Date: 03 October 1997
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October 3, 1997
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:
- contains suggestions for avoiding common UNIX system
configuration problems that have been exploited
- contains suggested steps for recovering from a root compromise on
a UNIX system
- 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.
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.
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
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
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
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
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
2. If you suspect that your system has been compromised, please review the
suggested steps in "Steps for Recovering from a UNIX 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,
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.
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 .
If you do not have FTP or web access, send mail to email@example.com with
"copyright" in the subject line.
CERT is registered in the U.S. Patent and Trademark Office.
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