AUSCERT External Security Bulletin Redistribution

                     ESB-97.160 -- CERT Advisory CA-97.27
                               FTP Bounce
                            11 December 1997


CERT has released the following advisory concerning a problem known as
the "FTP bounce" attack.  This is a well known multi-vendor problem which
is discussed in detail in the advisory.

The following security bulletin is provided as a service to AUSCERT's
members.  As AUSCERT did not write this document, AUSCERT has had no
control over its content.  As such, the decision to use any or all of this
information is the responsibility of each user or organisation, and should
be done so in accordance with site policies and procedures.

NOTE: This is only the original release of the security bulletin.  It will
not be updated when the original bulletin is.  If downloading at a later
date, it is recommended that the bulletin is retrieved from the original
authors to ensure that the information is still current.

Contact information for CERT is included in the Security Bulletin
below.  If you have any questions or need further information, please
contact them directly. 

Previous advisories and external security bulletins can be retrieved from:


If you believe that your system has been compromised, contact AUSCERT or your
representative in FIRST (Forum of Incident Response and Security Teams).

Internet Email: auscert@auscert.org.au
Telephone:      (07) 3365 4417 (International: +61 7 3365 4417)
                AUSCERT personnel answer during Queensland business hours
                which are GMT+10:00 (AEST).
                On call after hours for emergencies.
Facsimile:      (07) 3365 7031

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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

Phone    +1 412-268-7090 (24-hour hotline)
                CERT personnel answer 8:30-5:00 p.m. EST(GMT-5) / EDT(GMT-4)
                and are on call for emergencies during other hours.

Fax      +1 412-268-6989

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

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

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

This file: ftp://ftp.cert.org/pub/cert_advisories/CA-97.27.FTP_bounce
               click on "CERT Advisories"

Revision history

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