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This article created by Aditya Balapure, the author of Learning Metasploit Exploitation and Development,discusses about exploitation, which refers to the art of compromising a computer system. The basics of computer exploitation involves a deep understanding of the vulnerabilities and payloads. An exploit is a piece of well-written code, compiled and executed on a targeted system, which may compromise that system. An exploit usually targets a known vulnerability, a flaw in a service or a poorly written code. In this article, we will discuss the basics of how to find vulnerable systems and then exploit them.
(For more resources related to this topic, see here.)
Basic terms of exploitation
The basic terms of exploitation are explained as follows:
- Vulnerability: A vulnerability is a security hole in software or hardware, which allows an attacker to compromise a system. A vulnerability can be as simple as a weak password or as complex as a Denial of Service attack.
- Exploit: An exploit refers to a well-known security flaw or bug with which a hacker gains entry into a system. An exploit is the actual code with which an attacker takes advantage of a particular vulnerability.
- Payload: Once an exploit executes on the vulnerable system and the system has been compromised, the payload enables us to control the system. The payload is typically attached to the exploit and delivered.
- Shellcode: This is a set of instructions usually used as a payload when the exploitation occurs.
- Listener: A listener works as component waiting for an incoming connection.
How does exploitation work?
We consider the scenario of a computer lab in which we have two students doing work on their computers. After some time one of the students goes out for a coffee break and he responsibly locks down his computer. The password for that particular locked computer is Apple, which is a very simple dictionary word and is a system vulnerability. The other student starts to attempt a password guessing attack against the system of the student who left the lab. This is a classic example of an exploit. The controls that help the malicious user to control the system after successfully logging in to the computer are called the payload.
We now come to the bigger question of how exploitation actually works. An attacker basically sends an exploit with an attached payload to the vulnerable system. The exploit runs first and if it succeeds, the actual code of the payload runs. After the payload runs, the attacker gets fully privileged access to the vulnerable system, and then he may download data, upload malware, virus', backdoors, or whatever he wants.
A typical process for compromising a system
For compromising any system, the first step is to scan the IP address to find open ports and its operating system and services. Then we move on to identifying a vulnerable service and finding an exploit in Metasploit for that particular service. If the exploit is not available in Metasploit, we will go through the Internet databases such as www.securityfocus.com, www.exploitdb.com, www.1337day.com, and so on. After successfully finding an exploit, we launch the exploit and compromise the system.
The tools that are commonly used for port scanning are Nmap (Network Mapper), Autoscan, Unicorn Scan, and so on. For example, here we are using Nmap for scanning to show open ports and their services.
First open the terminal in your BackTrack virtual machine. Type in nmap –v –n 192.168.0.103 and press Enter to scan. We use the –v parameter to get verbose output and the –n parameter to disable reverse DNS resolutions.
Here we can see the results of Nmap, showing three open ports with their services running on them. If we need more detailed information such as the service version or Operating System type, we have to perform an intense scan using Nmap. For an intense scan, we use the command nmap –T4 –A –v 192.168.0.103. This shows us the complete results of the service version and the Operating System type.
The next step is to find an exploit according to the service or its version. Here, we can see that the first service running on port number 135 is msrpc, which is known as Microsoft Windows RPC. Now we will learn how to find an exploit for this particular service in Metasploit. Let's open our terminal and type in msfconsole to start Metasploit. On typing in search dcom, it searches all of the Windows RPC related exploits in its database.
In the following screenshot, we can see the exploit with its description and also the release date of this vulnerability. We are presented with a list of exploits according to their rank. From the three exploits related to this vulnerability, we select the first one since it is the most effective exploit with the highest rank. Now we have learned the technique of searching for an exploit in Metasploit through the search <service name> command.
Finding exploits from online databases
If the exploit is not available in Metasploit, then we have to search the Internet exploit databases for that particular exploit. Now we will learn how to search for an exploit on these online services such as www.1337day.com. We open the website and click on the Search tab. As an example, we will search for exploits on the Windows RPC service.
Now we have to download and save a particular exploit. For this, just click on the exploit you need.
After clicking on the exploit it shows the description of that exploit. Click on Open material to view or save the exploit.
The usage of this exploit is provided as a part of the documentation in the exploit code as marked in the following screenshot:
Now we will be exploiting our target machine with the particular exploit that we have downloaded. We have already scanned the IP address and found three open ports. The next step would be to exploit one of those ports. As an example, we will target the port number 135 service running on this target machine, which is msrpc. Let us start by compiling the downloaded exploit code. To compile the code, launch the terminal and type in gcc <exploit name with path> -o<exploitname>. For example, here we are typing gcc –dcom –o dcom.
After compiling the exploit we have a binary file of that exploit, which we use to exploit the target by running the file in the terminal by typing in ./<filename>.
From the preceding screenshot, we can see the requirements for exploiting the target. It requires the target IP address and the ID (Windows version). Let's have a look at our target IP address.
We have the target IP address, so let's start the attack. Type in ./dcom 6 192.168.174.129.
The target has been exploited and we already have the command shell. Now we check the IP address of the victim machine. Type in ipconfig.
The target has been compromised and we have actually gained access to it.
Now we will see how to use the internal exploits of Metasploit. We have already scanned an IP address and found three open ports. This time we target port number 445, which runs the Microsoft-ds service.
Let us start by selecting an exploit. Launch msfconsole, type in use exploit/windows/smb/ms08_067_netapi, and press Enter.
The next step will be to check the options for an exploit and what it requires in order to perform a successful exploitation. We type in show options and it will show us the requirements. We would need to set RHOST ( remote host), which is the target IP address, and let the other options keep their default values.
We set up the RHOST or the target address by typing in set RHOST 192.168.0.103.
After setting up the options, we are all set to exploit our target. Typing in exploit will give us the Meterpreter shell.
The following are some helpful references that shed further light on some of the topics covered in this article:
In this article, we covered the basics of vulnerability, a payload, and some tips on the art of exploitation. We also covered the techniques of how to search for vulnerable services and further query the Metasploit database for an exploit. These exploits were then used to compromise the vulnerable system. We also demonstrated the art of searching for exploits in Internet databases, which contain zero-day exploits on software and services.
Resources for Article :
- Understanding the True Security Posture of the Network Environment being Tested [Article]
- So, what is Metasploit? [Article]
- Tips and Tricks on BackTrack 4 [Article]
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About the Author :
Aditya Balapure is an information security researcher, consultant, and an author with expertise in the fields of Web Application Penetration Testing and Enterprise Server Security. Aditya has 3 years' of practical experience in the field of information security. He has quite a few credentials to his name, such as Associate of ISC2 (CISSP), CEH, ECSA, MCP, a few international publications, as well as a few research articles. His deep interest in vulnerability assessment and offensive penetration testing groups him among the white hats of the information security arena. Aditya is extensively involved in conducting corporate trainings in addition to his constant hobby of vulnerability disclosure and security research.