Format String Vulnerability is an important security issue which can be exploited by attackers to gain unauthorized access to a system or to execute malicious code. This type of attack is caused by a programming error in which the format specifiers (used for printing or reading data) are not properly sanitized or validated. As a result, an attacker can pass a specially crafted input to the application which may cause the execution of arbitrary code, memory corruption or even denial of service attacks.
To understand the mechanism behind Format String Vulnerability, it is important to know how the format specifiers are used in programming languages such as C or C++. In C programming, printf function is used for printing output on the screen or a file. For example, if we want to print the value of a variable "x" on the screen, we may use the following code:
printf("%d", x);
In this code, "%d" is the format specifier which tells the printf function to treat "x" as an integer and print its value on the screen.
However, if we do not properly sanitize or validate the input passed to the printf function, an attacker may use format string specifiers to exploit the application. For example, if an application reads input from the user and then uses printf function to print it on the screen, an attacker may pass a specially crafted input which contains format string specifiers such as "%x", "%s", etc. These specifiers may cause the printf function to print arbitrary values from the stack or heap, which may lead to a potential security breach.
For instance, consider the following code:
char* name;
scanf("%s", name); // read input from user
printf(name); // print input on the screen
In this code, we are reading an input from the user using scanf function and storing it in a character array "name". Then, we are printing this input on the screen using printf function. However, if an attacker passes a specially crafted input such as "%x %x %x %x", printf function may start printing values from the stack or heap memory segments, which may lead to a potential compromise of the application.
To mitigate the risk of Format String Vulnerability, it is recommended to properly sanitize or validate the input received from the user. One way to do this is to use functions such as fgets, which allow us to limit the number of characters read from the user input. Additionally, we can also use libraries such as "sprintf" or "snprintf" instead of printf function to avoid format string vulnerabilities. These functions allow us to specify the maximum size of output buffer, which prevents the buffer overflow attacks.
In conclusion, Format String Vulnerability is a serious security issue which can be exploited by attackers to gain unauthorized access to a system or to execute malicious code. It is caused by a programming error in which the format specifiers are not properly sanitized or validated. To avoid these kinds of vulnerabilities, it is essential to properly validate and sanitize the user input and to use secure functions which prevent buffer overflow attacks.
