=========================================================================== CA-93:14 CERT Advisory September 30, 1993 Internet Security Scanner (ISS) --------------------------------------------------------------------------- The CERT Coordination Center has received information concerning software that allows automated scanning of TCP/IP networked computers for security vulnerabilities. This software was posted to the comp.sources.misc Usenet newsgroup. 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. ISS does not attempt to gain access to a system being tested. However, given its wide distribution and ability to scan remote networks, CERT 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. CERT has analyzed the operation of the program and strongly recommends that administrators take this opportunity to re-examine systems for the vulnerabilities described below. 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. Vulnerabilities probed by ISS ----------------------------- The following vulnerabilities are currently 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 removed, or 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(1)" 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 removed, or disabled by placing a "#" at the beginning of the line and then executing the command "newaliases". Consult the manual page for "aliases(1)" 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 EDT 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 EDT 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 the following line to the sendmail.cf configuration file containing the string: OW* For this change to take effect, kill the sendmail process, refreeze the sendmail.cf file, and restart the sendmail process. 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 CERT Advisory CA-93:10 for more information. NIS ISS attempts to guess the NIS domainname. The program will try to grab the password file from ypserv. See CERT Advisory CA-92:13 for more information regarding SunOS 4.x machines using NIS. See CERT Advisory CA-93:01 for more information regarding HP machines using NIS. NFS Filesystems exported under NFS should be mountable only by a restricted set of hosts. The UNIX "showmount" command will display the filesystems currently 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(5)" or "NFS(4P)" 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 See CERT Advisory CA-92:05 for more information regarding IBM AIX machines using 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: COPS The COPS security tool will also detect the vulnerabilities described above. It is available via anonymous FTP from 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. 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 cert.org in the directory pub/tools/tcp_wrappers. Detecting an ISS Attack ----------------------- Given the wide distribution of the ISS tool, CERT 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 a 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 ----- --------------------------------------------------------------------------- The CERT Coordination Center would like to thank Steve Weeber from the Department of Energy's CIAC Team for his contribution to this advisory. --------------------------------------------------------------------------- If you believe that your system has been compromised, contact the CERT Coordination Center or your representative in FIRST (Forum of Incident Response and Security Teams). Internet E-mail: cert@cert.org Telephone: 412-268-7090 (24-hour hotline) CERT personnel answer 8:30 a.m.-5:00 p.m. EST(GMT-5)/EDT(GMT-4), and are on call for emergencies during other hours. CERT Coordination Center Software Engineering Institute Carnegie Mellon University Pittsburgh, PA 15213-3890 Past advisories, information about FIRST representatives, and other information related to computer security are available for anonymous FTP from cert.org (192.88.209.5).