********************************************************************** DDN MGT Bulletin 66 DCA DDN Defense Communications System 16 Aug 89 Published by: DDN Network Info Center (NIC@NIC.DDN.MIL) (800) 235-3155 DEFENSE DATA NETWORK MANAGEMENT BULLETIN The DDN MANAGEMENT BULLETIN is distributed online by the DDN Network Information Center under DCA contract as a means of communicating official policy, procedures and other information of concern to management personnel at DDN facilities. Back issues may be read through the TACNEWS server ("@n" command at the TAC) or may be obtained by FTP (or Kermit) from the NIC.DDN.MIL host [26.0.0.73 or 10.0.0.51] using login="anonymous" and password="guest". The pathname for bulletins is DDN-NEWS:DDN-MGT-BULLETIN-nn.TXT (where "nn" is the bulletin number). ********************************************************************** CONFIGURATION MANAGEMENT GUIDELINES This bulletin contains guidelines formulated by the Computer Emergency Response Team (CERT) regarding how to detect whether a UNIX host has been compromised in a particular manner and how to improve the security posture of UNIX hosts. These guidelines were written in response to specific breakin attempts recently experienced on the Internet. A description of the general method used is given below. There were a few MILNET hosts targetted for breakin attempts. The MILNET Host Administrators at these two locations were notified and have taken corrective action. We ask Host Administrators of gateways between MILNET and local area networks to promulgate this information to the system administrators of hosts connected to these local area networks. If you have questions regarding these procedures, please contact your vendor or the CERT: Computer Emergency Response Team CERT@SEI.CMU.EDU 1-412-268-7090 (24 hour hotline) GUIDELINES: Many computers connected to the Internet have recently experienced unauthorized system activity. Investigation shows that the activity has occurred for several months and is spreading. Several UNIX computers have had their "telnet" programs illicitly replaced with versions of "telnet" which log outgoing login sessions (including usernames and passwords to remote systems). It appears that access has been gained to many of the machines which have appeared in some of these session logs. (As a first step, frequent telnet users should change their passwords immediately.) While there is no cause for panic, there are a number of things that system administrators can do to detect whether the security on their machines has been compromised using this approach and to tighten security on their systems where necessary. At a minimum, all UNIX site administrators should do the following: o Test telnet for unauthorized changes by using the UNIX "strings" command to search for path/filenames of possible log files. Affected sites have noticed that their telnet programs were logging information in user accounts under directory names such as "..." and ".mail". In general, we suggest that site administrators be attentive to configuration management issues. These include the following: o Test authenticity of critical programs - Any program with access to the network (e.g., the TCP/IP suite) or with access to usernames and passwords should be periodically tested for unauthorized changes. Such a test can be done by comparing checksums of on-line copies of these programs to checksums of original copies. (Checksums can be calculated with the UNIX "sum" command.) Alternatively, these programs can be periodically reloaded from original tapes. o Privileged programs - Programs that grant privileges to users (e.g., setuid root programs/shells in UNIX) can be exploited to gain unrestricted access to systems. System administrators should watch for such programs being placed in places such as /tmp and /usr/tmp (on UNIX systems). A common malicious practice is to place a setuid shell (sh or csh) in the /tmp directory, thus creating a "back door" whereby any user can gain privileged system access. o Monitor system logs - System access logs should be periodically scanned (e.g., via UNIX "last" command) for suspicious or unlikely system activity. o Terminal servers - Terminal servers with unrestricted network access (that is, terminal servers which allow users to connect to and from any system on the Internet) are frequently used to camouflage network connections, making it difficult to track unauthorized activity. Most popular terminal servers can be configured to restrict network access to and from local hosts. o Passwords - Guest accounts and accounts with trivial passwords (e.g., username=password, password=none) are common targets. System administrators should make sure that all accounts are password protected and encourage users to use acceptable passwords as well as to change their passwords periodically, as a general practice. For more information on passwords, see Federal Information Processing Standard Publication (FIPS PUB) 112, available from the National Technical Information Service, U.S. Department of Commerce, Springfield, VA 22161. o Anonymous file transfer - Unrestricted file transfer access to a system can be exploited to obtain sensitive files such as the UNIX /etc/passwd file. If used, TFTP (Trivial File Transfer Protocol - which requires no username/password authentication) should always be configured to run as a non-privileged user and "chroot" to a file structure where the remote user cannot transfer the system /etc/passwd file. Anonymous FTP, too, should not allow the remote user to access this file, or any other critical system file. Configuring these facilities to "chroot" limits file access to a localized directory structure. o Apply fixes - Many of the old "holes" in UNIX have been closed. Check with your vendor and install all of the latest fixes. If MILNET system administrators do discover any unauthorized system activity, they are urged to contact the DDN Network Security Officer or the Computer Emergency Response Team (CERT). The DDN Network Security Officer can be contacted by phone through the MILNET Trouble Desk at 1-800-451-7413 (24 hour hotline) or through e-mail at DCAB615@DDN1.DCA.MIL. The CERT can be reached at CERT@SEI.CMU.EDU or by phone at 1-412-268-7090 (24 hour Hotline).