Why in the code we use ptr instead of using buff directly? Or if we can directly use buff?

The use of ptr alongside buff serves a specific purpose, particularly in managing the layout and content of the buffer memory for the exploit. Let's break down their roles and why both are used:

Role of buff:

• buff is the base pointer for the allocated memory. It's used to keep track of the start of the buffer. This is important because certain operations, like setting an environment variable (using putenv(buff)) or printing the buffer's address (printf("Using address: 0x%x\n", addr)), require the address of the beginning of the buffer.

Role of ptr:

• ptr is a secondary pointer used to manipulate the contents of the buffer without losing the original address stored in buff.
• Initially, ptr is set to point to the start of buff: ptr = buff;
• As the buffer is filled (first with the address to overwrite the return address, then with the shellcode), ptr is incremented to move through the buffer. This allows sequential writing into the buffer without altering the base address held in buff.

Why Both are Used:

1. Maintaining the Base Address:

   • Using buff directly for sequential writes would change the initial address stored in it, which is needed for setting the environment variable and other purposes.
   • By using ptr for writing, we ensure that buff still points to the start of the buffer, which is crucial for the exploit to work correctly.

2. Sequential Writing:

   • ptr allows for convenient and efficient writing into the buffer. It can be incremented as needed after each write operation.
   • This approach simplifies the process of first filling the buffer with the address and then appending the shellcode.

Direct Use of buff:

• In this specific exploit code, direct use of buff for sequential writing would make it difficult to keep track of the start of the buffer. However, in a simpler scenario or a different exploit method, you might be able to use buff directly, especially if there's no need to keep track of the original base address or if the writing operations are straightforward.

Conclusion:

The decision to use both buff and ptr is based on the need to manipulate the buffer contents while preserving the base address for later use. This approach provides flexibility and control in exploit development, especially in complex scenarios like buffer overflow exploits.

memcpy, putenv, system

These three lines in the exploit code are critical for setting up the environment in which the buffer overflow will be triggered and the exploit will be executed. Let's break down each line:

memcpy(buff,"EGG=",4);

• This line uses the memcpy function to copy the string "EGG=" into the beginning of the buff array. The 4 indicates that four characters (including the null terminator) are copied.
• The purpose of this is to create an environment variable with the name EGG. This is a common technique in exploits where the overflow string (containing the shellcode and other data) is stored in an environment variable.

putenv(buff);

• putenv is a standard library function that adds or changes the value of environment variables.
• By calling putenv(buff), the exploit adds the newly created EGG environment variable to the current environment. The value of this environment variable is the content of buff, which includes the address to overwrite, followed by the shellcode.
• The reason for using an environment variable is that it allows the shellcode and the overflow data to be easily passed to the target program (vulnerable.c in this case) in a way that is predictable and can be manipulated by the attacker.

system("/bin/bash");

• This line executes a new instance of the bash shell using the system function.
• The new bash shell inherits the environment of the current process, including the EGG environment variable set by putenv.
• The purpose of launching a new shell is two-fold: it allows the attacker to easily input commands for further actions, and it demonstrates that the attacker has successfully executed arbitrary code (in this case, launching a shell).

How They Work Together in the Exploit:

1. Preparing the Exploit Payload: The buffer (buff) is prepared with the necessary data to exploit the vulnerability (addresses and shellcode).

2. Setting up the Environment Variable: The EGG environment variable is created with the contents of the buffer. This variable is then passed to the vulnerable program.

3. Triggering the Exploit: When the vulnerable program is run with the EGG environment variable (as shown in the command line interaction with ./vulnerable $EGG), it triggers the buffer overflow with the prepared data, ideally leading to the execution of the shellcode.

This method of using environment variables is a common technique in exploitation, as it provides a controllable and predictable way to deliver the exploit payload to the target program.

Since the shell code has already spawn a shell, why we need system("/bin/bash"); at the end?

The system("/bin/bash"); call in the exploit2.c code serves a different purpose compared to the shellcode within the exploit. Here's why it's used:

Purpose of system("/bin/bash");

• Testing and Interaction: The call to system("/bin/bash"); in exploit2.c opens a new bash shell for the user running the exploit2 program. This allows the user to interact with their system normally while the EGG environment variable is set and ready for use. It's essentially a convenient way for the attacker to prepare and execute the exploit in a controlled environment.

• Environment Persistence: By launching a bash shell, the exploit environment (including the EGG environment variable) is maintained. When the vulnerable program is executed from this bash shell, it inherits the environment, including the EGG variable.

Difference from the Shellcode

• Shellcode's Role: The shellcode within the EGG environment variable is designed to exploit the vulnerable program. When vulnerable is executed with the overflow payload in EGG, the shellcode aims to spawn a shell as a result of exploiting the buffer overflow vulnerability in vulnerable.

• Sequence of Events: In the typical use of this exploit, the attacker would first run exploit2, which sets up the environment and then opens a bash shell. From this new shell, they would run the vulnerable program with the EGG environment variable. If the exploit is successful, the shellcode in EGG triggers when vulnerable overflows its buffer, potentially giving the attacker another shell, but this time with the privileges or context of the vulnerable program.

Conclusion

In summary, the system("/bin/bash"); call in exploit2.c is for setup and convenience—it's not the exploit itself. It allows the attacker to easily manage the exploit environment. The actual exploit occurs when the vulnerable program is run and processes the overflow payload, potentially leading to arbitrary code execution via the shellcode. The spawning of a shell by the shellcode is the primary goal of the exploit, potentially giving the attacker unauthorized access or control.

What is an environment variable and why?

An environment variable is a dynamic-named value that can affect the way running processes behave on a computer. They are part of the environment in which a process runs, and they provide a way to influence the behavior of software on the system.

Key Characteristics of Environment Variables:

1. Dynamic Values: Environment variables can be changed dynamically, meaning they can be adjusted without altering the application code.

2. System or User-Specific: They can be set at the system level (affecting all users) or at the user level (affecting only a particular user's processes).

3. Process Inheritance: When a process is created, it inherits the environment variables from its parent process. This inheritance is key in many software operations, including script execution and software development.

4. Configuration and Control: They are often used to configure and control the behavior of operating systems and applications. For instance, an environment variable might store the location of the user's home directory, the default text editor, or the path to executable files.

Common Uses of Environment Variables:

• Path Configuration: The PATH environment variable stores a list of directories that should be searched for executable files when a command is issued. This variable is fundamental in both Windows and Unix/Linux operating systems.

• Software Behavior: Many software programs use environment variables to determine various settings, like where to store log files, where to find necessary libraries, or configuration file locations.

• System Properties: They can hold system properties like the current user's name (USER in Unix/Linux), the home directory (HOME in Unix/Linux or UserProfile in Windows), and the system's hostname.

• Development and Debugging: In software development, environment variables can be used to set debugging levels, choose between different operation modes, or select which resources to use.

In the Context of Exploits:

In the context of software exploits like buffer overflows, environment variables can be used by attackers to deliver payload (like shellcode) to a vulnerable program. Since environment variables are inherited by child processes, an attacker can set an environment variable to contain malicious code and then run a vulnerable program that accesses this variable, potentially leading to code execution.

Conclusion:

Environment variables are a fundamental aspect of operating systems and software, offering a flexible way to influence the behavior of processes. Their ability to be set or changed without modifying code makes them valuable for configuration, but also potentially vulnerable if not properly managed or secured.

Why ptr += 4?

ptr is used to point to the address of buff and that ptr += 4 increases its value by 4.

Let's break down the purpose of this operation in the context of the exploit:

Understanding ptr += 4

1. Initial Pointer Assignment:

   • Initially, ptr is assigned the address of buff: ptr = buff;
   • At this point, ptr points to the start of buff.

2. First For Loop - Filling Buffer with Address:

   • The first for loop fills the buffer with the calculated address (addr). This address is supposed to be where the shellcode will eventually reside in memory.
   • addr_ptr, a long * pointer, is used for this purpose. It writes the address into the buffer in chunks of sizeof(long), which is typically 4 bytes on a 32-bit system or 8 bytes on a 64-bit system.

3. Incrementing ptr:

   • After the first loop, ptr is incremented by 4: ptr += 4;
   • This is done to move the pointer past the last address written into the buffer. The exact reason for this increment depends on how the buffer and stack are structured and what exactly the exploit is trying to achieve.
   • In many buffer overflow exploits, such adjustments are made to align the shellcode correctly in memory or to skip over certain parts of the buffer that might not be suitable for storing the shellcode (like null bytes or other control characters).

4. Second For Loop - Copying Shellcode:

   • After incrementing ptr, the second for loop copies the shellcode into the buffer, starting from the new position of ptr.
   • This placement ensures that the shellcode is located right after the repeated addresses in the buffer.

Purpose

The purpose of ptr += 4 is to align the start of the shellcode correctly within the buffer. This step is crucial in ensuring that when the buffer overflow occurs, the overwritten return address on the stack points precisely to the beginning of the shellcode, allowing the exploit to execute the shellcode successfully.

Note on Buffer Overflows

Buffer overflow exploits often require precise control of the memory layout and contents. The success of such exploits depends on various factors like the architecture of the system, the specific implementation of the stack and memory, and the presence of any security mechanisms like stack canaries or ASLR. Fine-tuning these exploits often involves a lot of trial and error to get the alignment and offsets just right.