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Now that we have discussed the two primary ways remote attackers gain access to a UNIX system, we need to describe several techniques used to obtain shell access It is important to keep in mind that a primary goal of any attacker is to gain command-line or shell access to the target system Traditionally, interactive shell access is achieved by remotely logging in to a UNIX server via telnet, rlogin, or ssh Additionally, you can execute commands via rsh, ssh, or rexec without having an interactive login At this point, you may be wondering what happens if remote login services are turned off or blocked by a firewall How can attackers gain shell access to the target system Good question Let s create a scenario and explore multiple ways attackers can gain interactive shell access to a UNIX system Figure 8-1 illustrates these methods

Using a simple text editor, the attackers will remove these entries and use the touch command to reset the last accessed date and time on the file Usually attackers will not generate history files because they disable the history feature of the shell by setting

Figure 8-1

Additionally, an intruder may link bash_history to /dev/null:

Suppose that attackers are trying to gain access to a UNIX-based web server that resides behind an industrial-based packet inspection firewall or router The brand is not important what is important is understanding that the firewall is a routing-based firewall and is not proxying any services The only services that are allowed through the firewall are HTTP, port 80, and HTTP over SSL (HTTPS), port 443 Now assume that the web server is vulnerable to an input validation attack such as the PHF attack mentioned earlier The web server is also running with the privileges of nobody, which is common and is considered a good security practice If attackers can successfully exploit the PHF input validation condition, they can execute code on the web server as the user nobody Executing commands on the target web server is critical, but it is only the first step in gaining interactive shell access

[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

Popularity: Simplicity: Impact: Risk Rating: 7 3 8 6

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:

8:

[schism]# gcc -Wall -D__DEBUG badattachK-03r2c -o badattach [schism]#

Because the attackers are able to execute commands on the web server via the PHF attack, one of the first techniques to obtain interactive shell access is to take advantage of the UNIX X Window System X is the windowing facility that allows many different programs to share a graphical display X is extremely robust and allows X-based client programs to display their output to the local X server or to a remote X server running on ports 6000 6063 One of the most useful X clients to attackers is xterm Xterm is used to start a local command shell when running X However, by enabling the display option, attackers can direct a command shell to the attackers X server Presto, instant shell access Let s take a look at how attackers might exploit PHF to do more than just display the contents of the passwd file Recall from earlier the original PHF exploit: