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Network Working Group M. Allman Request for Comments: 2577 NASA Glenn/Sterling Software Category: Informational S. Ostermann

                                                       Ohio University
                                                              May 1999
                    FTP Security Considerations

Status of this Memo

 This memo provides information for the Internet community.  It does
 not specify an Internet standard of any kind.  Distribution of this
 memo is unlimited.

Copyright Notice

 Copyright (C) The Internet Society (1999).  All Rights Reserved.


 The specification for the File Transfer Protocol (FTP) contains a
 number of mechanisms that can be used to compromise network security.
 The FTP specification allows a client to instruct a server to
 transfer files to a third machine.  This third-party mechanism, known
 as proxy FTP, causes a well known security problem.  The FTP
 specification also allows an unlimited number of attempts at entering
 a user's password.  This allows brute force "password guessing"
 attacks.  This document provides suggestions for system
 administrators and those implementing FTP servers that will decrease
 the security problems associated with FTP.

1 Introduction

 The File Transfer Protocol specification (FTP) [PR85] provides a
 mechanism that allows a client to establish an FTP control connection
 and transfer a file between two FTP servers.  This "proxy FTP"
 mechanism can be used to decrease the amount of traffic on the
 network; the client instructs one server to transfer a file to
 another server, rather than transferring the file from the first
 server to the client and then from the client to the second server.
 This is particularly useful when the client connects to the network
 using a slow link (e.g., a modem).  While useful, proxy FTP provides
 a security problem known as a "bounce attack" [CERT97:27].  In
 addition to the bounce attack, FTP servers can be used by attackers
 to guess passwords using brute force.

Allman & Ostermann Informational [Page 1] RFC 2577 FTP Security Considerations May 1999

 This document does not contain a discussion of FTP when used in
 conjunction with strong security protocols, such as IP Security.
 These security concerns should be documented, however they are out of
 the scope of this document.
 This paper provides information for FTP server implementers and
 system administrators, as follows.  Section 2 describes the FTP
 "bounce attack".  Section 3 provides suggestions for minimizing the
 bounce attack.  Section 4 provides suggestions for servers which
 limit access based on network address.  Section 5 provides
 recommendations for limiting brute force "password guessing" by
 clients.  Next, section 6 provides a brief discussion of mechanisms
 to improve privacy.  Section 7 provides a mechanism to prevent user
 identity guessing.  Section 8 discusses the practice of port
 stealing.  Finally, section 9 provides an overview of other FTP
 security issues related to software bugs rather than protocol issues.

2 The Bounce Attack

 The version of FTP specified in the standard [PR85] provides a method
 for attacking well known network servers, while making the
 perpetrators difficult to track down.  The attack involves sending an
 FTP "PORT" command to an FTP server containing the network address
 and the port number of the machine and service being attacked.  At
 this point, the original client can instruct the FTP server to send a
 file to the service being attacked.  Such a file would contain
 commands relevant to the service being attacked (SMTP, NNTP, etc.).
 Instructing a third party to connect to the service, rather than
 connecting directly, makes tracking down the perpetrator difficult
 and can circumvent network-address-based access restrictions.
 As an example, a client uploads a file containing SMTP commands to an
 FTP server.  Then, using an appropriate PORT command, the client
 instructs the server to open a connection to a third machine's SMTP
 port.  Finally, the client instructs the server to transfer the
 uploaded file containing SMTP commands to the third machine.  This
 may allow the client to forge mail on the third machine without
 making a direct connection.  This makes it difficult to track

3 Protecting Against the Bounce Attack

 The original FTP specification [PR85] assumes that data connections
 will be made using the Transmission Control Protocol (TCP) [Pos81].
 TCP port numbers in the range 0 - 1023 are reserved for well known
 services such as mail, network news and FTP control connections
 [RP94].  The FTP specification makes no restrictions on the TCP port
 number used for the data connection.  Therefore, using proxy FTP,

Allman & Ostermann Informational [Page 2] RFC 2577 FTP Security Considerations May 1999

 clients have the ability to tell the server to attack a well known
 service on any machine.
 To avoid such bounce attacks, it is suggested that servers not open
 data connections to TCP ports less than 1024.  If a server receives a
 PORT command containing a TCP port number less than 1024, the
 suggested response is 504 (defined as "Command not implemented for
 that parameter" by [PR85]).  Note that this still leaves non-well
 known servers (those running on ports greater than 1023) vulnerable
 to bounce attacks.
 Several proposals (e.g., [AOM98] and [Pis94]) provide a mechanism
 that would allow data connections to be made using a transport
 protocol other than TCP.  Similar precautions should be taken to
 protect well known services when using these protocols.
 Also note that the bounce attack generally requires that a
 perpetrator be able to upload a file to an FTP server and later
 download it to the service being attacked.  Using proper file
 protections will prevent this behavior.  However, attackers can also
 attack services by sending random data from a remote FTP server which
 may cause problems for some services.
 Disabling the PORT command is also an option for protecting against
 the bounce attack.  Most file transfers can be made using only the
 PASV command [Bel94].  The disadvantage of disabling the PORT command
 is that one loses the ability to use proxy FTP, but proxy FTP may not
 be necessary in a particular environment.

4 Restricted Access

 For some FTP servers, it is desirable to restrict access based on
 network address.  For example, a server might want to restrict access
 to certain files from certain places (e.g., a certain file should not
 be transferred out of an organization).  In such a situation, the
 server should confirm that the network address of the remote hosts on
 both the control connection and the data connection are within the
 organization before sending a restricted file.  By checking both
 connections, a server is protected against the case when the control
 connection is established with a trusted host and the data connection
 is not.  Likewise, the client should verify the IP address of the
 remote host after accepting a connection on a port opened in listen
 mode to verify that the connection was made by the expected server.
 Note that restricting access based on network address leaves the FTP
 server vulnerable to "spoof" attacks.  In a spoof attack, for
 example, an attacking machine could assume the host address of
 another machine inside an organization and download files that are

Allman & Ostermann Informational [Page 3] RFC 2577 FTP Security Considerations May 1999

 not accessible from outside the organization.  Whenever possible,
 secure authentication mechanisms should be used, such as those
 outlined in [HL97].

5 Protecting Passwords

 To minimize the risk of brute force password guessing through the FTP
 server, it is suggested that servers limit the number of attempts
 that can be made at sending a correct password.  After a small number
 of attempts (3-5), the server should close the control connection
 with the client.  Before closing the control connection the server
 must send a return code of 421 ("Service not available, closing
 control connection." [PR85]) to the client.  In addition, it is
 suggested that the server impose a 5 second delay before replying to
 an invalid "PASS" command to diminish the efficiency of a brute force
 attack.  If available, mechanisms already provided by the target
 operating system should be used to implement the above suggestions.
 An intruder can subvert the above mechanisms by establishing
 multiple, parallel control connections to a server.  To combat the
 use of multiple concurrent connections, the server could either limit
 the total number of control connections possible or attempt to detect
 suspicious activity across sessions and refuse further connections
 from the site.  However, both of these mechanisms open the door to
 "denial of service" attacks, in which an attacker purposely initiates
 the attack to disable access by a valid user.
 Standard FTP [PR85] sends passwords in clear text using the "PASS"
 command.  It is suggested that FTP clients and servers use alternate
 authentication mechanisms that are not subject to eavesdropping (such
 as the mechanisms being developed by the IETF Common Authentication
 Technology Working Group [HL97]).

6 Privacy

 All data and control information (including passwords) is sent across
 the network in unencrypted form by standard FTP [PR85].  To guarantee
 the privacy of the information FTP transmits, a strong encryption
 scheme should be used whenever possible.  One such mechanism is
 defined in [HL97].

7 Protecting Usernames

 Standard FTP [PR85] specifies a 530 response to the USER command when
 the username is rejected.  If the username is valid and a password is
 required FTP returns a 331 response instead.  In order to prevent a
 malicious client from determining valid usernames on a server, it is
 suggested that a server always return 331 to the USER command and

Allman & Ostermann Informational [Page 4] RFC 2577 FTP Security Considerations May 1999

 then reject the combination of username and password for an invalid

8 Port Stealing

 Many operating systems assign dynamic port numbers in increasing
 order.  By making a legitimate transfer, an attacker can observe the
 current port number allocated by the server and "guess" the next one
 that will be used.  The attacker can make a connection to this port,
 thus denying another legitimate client the ability to make a
 transfer.  Alternatively, the attacker can steal a file meant for a
 legitimate user.  In addition, an attacker can insert a forged file
 into a data stream thought to come from an authenticated client.
 This problem can be mitigated by making FTP clients and servers use
 random local port numbers for data connections, either by requesting
 random ports from the operating system or using system dependent

9 Software-Base Security Problems

 The emphasis in this document is on protocol-related security issues.
 There are a number of documented FTP security-related problems that
 are due to poor implementation as well.  Although the details of
 these types of problems are beyond the scope of this document, it
 should be pointed out that the following FTP features has been abused
 in the past and should be treated with great care by future
 Anonymous FTP
    Anonymous FTP refers to the ability of a client to connect to an
    FTP server with minimal authentication and gain access to public
    files.  Security problems arise when such a user can read all
    files on the system or can create files. [CERT92:09] [CERT93:06]
 Remote Command Execution
    An optional FTP extension, "SITE EXEC", allows clients to execute
    arbitrary commands on the server.  This feature should obviously
    be implemented with great care.  There are several documented
    cases of the FTP "SITE EXEC" command being used to subvert server
    security [CERT94:08] [CERT95:16]
 Debug Code
    Several previous security compromises related to FTP can be
    attributed to software that was installed with debugging features
    enabled [CERT88:01].

Allman & Ostermann Informational [Page 5] RFC 2577 FTP Security Considerations May 1999

 This document recommends that implementors of FTP servers with these
 capabilities review all of the CERT advisories for attacks on these
 or similar mechanisms before releasing their software.

10 Conclusion

 Using the above suggestions can decrease the security problems
 associated with FTP servers without eliminating functionality.

11 Security Considerations

 Security issues are discussed throughout this memo.


 We would like to thank Alex Belits, Jim Bound, William Curtin, Robert
 Elz, Paul Hethmon, Alun Jones and Stephen Tihor for their helpful
 comments on this paper.  Also, we thank the FTPEXT WG members who
 gave many useful suggestions at the Memphis IETF meeting.


 [AOM98]     Allman, M., Ostermann, S. and C. Metz, "FTP Extensions
             for IPv6 and NATs", RFC 2428, September 1998.
 [Bel94]     Bellovin. S., "Firewall-Friendly FTP", RFC 1579, February
 [CERT88:01] CERT Advisory CA-88:01. ftpd Vulnerability. December,
 [CERT92:09] CERT Advisory CA-92:09. AIX Anonymous FTP Vulnerability.
             April 27, 1992.
 [CERT93:06] CERT Advisory CA-93:06. Wuarchive ftpd Vulnerability.
             September 19,1997
 [CERT94:08] CERT Advisory CA-94:08. ftpd Vulnerabilities. September
             23, 1997.
 [CERT95:16] CERT Advisory CA-95:16. wu-ftpd Misconfiguration
             Vulnerability.  September 23, 1997
 [CERT97:27] CERT Advisory CA-97.27. FTP Bounce.  January 8, 1998.

Allman & Ostermann Informational [Page 6] RFC 2577 FTP Security Considerations May 1999

 [HL97]      Horowitz, M. and S. Lunt, "FTP Security Extensions", RFC
             2228, October 1997.
 [Pis94]     Piscitello, D., "FTP Operation Over Big Address Records
             (FOOBAR), RFC 1639, June 1994.
 [Pos81]     Postel, J., "Transmission Control Protocol", STD 7, RFC
             793, September 1981.
 [PR85]      Postel, J. and J. Reynolds, "File Transfer Protocol
             (FTP)", STD 9, RFC 959, October 1985.
 [RP94]      Reynolds, J. and J. Postel, "Assigned Numbers", STD 2,
             RFC 1700, October 1994.  See also:

Authors' Addresses

 Mark Allman
 NASA Glenn Research Center/Sterling Software
 21000 Brookpark Rd.  MS 54-2
 Cleveland, OH  44135
 Shawn Ostermann
 School of Electrical Engineering and Computer Science
 Ohio University
 416 Morton Hall
 Athens, OH  45701

Allman & Ostermann Informational [Page 7] RFC 2577 FTP Security Considerations May 1999

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 Internet Society.

Allman & Ostermann Informational [Page 8]

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