TUCoPS :: Unix :: General :: intrud~1.txt

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October 3, 1997	
Version 1.2
ftp://info.cert.org/pub/tech_tips/intruder_detection_checklist
 CERT(*) Coordination Center
 Intruder Detection Checklist
This document outlines suggested steps for determining if your system has
been compromised. System administrators can use this information to look
for several types of break-ins. We encourage you to review all sections of
this document and modify your systems to close potential weaknesses.
In addition to the information in this document, we provide three companion
documents that may help you:
 ftp://info.cert.org/pub/tech_tips/UNIX_configuration_guidelines
 - contains suggestions for avoiding common UNIX system
 configuration problems that have been exploited
 ftp://info.cert.org/pub/tech_tips/root_compromise
 - contains suggested steps for recovering from a root compromise on
 a UNIX system
 ftp://info.cert.org/pub/tech_tips/security_tools
 - contains descriptions of tools that can be used to help secure a
 system and deter break-ins
We also encourage you to check with your vendor(s) regularly for any
updates or new patches that relate to your systems.
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A. Look For Signs That Your System May Have Been Compromised
Note that all action taken during the course of an investigation should be
in accordance with your organization's policies and procedures.
 1. Examine log files for connections from unusual locations or other
 unusual activity. For example, look at your 'last' log, process
 accounting, all logs created by syslog, and other security logs.
 If your firewall or router writes logs to a different location than the
 compromised system, remember to check these logs also. Note that this is
 not foolproof unless you log to append-only media; many intruders edit
 log files in an attempt to hide their activity.
 2. Look for setuid and setgid files (especially setuid root files)
 everywhere on your system. Intruders often leave setuid copies of
 /bin/sh or /bin/time around to allow them root access at a later
 time. The UNIX find(1) program can be used to hunt for setuid and/or
 setgid files. For example, you can use the following commands to find
 setuid root files and setgid kmem files on the entire file system:
 find / -user root -perm -4000 -print
 find / -group kmem -perm -2000 -print
 Note that the above examples search the entire directory tree,
 including NFS/AFS mounted file systems. Some find(1) commands
 support an "-xdev" option to avoid searching those hierarchies.
 For example:
 find / -user root -perm -4000 -print -xdev
 Another way to search for setuid files is to use the ncheck(8)
 command on each disk partition. For example, use the following command
 to search for setuid files and special devices on the disk partition
 /dev/rsd0g:
 ncheck -s /dev/rsd0g
 3. Check your system binaries to make sure that they haven't been
 altered. We've seen intruders change programs on UNIX systems such as
 login, su, telnet, netstat, ifconfig, ls, find, du, df, libc, sync,
 any binaries referenced in /etc/inetd.conf, and other critical
 network and system programs and shared object libraries. Compare the
 versions on your systems with known good copies, such as those from
 your initial installation media. Be careful of trusting backups; your
 backups could also contain Trojan horses.
 Trojan horse programs may produce the same standard checksum and
 timestamp as the legitimate version. Because of this, the standard
 UNIX sum(1) command and the timestamps associated with the programs
 are not sufficient to determine whether the programs have been
 replaced. The use of cmp(1), MD5, Tripwire, and other cryptographic
 checksum tools is sufficient to detect these Trojan horse programs,
 provided the checksum tools themselves are kept secure and are not
 available for modification by the intruder. Additionally, you may
 want to consider using a tool (PGP, for example) to "sign" the output
 generated by MD5 or Tripwire, for future reference.
 4. Check your systems for unauthorized use of a network monitoring
 program, commonly called a sniffer or packet sniffer. Intruders may
 use a sniffer to capture user account and password information. For
 related information, see CERT advisory CA-94:01 available in
 ftp://info.cert.org/pub/cert_advisories/CA-94:01.network.monitoring.attacks
 5. Examine all the files that are run by 'cron' and 'at.' We've seen
 intruders leave back doors in files run from 'cron' or submitted to
 'at.' These techniques can let an intruder back on the system (even
 after you believe you had addressed the original compromise). Also,
 verify that all files/programs referenced (directly or indirectly) by
 the 'cron' and 'at' jobs, and the job files themselves, are not
 world-writable.
 6. Check for unauthorized services. Inspect /etc/inetd.conf for
 unauthorized additions or changes. In particular, search for entries
 that execute a shell program (for example, /bin/sh or /bin/csh) and
 check all programs that are specified in /etc/inetd.conf to verify
 that they are correct and haven't been replaced by Trojan horse
 programs.
 Also check for legitimate services that you have commented out in
 your /etc/inetd.conf. Intruders may turn on a service that you
 previously thought you had turned off, or replace the inetd program
 with a Trojan horse program.
 7. Examine the /etc/passwd file on the system and check for modifications
 to that file. In particular, look for the unauthorized creation of new
 accounts, accounts with no passwords, or UID changes (especially UID 0)
 to existing accounts.
 8. Check your system and network configuration files for unauthorized
 entries. In particular, look for '+' (plus sign) entries and
 inappropriate non-local host names in /etc/hosts.equiv, /etc/hosts.lpd,
 and in all .rhosts files (especially root, uucp, ftp, and other system
 accounts) on the system. These files should not be world-writable.
 Furthermore, confirm that these files existed prior to any intrusion and
 were not created by the intruder.
 9. Look everywhere on the system for unusual or hidden files (files that
 start with a period and are normally not shown by 'ls'), as these can
 be used to hide tools and information (password cracking programs,
 password files from other systems, etc.). A common technique on UNIX
 systems is to put a hidden directory in a user's account with an unusual
 name, something like '...' or '.. ' (dot dot space) or '..^G' (dot dot
 control-G). Again, the find(1) program can be used to look for hidden
 files, for example:
 find / -name ".. " -print -xdev
 find / -name ".*" -print -xdev | cat -v
 Also, files with names such as '.xx' and '.mail' have been used
 (that is, files that might appear to be normal).
 10. Examine all machines on the local network when searching for signs of
 intrusion. Most of the time, if one host has been compromised, others
 on the network have been, too. This is especially true for networks
 where NIS is running or where hosts trust each other through the use
 of .rhosts files and/or /etc/hosts.equiv files. Also, check hosts for
 which your users share .rhosts access.
B. Review Other CERT Documents
 1. For further information about the types of attack that have recently
 been reported to the CERT Coordination Center and for a list of new
 or updated files that are available for anonymous FTP, see our past
 CERT Summaries, available in the directory
 ftp://info.cert.org/pub/cert_summaries/
 2. If you suspect that your system has been compromised, please review the
 suggested steps in "Steps for Recovering from a UNIX Root Compromise,"
 available from
 ftp://info.cert.org/pub/tech_tips/root_compromise
 Also review other appropriate files in our tech_tips directory.
 3. To report a computer security incident to the CERT Coordination
 Center, please complete and return a copy of our Incident Reporting Form,
 available from
 ftp://info.cert.org/pub/incident_reporting_form
 The information on the form helps us provide the best assistance, as
 it enables us to understand the scope of the incident, to determine
 if your incident may be related to any other incidents that have been
 reported to us, and to identify trends in intruder activities.
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Copyright 1996 Carnegie Mellon University. Conditions for use, disclaimers,
and sponsorship information can be found in
http://www.cert.org/legal_stuff.html and ftp://ftp.cert.org/pub/legal_stuff .
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