Buffer overflow vulnerabilities are software security vulnerabilities that occur when a program tries to store more data in a buffer than it can hold. Attackers can exploit a buffer overflow to overwrite data on the stack or heap, execute arbitrary code, or crash the program. As such, buffer overflow exploits are a popular technique attackers use to compromise computer systems. This article will explore the basics of buffer overflow vulnerabilities and how attackers can exploit them.

Introduction to Buffer Overflow Vulnerabilities

A buffer is a temporary storage area in a computer’s memory where data can be held while being processed. Programs commonly use Buffers to store user input or data from other sources. The size of a buffer is typically fixed at the time of its creation, and it is the programmer’s responsibility to ensure that the buffer is large enough to accommodate all the data that will be stored in it.

Buffer overflow vulnerabilities occur when a program tries to store more data in a buffer than it can hold. If the data being stored exceeds the size of the buffer, it overflows into adjacent memory locations, potentially overwriting data critical to the program’s operation. Attackers can exploit buffer overflow vulnerabilities to overwrite data on the stack or heap, execute arbitrary code, or crash the program.

Stack-Based Buffer Overflow Vulnerabilities

One common type of buffer overflow vulnerability is the stack-based buffer overflow. In this type of vulnerability, a program stores data on the stack without first checking the size of the buffer. If an attacker can provide more data than the buffer can hold, the data will overflow into adjacent memory locations, potentially overwriting critical data.

Consider the following code snippet:

void foo(char* input) {
  char buffer[8];
  strcpy(buffer, input);
  // other code
}

In this code, the function foo takes a string input and copies it to a buffer of size 8. If an attacker provides more than 8 bytes of data as input, the data will overflow into adjacent memory locations, potentially overwriting data on the stack. An attacker can use this vulnerability to overwrite the function’s return address with an address of their choice, allowing them to execute arbitrary code.

Heap-Based Buffer Overflow Vulnerabilities

Heap-based buffer overflow vulnerabilities occur when a program stores data on the heap without first checking the size of the buffer. If an attacker can provide more data than the buffer can hold, the data will overflow into adjacent memory locations, potentially overwriting critical data.

Consider the following code snippet:

void foo(int size) {
  char* buffer = (char*) malloc(size);
  strcpy(buffer, "Hello, World!");
  // other code
  free(buffer);
}

In this code, the function foo dynamically allocates a buffer of size size on the heap and copies the string “Hello, World!” to it using the strcpy function. If the size provided by the caller is not large enough to hold the string, a buffer overflow vulnerability occurs. An attacker can use this vulnerability to overwrite critical data on the heap, potentially allowing them to execute arbitrary code.

Exploiting Buffer Overflow Vulnerabilities

To exploit a buffer overflow vulnerability, an attacker must understand the program’s memory layout and how the buffer is stored. The attacker also needs to know the address of the code they want to execute, which they can obtain through a technique called “return-oriented programming” (ROP).

Return-oriented programming is used to bypass data execution prevention (DEP) and code signing protections. In return-oriented programming, the attacker uses small code snippets called “gadgets” in the program’s code to construct a chain of instructions that achieves their objective. Gadgets are typically short sequences of assembly instructions that perform a specific operation, such as adding two numbers or moving data from one register to another. By chaining together a series of gadgets, an attacker can construct arbitrary code that achieves their objective.

To exploit a stack-based buffer overflow vulnerability, an attacker typically overwrites the function’s return address with the address of a gadget in the program’s code. The gadget then returns to another gadget, which in turn returns to another gadget until the attacker has constructed a chain of gadgets that performs their desired action. For example, an attacker could build a chain of gadgets that opens a shell on the target system, allowing them to execute arbitrary commands.

To exploit a heap-based buffer overflow vulnerability, an attacker typically overwrites a function pointer or virtual method table pointer with the address of a gadget. When the program later calls the function or virtual method, it inadvertently jumps to the gadget, which starts the chain of gadgets described above.

Example: Exploiting a Stack-Based Buffer Overflow Vulnerability

To illustrate how a stack-based buffer overflow vulnerability can be exploited, consider the following C program:

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

void foo(char* input) {
  char buffer[8];
  strcpy(buffer, input);
  printf("%s\n", buffer);
}

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

  foo(argv[1]);
  return 0;
}

This program takes a string input from the command line and copies it to a buffer of size 8 using the strcpy function. The program then prints the contents of the buffer to the console.

To exploit this program, an attacker could provide more than 8 bytes of input, causing the buffer to overflow into adjacent memory locations. The attacker could then overwrite the return address of the foo function with the address of a gadget in the program’s code.

The attacker could use a tool like ROPgadget to find a suitable gadget, which searches the program’s code for gadgets that match specific criteria. For example, the following command searches the program’s code for gadgets that contain the instruction pop eax; ret;, which can be used to pop a value off the stack into the eax register:

ROPgadget --binary program --only "pop eax; ret;"

Assuming the tool finds a suitable gadget at address 0xdeadbeef, the attacker could construct a payload that overwrites the return address of the foo function with 0xdeadbeef, causing the program to return to the gadget.

The payload would look something like this:

python -c 'print "A"*8 + "\xef\xbe\xad\xde"'

This payload consists of 8 bytes of arbitrary data followed by the gadget’s address in little-endian byte order.

When the program is run with this payload as input, it will overflow the buffer and overwrite the foo function’s return address with the gadget’s address. The gadget will then pop a value off the stack into the eax register and return to the next gadget in the chain. Assuming the attacker has constructed a chain of gadgets that opens a shell, they will now have a shell on the target system.

Example: Exploiting a Heap-Based Buffer Overflow Vulnerability

To illustrate how a heap-based buffer overflow vulnerability can be exploited, consider the following C++ program:

#include <iostream>
#include <cstring>

class MyClass {
    public:
    virtual void func() {
        std::cout << "MyClass::func()" << std::endl;
    }

    virtual void print() {
    std::cout << "MyClass" << std::endl;
    }
};

void foo(MyClass* obj) {
    char* buffer = new char[8];
    strcpy(buffer, "Hello, World!");
    obj->print();
    delete[] buffer;
}

int main() {
    MyClass* obj = new MyClass();
    foo(obj);
    delete obj;
    return 0;
}

This program defines a class MyClass with two virtual methods, func and print. The foo function takes a pointer to a MyClass object and dynamically allocates a buffer of size 8 on the heap using the new operator. The function then copies the string “Hello, World!” to the buffer using the strcpy function. Finally, the function calls the print method on the MyClass object and frees the buffer using the delete[] operator.

To exploit this program, an attacker could provide a size argument to the foo function smaller than the size required to hold the string “Hello, World!”. This would cause the strcpy function to overflow the buffer and overwrite adjacent memory locations, potentially including a function pointer or virtual method table pointer.

Assuming the attacker knows the address of a suitable gadget in the program’s code, they could construct a payload that overwrites the function pointer or virtual method table pointer with the address of the gadget. The payload would look something like this:

python -c 'print "\xef\xbe\xad\xde" + "A"\*4 + "\xef\xbe\xad\xde"'

This payload consists of the gadget’s address in little-endian byte order, followed by 4 bytes of arbitrary data to fill the space between the buffer and the function pointer or virtual method table pointer, followed by the gadget’s address again.

When the program runs with this payload as input, it will overflow the buffer and overwrite the function or virtual method table pointer with the gadget’s address. When the program later calls the print method on the MyClass object, it will inadvertently jump to the gadget and start the chain of gadgets as described above.

Mitigating Buffer Overflow Vulnerabilities

Buffer overflow vulnerabilities can be difficult to mitigate, requiring a deep understanding of the program’s memory layout and the specific conditions that trigger the vulnerability. However, some general techniques can be used to reduce the risk of buffer overflow vulnerabilities.

Bounds Checking

Bounds checking is a technique used to ensure that data being stored in a buffer does not exceed the size of the buffer. This can be done at compile time by using a language that supports array bounds checking, such as Java or C#, or at runtime by using libraries that provide bounds checking, such as SafeStack or AddressSanitizer.

Stack Canaries

Stack canaries are a technique used to detect stack-based buffer overflow vulnerabilities. A stack canary is a random value placed on the stack between the buffer and the return address. Before the function returns, the value of the canary is checked to ensure that it has not been overwritten. If the canary has been overwritten, the program terminates.

Data Execution Prevention (DEP)

Data execution prevention is a technique used to prevent attackers from executing code on the stack or heap. DEP works by marking certain memory pages as non-executable, preventing attackers from executing code that they have injected into the program’s memory.

Conclusion

Buffer overflow vulnerabilities are common software security vulnerabilities that attackers can exploit to execute arbitrary code or crash a program. Stack-based buffer overflow vulnerabilities occur when a program stores data on the stack without first checking the size of the buffer. At the same time, heap-based buffer overflow vulnerabilities arise when a program stores data on the heap without first checking the buffer size. To exploit a buffer overflow vulnerability, an attacker needs to understand the layout of the program’s memory and how the buffer is stored. The attacker also needs to know the address of the code they want to execute, which they can obtain through return-oriented programming.

To mitigate buffer overflow vulnerabilities, developers can use techniques such as bounds checking, stack canaries, and data execution prevention. However, these techniques are not foolproof, and attackers can still find ways to exploit buffer overflow vulnerabilities.

As a red team or pen tester, it’s essential to be familiar with buffer overflow vulnerabilities and how they can be exploited. By understanding the basics of buffer overflow vulnerabilities and how to exploit them, you can better assess the security of a system and help to identify vulnerabilities that need to be addressed. Additionally, using tools like ROPgadget and Metasploit, you can test a system’s defenses against buffer overflow attacks and identify areas for improvement.

Overall, buffer overflow vulnerabilities are a severe threat to the security of computer systems, and developers, security professionals, and pen testers need to be aware of them and take steps to mitigate them. While it can be challenging to defend against buffer overflow attacks, the right tools and techniques can reduce the risk of these vulnerabilities and help protect critical systems and data.