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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
   General.
   
   A complete revision history is at the end of this file.
     _________________________________________________________________
   
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
   
   ftp://ftp.cert.org/pub/tech_tips/FTP_PORT_attacks
   
   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.
   
     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
   
   ftp://ftp.academ.com/pub/wu-ftpd/private/wu-ftpd-2.4.2-beta-16.tar.Z
   
   MD5 (wu-ftpd-2.4.2-beta-16.tar.Z) = c18c083c2a82eef1ccba6df9a406f026
   
   Further information on this package can be obtained from
   
   https://www.academ.com/academ/wu-ftpd.
   
     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
   
   ftp://ftp.cert.org/pub/tech_tips/anonymous_ftp_config
   
   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, Inc.

   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
   obtained from:
   
   ftp://ftp.caldera.com/pub/openlinux/updates/1.1/current/
   
   Other Caldera security resources are located at:
   
   https://www.caldera.com/tech-ref/security/
   
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.
   
DATA GENERAL


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:

       https://www.service.digital.com/html/patch_service.html

    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.
   
Hewlett-Packard Company

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

     https://us-support.external.hp.com
       (for US, Canada, Asia-Pacific, & Latin-America)

     https://europe-support.external.hp.com
       (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:

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

         https://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:
   
   https://service.software.ibm.com/aixsupport/
   
   or send e-mail to aixserv@austin.ibm.com with a subject of "FixDist".
   
MadGoat

   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 value.
   
NEC Corporation

   Several NEC Unix systems have proven vulnerable. Work is currently
   underway to identify all affected systems. Patches are forthcoming.
   
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 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:
   ftp.netbsd.org:/pub/NetBSD/NetBSD-current/src/libexec/ftpd
   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
   CMW+
   
   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
   Microsystems.
   ______________________________________________________________________
   
   This document is available from:
   https://www.cert.org/advisories/CA-97.27.FTP_bounce.html.
   ______________________________________________________________________
   
CERT/CC Contact Information

   Email: cert@cert.org
          Phone: +1 412-268-7090 (24-hour hotline)
          Fax: +1 412-268-6989
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          U.S.A.
          
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   ______________________________________________________________________
   
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     _________________________________________________________________
   
   Revision history
   
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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