||Home : Advisories : FTP Bounce|
||10th December 1997
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CERT Advisory CA-97.27.FTP_bounce
Original issue date: Dec. 10, 1997
Last revised: March 08, 1999 Added vendor information for Data
A complete revision history is at the end of this file.
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
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
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.
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
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
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
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.
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
MD5 (wu-ftpd-2.4.2-beta-16.tar.Z) = c18c083c2a82eef1ccba6df9a406f026
Further information on this package can be obtained from
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
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.
Caldera OpenLinux(tm) 1.2 ships with wu-ftpd-2.4.2 beta 15. For those
with earlier versions of wu-ftpd, updates to this package can be
Other Caldera security resources are located at:
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.
DGUX documents a "-p" switch for ftpd, which appears to prevent
exploitation of the problem described. Revision R4.20MU04 and later
will be configured to include this switch in the /etc/inetd.conf file.
Customers running earlier revisions should change the ftp line in their
inetd.conf file to the following:
ftp stream tcp nowait root /usr/bin/ftpd ftpd -p -t900
DIGITAL EQUIPMENT CORPORATION
A DIGITAL EQUIPMENT CORPORATION ADVISORY VB#SSRT0452, concerning "DIGITAL UNIX
ftpd V3.2g, V4.0, V4.0a, V4.0b, V4.0c" was issued APR 30, 1998. For more
information, please see
the World Wide Web at the following FTP address:
Use the FTP access option, select DIGITAL_UNIX directory
then choose the appropriate version directory
and download the patch accordingly.
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.
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)
Current patches for SB#28 as of 11/5/97 from security patch matrix
Security Bulletin 028: Security Vulnerability in FTP
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
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:
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.
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
APARs may be ordered using Electronic Fix Distribution (via FixDist)
or from the IBM Support Center. For more information on FixDist,
or send e-mail to email@example.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.
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
Several NEC Unix systems have proven vulnerable. Work is currently
underway to identify all affected systems. Patches are forthcoming.
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 05.01.01.64)
For inet 6.01.xx.xx: - PINET601 (Version later than 06.01.00.24)
For inet 6.02.xx.xx: - PINET602 (Version later than 06.02.00.05)
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 :-
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 5.6, Sun's
FTP server software also allows data connections to arbitrary ports.
In SunOS 5.6, 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,
Caldera, Hewlett-Packard, Livingston, NetBSD, OpenBSD, Sun
This document is available from:
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Mar. 8, 1999 Added vendor information for Data General.
Jul. 9, 1998 Updated information for Digital Equipment Corporation
Jan. 8, 1998 Updates to Section III.B.
Jan. 7, 1998 Updated vendor information for NCR. Updates to Section III.B.
Dec. 19, 1997 Updates to Section III-B and Acknowledgments.
Dec. 16, 1997 Vendor updates for Sun Microsystems, Inc.
Dec. 11, 1997 Vendor updates for Caldera, Digital Equipment
Corporation, NEC Corporation.
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