************************************************************************** Security Bulletin 9550 DISA Defense Communications System December 5, 1995 Published by: DISN Security Coordination Center (SCC@NIC.DDN.MIL) 1-(800) 365-3642 DEFENSE INFORMATION SYSTEM NETWORK SECURITY BULLETIN The DISN SECURITY BULLETIN is distributed by the DISN SCC (Security Coordination Center) under DISA contract as a means of communicating information on network and host security exposures, fixes, and concerns to security and management personnel at DISN facilities. Back issues may be obtained via FTP (or Kermit) from NIC.DDN.MIL [192.112.36.5] using login="anonymous" and password="guest". The bulletin pathname is scc/sec-yynn (where "yy" is the year the bulletin is issued and "nn" is a bulletin number, e.g. scc/sec-9544.txt). These are also available at our WWW site, http://nic.ddn.mil. ************************************************************************** + - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - + ! ! ! The following important advisory was issued by the Automated ! ! Systems Security Incident Support Team (ASSIST) and is being ! ! relayed unedited via the Defense Information Systems Agency's ! ! Security Coordination Center distribution system as a means ! ! of providing DISN subscribers with useful security information. ! ! ! + - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - + -----BEGIN PGP SIGNED MESSAGE----- <<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>> Automated Systems Security Incident Support Team _____ ___ ___ _____ ___ _____ | / /\ / \ / \ | / \ | | / Integritas / \ \___ \___ | \___ | | < et /____\ \ \ | \ | | \ Celeritas / \ \___/ \___/ __|__ \___/ | |_____\ <<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>> Bulletin 95-47 Release date: 4 December, 1995, 4:15 PM EST (GMT -5) SUBJECT: wu-ftpd Misconfiguration Vulnerability SUMMARY: A vulnerability exists with certain configurations of the SITE EXEC command in the Washington University ftpd, also known as wu-ftpd. Exploitation of this vulnerability may allow root access from any account on the system. The vulnerable configuration is known to exist in numerous Linux distributions and is currently being actively exploited by intruders. It should be noted that this vulnerability is not necessarily limited to Linux but may exist on any wu-ftpd installation. Thus, all users of the wu-ftpd program, not just the Linux users, should take this opportunity to verify the configuration of their daemons. Note that versions of wu-ftpd before the 2.4 release contain serious security vulnerabilities and should be updated immediately. BACKGROUND: There is a problem with the default configuration of the Washington University FTP Server version 2.4 in major Linux distributions, including but not limited to Slackware 2.0, 2.1, 2.2, 2.3, Yggdrasil Plug&Play Fall'94, and the Debian Distribution. By exploiting this problem, any user who is able to log into a system having the vulnerable configuration via FTP using their login, and not the anonymous login, may gain root access. Other systems besides Linux can be configured to be vulnerable although the standard wu-ftpd 2.4 source code as distributed is not vulnerable. The problem is that the variable _PATH_EXECPATH was set to "/bin" in the configuration file src/pathnames.h when the distribution binary was built. _PATH_EXECPATH should be set to "/bin/ftp-exec" or a similar directory that does not contain a shell or command interpreter, for example. The source code shipped with the Linux distributions contains the correct value ("/bin/ftp-exec") despite the incorrect distribution binary. You should verify that _PATH_EXECPATH has the correct value before recompiling. Note that the documentation for wu-ftpd states that the directory defined by _PATH_EXECPATH is relative to ~ftp, the ftp home directory as specified in the password file. This is misleading. The pathname is relative to ~ftp for anonymous users only. This pathname is relative to "/" for other user sessions. How to determine if you are vulnerable All systems running wu-ftpd should be checked to determine if the configuration is vulnerable. To test your configuration, access the ftp server using a legitimate user account (not an anonymous ftp login) and execute the following commands: srchost> ftp ftphost Connected to ftphost 220 ftphost FTP server (Version wu-2.4(2) Mon Apr 18 09:12:35 GMT+1000 1994) ready. Name (srchost:joe): 331 Password required for joe. Password: 230 User joe logged in. ftp> quote site exec echo problem 200-echo problem 200-problem 200 (end of 'echo problem') ftp> quit 221 Goodbye. srchost> If you receive the line "200-problem", your site is vulnerable. IMPACT: Any user with a local account on a system offering FTP services with the vulnerable configuration may gain root access. Support for anonymous FTP access is not required to exploit this vulnerability. RECOMMENDED SOLUTIONS: If you have the vulnerability, we recommend that you turn off ftpd immediately using the method described in Section A below. Once you have done that, you can then decide whether to rebuild or fetch a new ftpd binary. If you have built wu-ftpd from a source distribution, follow the steps in Sections B.2 and B.3 below. Once you have eliminated this vulnerability, turn on ftpd with the method described in Section C below. A. Disable ftpd To disable ftpd, do the following as root. 1. Shut down the FTP server using the ftpshut command. This command blocks all connections to the FTP server. For ftpshut to work correctly, the ftpaccess(5) file will need a shutdown directive that names a file used by wu-ftpd to indicate that the server is shutdown. If your ftpaccess file does not have such a directive, add one to that file. When added, use ftpshut(8) to shut down the server. Once the server has been shutdown, all new incoming FTP requests will fail. Here is an example of the ftpshut command: ftpshut now 2. Verify that the FTP service has been shut down by attempting to connect to it. You should see a message that contains a line similar to the following: hostname FTP server shut down -- please try again later where hostname is the host from which you are requesting FTP service. B. Correct the configuration Item 1 below applies to those running Debian Linux. Item 2 applies to all other Linux systems. Item 3 applies to those who are building wu-ftpd from source on systems other than Linux. 1. If you are running Debian Linux, obtain a fixed binary, available from the following location, and install this binary. ftp://ftp.debian.org/debian/debian-0.93/binary/net/wu-ftpd-2.4-14.deb MD5 (wu-ftpd-2.4-14.deb) = c00a0aac75216bf83568aee4c2e7d168 2. If you are running any version of Linux, there is a version of the source code available that has been improved to compile more cleanly. It too is correctly configured for SITE EXEC. It is available from (file wu-ftpd-2.4-fixed.tar.gz) ftp://bach.cis.temple.edu/pub/Linux/security/wu-ftpd-2.4-fix/ MD5 (wu-ftpd-2.4-fixed.tar.gz) = 3e1c6fd7cd6757e45894df0d3638b524 This version is also correctly configured for the SITE EXEC command and can be compiled and installed. Consult Section D below for suggestions on how to configure wu-ftpd. 3. If you are running a version of wu-ftpd before version 2.4, you should upgrade to version 2.4 first. That version is available from ftp://wuarchive.wustl.edu/packages/wuarchive-ftpd/wu-ftpd-2.4.tar.Z MD5 (wu-ftpd-2.4.tar.Z) = 57f1a962c90a9b12825d39af518df433 Version 2.4 is correctly configured for the SITE EXEC command and can be compiled and installed. Consult Section D below for suggestions on how to configure wu-ftpd. C. Enabling ftpd 1. To turn ftpd back on, delete the file referenced by the shutdown directive in your ftpaccess file. 2. Verify that the FTP service has been enabled by attempting to connect to it. You should see a message that contains lines similar to the following: srchost> ftp ftphost Connected to ftphost 220 ftphost FTP server (Version wu-2.4(3) Mon Apr 3 16:53:11 EDT 1995) ready. Name (srchost:joe): D. Advice on configuring the FTP Daemon for SITE EXEC Here are some configuration guidelines for the directories named by the _PATH_EXECPATH variable. 1. Directories used by SITE EXEC: The documentation for wu-ftpd states that the directory defined by the _PATH_EXECPATH variable is relative to ~ftp, the ftp home directory as specified in the password file. This is misleading. The pathname is relative to ~ftp for anonymous users only. The pathname is relative to "/" for all other user sessions. Therefore, you need to check the two directories used by the SITE EXEC command. For example, if the _PATH_EXECPATH variable is set to /bin/ftp-exec, then wu-ftpd searches the ~ftp/bin/ftp-exec directory for programs specified by SITE EXEC when the anonymous login is used, and the /bin/ftp-exec directory specified by SITE EXEC when any other login is used. 2. Contents of the directories used by SITE EXEC: The commands installed in these directories can be executed by the SITE EXEC command. We strongly recommend that this directory contain only those programs that you wish to be executed by those users who connect to your FTP server. An example of a program to install in these directories is the ls program. Programs that should not be installed in these directories are shells, for example sh or csh, and command interpreters, for example awk and perl. <<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>> ASSIST would like to thank the CERT Coordination Center for information contained in this bulletin. <<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>> ASSIST is an element of the Defense Information Systems Agency (DISA), and provides service to the entire DoD community. Constituents of the DoD with questions about ASSIST or computer security issues, can contact ASSIST using one of the methods listed below. Non-DoD organizations/institutions, contact the Forum of Incident Response and Security Teams (FIRST) representative. To obtain a list of FIRST member organizations and their constituencies send an email to docserver@first.org with an empty "subject" line and a message body containing the line "send first-contacts". ASSIST Information Resources: To be included in the distribution list for the ASSIST bulletins, send your Milnet (Internet) e-mail address to assist-request@assist.mil. Back issues of ASSIST bulletins, and other security related information, are available from the ASSIST BBS at 703-607-4710, 327-4710, and through anonymous FTP from assist.mil (IP address 199.211.123.11). Note: assist.mil will only accept anonymous FTP connections from Milnet addresses that are registered with the NIC or DNS. If your system is not registered, you must provide your MILNET IP address to ASSIST before access can be provided. ASSIST Contact Information: PHONE: 800-357-4231, COMM 703-607-4700, DSN 327-4700. ELECTRONIC MAIL: assist@assist.mil. ASSIST BBS: COMM 703-607-4710, DSN 327-4710, leave a message for the "sysop". FAX: COMM 703-607-4735, DSN 607-4735 ASSIST uses Pretty Good Privacy (PGP) 2.6.2 as the digital signature mechanism for bulletins. PGP 2.6.2 incorporates the RSAREF(tm) Cryptographic Toolkit under license from RSA Data Security, Inc. A copy of that license is available via anonymous FTP from net-dist.mit.edu (IP 18.72.0.3) in the file /pub/PGP/rsalicen.txt, and through the world wide web from http://net-dist.mit.edu/pgp.html. In accordance with the terms of that license, PGP 2.6.2 may be used for non-commercial purposes only. Instructions for downloading the PGP 2.6.2 software can also be obtained from net-dist.mit.edu in the pub/PGP/README file. PGP 2.6.2 and RSAREF may be subject to the export control laws of the United States of America as implemented by the United States Department of State Office of Defense Trade Controls. The PGP signature information will be attached to the end of ASSIST bulletins. Reference herein to any specific commercial product, process, or service by trade name, trademark manufacturer, or otherwise, does not constitute or imply its endorsement, recommendation, or favoring by ASSIST. 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If you are not part of the DOD community, please contact your agency's incident response team to report incidents. Your agency's team will coordinate with DOD. The Forum of Incident Response and Security Teams (FIRST) is a world-wide organization. A list of FIRST member organizations and their constituencies can be obtained by sending email to docserver@first.org with an empty subject line and a message body containing the line: send first-contacts. This document was prepared as an service to the DOD community. Neither the United States Government nor any of their employees, makes any warranty, expressed or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial products, process, or service by trade name, trademark manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government. The opinions of the authors expressed herein do not necessarily state or reflect those of the United States Government, and shall not be used for advertising or product endorsement purposes.