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************************************************************************** Security Bulletin 9318 DISA Defense Communications System September 30, 1993 Published by: DDN Security Coordination Center (SCC@NIC.DDN.MIL) 1-(800) 365-3642 DEFENSE DATA NETWORK SECURITY BULLETIN The DDN SECURITY BULLETIN is distributed by the DDN 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 DDN 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/ddn-security-yynn (where "yy" is the year the bulletin is issued and "nn" is a bulletin number, e.g. scc/ddn-security-9302). ************************************************************************** + - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - + ! ! ! The following important advisory was issued by the Computer ! ! Emergency Response Team (CERT) and is being relayed unedited ! ! via the Defense Information Systems Agency's Security ! ! Coordination Center distribution system as a means of ! ! providing DDN subscribers with useful security information. ! ! ! + - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - + Below Incident Advisory is provided by Department of Energy, CIAC Team. ======================================================================== DISTRIBUTION RESTRICTIONS: NONE -- PUBLIC RELEASE _____________________________________________________ The Computer Incident Advisory Capability ___ __ __ _ ___ / | / \ / \___ __|__ /___\ \___ _____________________________________________________ ADVISORY NOTICE Automated Scanning of Network Vulnerabilities September 30, 1993 1000 PDT Number D-25 __________________________________________________________________________ PROBLEM: Automated attacks on networked computers. PLATFORM: All systems supporting TCP/IP networking. DAMAGE: Unauthorized access to information and computer resources. SOLUTION: Examine machines for vulnerabilities detailed below and apply fixes as needed. __________________________________________________________________________ Critical Information about Automated Network Scanning Software CIAC has learned that software allowing automated scanning of networked computers for security vulnerabilities was recently made publicly available on the Internet. The software package, known as ISS or Internet Security Scanner, will interrogate all computers within a specified IP address range, determining the security posture of each with respect to several common system vulnerabilities. The software was designed as a security tool for system and network administrators. However, given its wide distribution and ability to scan remote networks, CIAC feels that it is likely ISS will also be used to locate vulnerable hosts for malicious reasons. While none of the vulnerabilities ISS checks for are new, their aggregation into a widely available automated tool represents a higher level of threat to networked machines. CIAC has analyzed the operation of the program and strongly recommends that administrators take this opportunity to re-examine systems for the vulnerabilities described below. Also detailed below are available security tools that may assist in the detection and prevention of malicious use of ISS. Finally, common symptoms of an ISS attack are outlined to allow detection of malicious use. ISS Vulnerabilities ------------------- The following vulnerabilities are tested for by the ISS tool. Administrators should verify the state of their systems and perform corrective actions as indicated. Default Accounts The accounts "guest" and "bbs", if they exist, should have non-trivial passwords. If login access to these accounts is not needed, they should be disabled by placing a "*" in the password field and the string "/bin/false" in the shell field in /etc/passwd. See the system manual entry for "passwd" for more information on changing passwords and disabling accounts. For example, the /etc/passwd entry for a disabled guest account should resemble the following: guest:*:2311:50:Guest User:/home/guest:/bin/false lp Account The account "lp", if it exists, should not allow logins. It should be disabled by placing a "*" in the password field and the string "/bin/false" in the shell field in /etc/passwd. Decode Alias Mail aliases for decode and uudecode should be disabled on UNIX systems. If the file /etc/aliases contains entries for these programs, they should be disabled by placing a "#" at the beginning of the line and then executing the command "newaliases". Consult the manual page for "aliases" for more information on UNIX mail aliases. A disabled decode alias should appear as follows: # decode: "|/usr/bin/uudecode" Sendmail The sendmail commands "wiz" and "debug" should be disabled. This may be verified by executing the following commands: % telnet hostname 25 220 host Sendmail 5.65 ready at Wed, 29 Sep 93 20:28:46 PDT wiz You wascal wabbit! Wandering wizards won't win! (or 500 Command unrecognized) quit % telnet hostname 25 220 host Sendmail 5.65 ready at Wed, 29 Sep 93 20:28:46 PDT debug 500 Command unrecognized quit If the "wiz" command returns "Please pass, oh mighty wizard", your system is vulnerable to attack. The command should be disabled by adding a line to the sendmail.cf configuration file containing the string: OW* If the "debug" command responds with the string "200 Debug set", you should immediately obtain a newer version of sendmail software from your vendor. Anonymous FTP Anonymous FTP allows users without accounts to have restricted access to certain directories on the system. The availability of anonymous FTP on a given system may be determined by executing the following commands: % ftp hostname Connected to hostname. 220 host FTP server ready. Name (localhost:jdoe): anonymous 530 User anonymous unknown. Login failed. The above results indicate that anonymous FTP is not enabled. If the system instead replies with the string "331 Guest login ok" and then prompts for a password, anonymous FTP access is enabled. The configuration of systems allowing anonymous FTP should be checked carefully, as improperly configured FTP servers are frequently attacked. Refer to CIAC Bulletin D-19 for more information. NIS SunOS 4.x machines using NIS are vulnerable unless the patch 100482 has been installed. See CIAC Bulletin C-25 for more information regarding this patch. NFS Filesystems exported under NFS should be mountable only by a restricted set of hosts. The UNIX "showmount" command will display the filesystems exported by a given host: % /usr/etc/showmount -e hostname export list for hostname: /usr hosta:hostb:hostc /usr/local (everyone) The above output indicates that this NFS server is exporting two partitions: /usr, which can be mounted by hosta, hostb, and hostc; and /usr/local which can be mounted by anyone. In this case, access to the /usr/local partition should be restricted. Consult the system manual entry for "exports" or "NFS" for more information. rusers The UNIX rusers command displays information about accounts currently active on a remote system. This may provide an attacker with account names or other information useful in mounting an attack. To check for the availability of rusers information on a particular machine, execute the following command: % rusers -l hostname hostname: RPC: Program not registered If the above example had instead generated a list of user names and login information, a rusers server is running on the host. The server may be disabled by placing a "#" at the beginning of the appropriate line in the file /etc/inetd.conf and then sending the SIGHUP signal to the inetd process. For example, a disabled rusers entry might appear as follows: #rusersd/2 dgram rpc/udp wait root /usr/etc/rusersd rusersd rexd The UNIX remote execution server rexd provides only minimal authentication and is easily subverted. It should be disabled by placing a "#" at the beginning of the rexd line in the file /etc/inetd.conf and then sending the SIGHUP signal to the inetd process. The disabled entry should resemble the following: #rexd/1 stream rpc/tcp wait root /usr/etc/rexd rexd Available Tools --------------- There are several available security tools that may be used to prevent or detect malicious use of ISS. They include the following: SPI SPI, the Security Profile Inspector, will detect the system vulnerabilities described above, as well as many others. U.S. Government agencies interested in obtaining SPI should send E-mail to spi@cheetah.llnl.gov or call (510) 422-3881 for more information. COPS The COPS security tool will also detect the vulnerabilities described above. It is available via anonymous FTP from ftp.cert.org in the directory /pub/tools/cops/1.04. ISS Running ISS on your systems will provide you with the same information an attacker would obtain, allowing you to correct vulnerabilities before they can be exploited. Note that the current version of the software is known to function poorly on some operating systems. If you should have difficulty using the software, please contact CIAC for assistance. ISS may be obtained via anonymous FTP from ftp.uu.net in the directory /usenet/comp.sources.misc/volume39/iss. TCP Wrappers Access to most UNIX network services can be more closely controlled using software known as a TCP wrapper. The wrapper provides additional access control and flexible logging features that may assist in both the prevention and detection of network attacks. This software is available via anonymous FTP from ftp.win.tue.nl in the file /pub/security/tcp_wrappers_6.0.shar.Z Detecting an ISS Attack ----------------------- Given the wide distribution of the ISS tool, CIAC feels that remote attacks are likely to occur. Such attacks can cause system warnings to be generated that may prove useful in tracking down the source of the attack. The most probable indicator of an ISS attack is a mail message sent to "postmaster" on the scanned system similar to the following: From: Mailer-Daemon@hostname (Mail Delivery Subsystem) Subject: Returned mail: Unable to deliver mail Message-Id: <9309291633.AB04591@> To: Postmaster@hostname ----- Transcript of session follows ----- <<< VRFY guest 550 guest... User unknown <<< VRFY decode 550 decode... User unknown <<< VRFY bbs 550 bbs... User unknown <<< VRFY lp 550 lp... User unknown <<< VRFY uudecode 550 uudecode... User unknown <<< wiz 500 Command unrecognized <<< debug 500 Command unrecognized 421 Lost input channel to remote.machine ----- No message was collected ----- If you should receive such a message, it is likely that your machine and others on your network have been scanned for vulnerabilities. You should immediately contact your computer security officer or CIAC for assistance in assessing the damage and taking corrective action. For additional information or assistance, please contact CIAC at (510) 423-9878 or send E-mail to ciac@llnl.gov. FAX messages to (510) 423-8002. PLEASE NOTE: Many users outside of the DOE and ESnet computing communities receive CIAC bulletins. If you are not part of these communities, please contact your agency's response team to report incidents. Your agency's team will coordinate with CIAC. 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 account of work sponsored by an agency of the United States Government. Neither the United States Government nor the University of California 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 or the University of California. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government nor the University of California, and shall not be used for advertising or product endorsement purposes. **************************************************************************** * * * The point of contact for MILNET security-related incidents is the * * Security Coordination Center (SCC). * * * * E-mail address: SCC@NIC.DDN.MIL * * * * Telephone: 1-(800)-365-3642 * * * * NIC Help Desk personnel are available from 7:00 a.m.-7:00 p.m. EST, * * Monday through Friday except on federal holidays. * * * ****************************************************************************