xx.xx: - PINET602 (Version later than

After installation of the respective patch, the default behavior will be
to protect from this vulnerability.. A new ftpd man-page describe how to
enable the old RFC compliant behavior.

The NetBSD Project
- ------------------

There are no patches for NetBSD 1.2.1 or prior, however the ftpd
sources available from:
should work on a NetBSD 1.2.1 machine.

The OpenBSD project
- -------------------

FTP bounce can be fixed in the operating system by fixing all vulnerable
services by checking for connections from port 20. Since this has been
done in OpenBSD, OpenBSD is not vulnerable and does NOT NEED the
variable port command. The solution applies since OpenBSD 2.1 (ie. it
applies for both 2.1 and for 2.2).

Red Hat Software
- ----------------

We ship wu-ftpd, so this isn't a problem for us.

The Santa Cruz Operation, Inc.
- ------------------------------

SCO has determined that the following Operating systems are vulnerable
to the ftp-bounce attack :-

OpenServer 5.0.4
UnixWare 2.1
ODT 3.0

We are currently working on a fix to this problem.

Siemens-Nixdorf Informationssysteme AG
- --------------------------------------

ReliantUNIX is vulnerable.
The problem has been corrected in the current sources.
Patches will be developed (as necessary) and made available via your
Siemens-Nixdorf customers service.

Sun Microsystems, Inc.
- ----------------------

Sun's FTP server software in SunOS 4.1.x and 5.x allow PORT requests
to make data connections to arbitrary hosts. Prior to SunOS 2.6, Sun's
FTP server software also allows data connections to arbitrary ports.

In SunOS 2.6, the FTP server software does not accept PORT requests to make
data connections to well-known (privileged) ports. Sun has also released
the following patches that prevent Sun's FTP server software from accepting
PORT requests to make data connections to well-known ports for the
following SunOS releases:

103603-05 SunOS 5.5.1
103604-05 SunOS 5.5.1_x86
103577-06 SunOS 5.5
103578-06 SunOS 5.5_x86
101945-51 SunOS 5.4
101946-45 SunOS 5.4_x86
104938-01 SunOS 5.3
104477-03 SunOS 4.1.4
104454-03 SunOS 4.1.3_U1

Sun recommends that sites that do not require their FTP server make
connections to arbitrary hosts consider using wu-ftpd as a workaround.

- -----------------------------------------------------------------------------

The CERT Coordination Center thanks AUSCERT and DFN-CERT for helping
develop this advisory. We also thank Steve Bellovin and the vendors who
offered valuable comments on the problem and solutions: BSDI, Hewlett-Packard,
Livingston, NetBSD, OpenBSD, Sun Microsystems.

- -----------------------------------------------------------------------------

If you believe that your system has been compromised, contact the CERT
Coordination Center or your representative in the Forum of Incident Response
and Security Teams (see http://www.first.org/team-info/).

CERT/CC Contact Information
- ----------------------------
Email cert@cert.org

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Fax +1 412-268-6989

Postal address
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Software Engineering Institute
Carnegie Mellon University
Pittsburgh PA 15213-3890

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- ---------------------------------------------------------------------------

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- ---------------------------------------------------------------------------

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Revision history

Version: 2.6.2