National Cyber Warfare Foundation (NCWF)

Welcome back, my aspiring cyber warriors! In the world of cybersecurity, some vulnerabilities persist despite being well-known for decades. Format string vulnerabilities are among these enduring security issues that continue to plague software systems. Despite being over twenty years old, format string vulnerabilities are still found in dozens of real systems every year, potentially leading […]

The post Exploit Development: Format String Vulnerability first appeared on Hackers Arise.

Welcome back, my aspiring cyber warriors!

In the world of cybersecurity, some vulnerabilities persist despite being well-known for decades. Format string vulnerabilities are among these enduring security issues that continue to plague software systems. Despite being over twenty years old, format string vulnerabilities are still found in dozens of real systems every year, potentially leading to devastating consequences.

A format string vulnerability occurs when an attacker can input text that is subsequently evaluated differently than originally intended by the program. This vulnerability is particularly common in C and C++ programming, but can also affect other languages that use similar formatting functions.

Format strings are special types of strings used by functions like printf to output formatted text. When these functions are used incorrectly, they can open the door to various attacks, from information disclosure to arbitrary code execution.

Format String Basics

Format strings are composed of three main components that we need to understand:

Format Function

These are special types of ANSI C functions that accept a variable number of arguments, one of which is called a format string. Common examples include printf, fprintf, sprintf, snprintf, and many others from the printf family. Other vulnerable functions include syslog, setproctitle, and various error handling functions.

Format String

This is the string that contains text mixed with format specifiers. It’s the first argument to the format function and determines how the output will be formatted.

Format Parameters

These are special characters preceded by the % symbol that define how variables should be output. Some common format parameters include:

Parameter	Input	Output
%d	Value	Number
%u	Value	Unsigned number
%x	Value	Number in hexadecimal
%s	Pointer	String
%n	Pointer	Numbers of bytes already written

The %n parameter is particularly dangerous as it writes to a variable rather than reading from it.

Step #1: Understanding the Vulnerability

The vulnerability occurs when user input is directly passed as the format string to a formatting function. Let’s look at two code examples to understand the difference between secure and vulnerable code:

Secure code:

kali > cat good.c

#include 

int main(int argc, char* argv[]){
    char* var = argv[1];
    printf("Name: %s
", var);
    return 0;
}

kali > ./good John

Name: John

kali > ./good %x

Name: %x

Vulnerable code:

kali > cat bad.c

#include 

int main(int argc, char* argv[]){
    char* var = argv[1];
    printf("Name: ");
    printf(var); //vulnerability!
    printf("
");
    return 0;
}

kali > ./bad John

Name: John

kali > ./bad %d

Name: 32

Notice how in the vulnerable code, when we input %d, the function interprets it as a format specifier and prints a value from the stack. This is the essence of the format string vulnerability.

Step #2: Exploiting the Vulnerability

Format string vulnerabilities can be exploited in several ways. Let’s examine the most common attack vectors:

Crashing the Program

The simplest attack is to cause a segmentation fault by providing format specifiers that attempt to access invalid memory addresses:

kali > ./bad %s%s%s%s%s%s%s

This happens because the %s parameter expects a pointer to a string, and when there are no corresponding arguments, it reads from arbitrary memory locations that may be invalid.

Reading Data from the Stack

An attacker can use format specifiers to read values from the stack:

kali > ./bad %08x.%08x.%08x.%08x

This reveals four values from the stack in hexadecimal format, potentially exposing sensitive information.

Summary

Format string vulnerabilities represent a serious security risk that has persisted for decades. Despite being well-understood, they continue to appear in real systems, potentially leading to information disclosure, memory corruption, and even arbitrary code execution.

By understanding how format string vulnerabilities work, you can effectively protect systems from these persistent and dangerous security flaws—or exploit them to your advantage.