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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.
+
+Abstract
+
+ 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
+ attackers.
+
+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
+ username.
+
+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
+ mechanisms.
+
+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
+ implementers:
+
+ 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.
+
+Acknowledgments
+
+ 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.
+
+References
+
+ [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
+ 1994.
+
+ [CERT88:01] CERT Advisory CA-88:01. ftpd Vulnerability. December,
+ 1988 ftp://info.cert.org/pub/cert_advisories/
+
+ [CERT92:09] CERT Advisory CA-92:09. AIX Anonymous FTP Vulnerability.
+ April 27, 1992. ftp://info.cert.org/pub/cert_advisories/
+
+ [CERT93:06] CERT Advisory CA-93:06. Wuarchive ftpd Vulnerability.
+ September 19,1997
+ ftp://info.cert.org/pub/cert_advisories/
+
+ [CERT94:08] CERT Advisory CA-94:08. ftpd Vulnerabilities. September
+ 23, 1997. ftp://info.cert.org/pub/cert_advisories/
+
+ [CERT95:16] CERT Advisory CA-95:16. wu-ftpd Misconfiguration
+ Vulnerability. September 23, 1997
+ ftp://info.cert.org/pub/cert_advisories/
+
+ [CERT97:27] CERT Advisory CA-97.27. FTP Bounce. January 8, 1998.
+ ftp://info.cert.org/pub/cert_advisories/
+
+
+
+
+Allman & Ostermann Informational [Page 6]
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+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:
+ http://www.iana.org/numbers.html
+
+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
+
+
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+Allman & Ostermann Informational [Page 7]
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+RFC 2577 FTP Security Considerations May 1999
+
+
+Full Copyright Statement
+
+ Copyright (C) The Internet Society (1999). All Rights Reserved.
+
+ This document and translations of it may be copied and furnished to
+ others, and derivative works that comment on or otherwise explain it
+ or assist in its implementation may be prepared, copied, published
+ and distributed, in whole or in part, without restriction of any
+ kind, provided that the above copyright notice and this paragraph are
+ included on all such copies and derivative works. However, this
+ document itself may not be modified in any way, such as by removing
+ the copyright notice or references to the Internet Society or other
+ Internet organizations, except as needed for the purpose of
+ developing Internet standards in which case the procedures for
+ copyrights defined in the Internet Standards process must be
+ followed, or as required to translate it into languages other than
+ English.
+
+ The limited permissions granted above are perpetual and will not be
+ revoked by the Internet Society or its successors or assigns.
+
+ This document and the information contained herein is provided on an
+ "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
+ TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
+ BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
+ HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
+ MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
+
+Acknowledgement
+
+ Funding for the RFC Editor function is currently provided by the
+ Internet Society.
+
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