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Hacking Exposed 6: Network Security Secrets & Solutions
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Additionally, an intruder may link bash_history to /dev/null:
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[rumble]# ln -s /dev/null ~/bash_history [rumble]# ls -l bash_history lrwxrwxrwx 1 root root 9 Jul 26 22:59 bash_history -> /dev/null
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The approaches illustrated above will aide in covering a hacker s tracks provided two conditions are met: Log les are kept on the local server Logs are not monitored or alerted on in real-time In today s enterprise environments this scenario is unlikely Shipping log files to a remote syslog server has become part of best practice, and several software products are also available for log scraping and alerting Because events can be captured in real time and stored remotely, clearing local files after the fact can no longer ensure all traces of the event have been removed This presents a fundamental problem for classic log wipers For this reason, advanced cleaners are taking a more proactive approach Rather than clearing log entries post factum, entries are intercepted and discarded before they are ever written A popular method for accomplishing this is via the ptrace() system call Ptrace is a powerful API for debugging and tracing processes and has been used in utilities such as gdb Because the ptrace system call allows one process to control the execution of another, it is also very useful to log cleaning authors to attach and control logging daemons such as syslogd The badattachK log cleaner by Matias Sedalo will be used to demonstrate this technique The first step is to compile the source of the program:
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[schism]# gcc -Wall -D__DEBUG badattachK-03r2c -o badattach [schism]#
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We need to define a list of strings values that, when found in a syslog entry, are discarded before they are written The default file, stringslist, stores these values We want to add the IP address of the system we will be coming from and the compromised account we will be using to authenticate to this list:
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[schism]# echo "1921681102" >> stringslist [schism]# echo "w00t" >> stringslist
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Now that we have compiled the log cleaner and created our list, let s run the program The program will attach to the process ID of syslogd and stop any entries from being logged when they are matched to any value in our list:
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[schism]# /badattach (c)2004 badattachK Version 03r2 by Matias Sedalo <s0t4ipv6@shellcodecomar> Use: /badattach <pid of syslog>
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5:
Hacking Unix
[schism]# /badattach `ps -C syslogd -o pid=` * syslogd on pid 9171 atached + SYS_socketcall:recv(0, 0xbf862e93, 1022, 0) == 93 bytes - Found '1921681102 port 24537 ssh2' at 0xbf862ed3 - Found 'w00t from 1921681102 port 24537 ssh2' at 0xbf862ec9 - Discarding log line received + SYS_socketcall:recv(0, 0xbf862e93, 1022, 0) == 82 bytes - Found 'w00t by (uid=0)' at 0xbf862ed6 - Discarding log line received
If we grep through the auth logs on the system, you will see no entry has been created for this recent connection The same will hold true if syslog forwarding is enabled:
[schism]# grep 1921681102 /var/log/authlog [schism]#
We should note that the debug option was enabled at compile-time to allow you to see the entries as they are intercepted and discarded; however, a hacker would want the log cleaner to be as stealthy as possible and would not output any information to the console or anywhere else The malicious user would also use a kernel level rootkit to hide all files and processes relating to the log cleaner We will discuss kernel rootkits in detail in the next section
Log Cleaning Countermeasure
It is important to write log file information to a medium that is difficult to modify Such a medium includes a file system that supports extend attributes such as the append-only flag Thus, log information can only be appended to each log file, rather than altered by attackers This is not a panacea, because it is possible for attackers to circumvent this mechanism The second method is to syslog critical log information to a secure log host Keep in mind that if your system is compromised, it is very difficult to rely on the log files that exist on the compromised system due to the ease with which attackers can manipulate them
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