What is a Buffer Overflow?

In a computer program, variables are allocated with fixed-size blocks of memory. After this memory is allocated, the program can store and retrieve data from these locations. Buffer overflows occur when the amount of data written to one of these blocks of memory exceeds its size. As a result, memory allocated for other purposes is overwritten, which can have various effects on the program.

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What is a Buffer Overflow?

A Buffer Overflow Attack

A buffer overflow attack is a common cyberattack that deliberately exploits a buffer overflow vulnerability where user-controlled data is written to memory. By submitting more data than can fit in the allocated memory block, the attacker can overwrite data in other parts of memory.

Attackers can perform buffer overflow attacks for various reasons, such as overwriting critical code or data to crash the program, injecting malicious code to be run to the program, or modifying critical values, changing the execution flow of the program.

Buffer Overflow Threat

Buffer overflow attacks can be used to achieve various objectives, including:

  • Denial of Service (DoS) Attacks: Within an application’s memory space are pointers, code, and other pieces of data that are critical to the program’s ability to execute. Overwriting this data could cause the program to crash, resulting in a DoS attack.
  • Code Execution: A common goal of buffer overflow exploits is to force the vulnerable application to execute attacker-provided code. This allows the attacker to run code on the affected system with the same access and permissions as the exploited application.
  • Access Control Bypasses: Exploitation of buffer overflows to run code can elevate an attacker’s access to a target system. This expanded access can then be used to perform follow-on attacks.

Types of Buffer Overflow Attacks

A buffer overflow attack can be performed in a few different ways, but some of the most common examples include:

  • Stack-Based Buffer Overflow: The program stack contains critical control flow data for an application — such as function return pointers — and is a common target of buffer overflow attacks. Overwriting a return pointer can cause the program to jump to attacker-controlled data and execute it as code, allowing the attacker to run code with the same permissions as the application.
  • Heap-Based Buffer Overflow: The program heap is used to dynamically allocate memory to variables whose size is not defined when the program compiles. By exploiting a buffer overflow vulnerability and flooding the system heap, an attacker can overwrite critical application data.
  • Format String Attacks: Functions in the printf family in C/C++ can use format strings, which allow reading and writing of memory. If user-provided data is interpreted as a format string, it can be used to leak or modify sensitive values.

Buffer Overflow Attack Examples

Buffer overflow vulnerabilities are common in C/C++ and occur when a program allocates a fixed-size chunk of memory and then insecurely copies data into it. The following code sample contains a buffer overflow vulnerability:

char buf[BUFSIZE];


In this code sample, the variable buf has a fixed size of BUFSIZE. However, the gets function will read data and store it within buf until it reaches a null terminator (0x00). An attacker can exploit this to overwrite data beyond the end of buf.

Format string attacks are a special case of buffer overflows. The following code sample contains a format string vulnerability.

#include <stdio.h>

void main(int argc, char **argv)




In this example, the program takes user-provided input from argv[1] and prints it to the terminal. However, if the user’s input contains a format string, this could allow an attacker to read or edit memory on the system.

How to Prevent Buffer Overflows

Buffer overflow vulnerabilities can be prevented by:

  • Performing Input Validation: Buffer overflow vulnerabilities occur when a program makes assumptions about user-provided input without validating these assumptions. Checking the length of data or only copying a certain number of bytes to a memory location can help avoid buffer overflows.
  • Enabling Runtime Memory Protection: Most computers have built-in protections against buffer overflows such as Address Space Layout Randomization (ASLR), Data Execution Prevention (DEP), and Structured Exception Handling Overwrite Protection. Enabling these protections makes buffer overflow attacks much more difficult to perform.
  • Avoiding Vulnerable Functions: Buffer overflows are made possible by vulnerable functions such as gets, scanf, and strcpy in C/C++. Buffer overflow vulnerabilities can be avoided by properly using the safe versions of these functions.
  • Using Memory-Safe Languages: Buffer overflows occur in programming languages with fixed-size variables and no memory protections. The use of other programming languages such as Python, Java, or C# makes buffer overflows difficult or impossible.
  • Preventing Vulnerability Exploitation: Web application firewalls (WAFs) and Web Application and API Protection (WAAP) solutions can identify and block attempted exploitation of buffer overflow vulnerabilities. This reduces the risk that buffer overflow attacks pose to corporate application security (AppSec).

Buffer Overflow Protection with CloudGuard AppSec

The exploitation of buffer overflow vulnerabilities can cause program crashes, data leaks, or execution of malicious code on an organization’s systems. Learn how CloudGuard AppSec can help protect against buffer overflow attacks and other threats in this eBook. Then, sign up for a free demo to see for yourself how CloudGuard AppSec can improve your cloud application security.

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