Format String Vulnerabilities

Format String Vulnerabilities

Format string vulnerabilities are one of the most common security flaws in software applications. They occur when an attacker is able to control the format string argument of a vulnerable function, such as printf(). This can allow the attacker to gain control of the program and potentially execute arbitrary code.

What are Format String Vulnerabilities?

Format string vulnerabilities occur when an attacker is able to control the format string argument of a vulnerable function, such as printf(). This can allow the attacker to gain control of the program and potentially execute arbitrary code. The vulnerability is caused by the incorrect use of the printf() function and other related functions.

The printf() function is used to output a formatted string to the console. It takes a format string as its first argument, followed by a variable number of additional arguments. The format string contains placeholders, which are replaced with the values of the additional arguments. For example, the following code will output the string “Hello, World!” to the console:

printf("Hello, World!");

The vulnerability occurs when the format string is not properly sanitized before being passed to the printf() function. If the attacker can control the format string, they can inject malicious code into the program. This can allow the attacker to gain control of the program and potentially execute arbitrary code.

Exploiting Format String Vulnerabilities

Format string vulnerabilities can be exploited in a variety of ways. The most common method is to overwrite the saved return address with a pointer to malicious code. This can be done by writing the address of the malicious code into the stack.

To do this, the attacker must first identify the location of the saved return address on the stack. This can be done by using a debugging tool, such as GDB. Once the location of the saved return address is known, the attacker can use the format string to write the address of the malicious code into the stack.

For example, the following code will overwrite the saved return address with 0x41414141:

#include <stdio.h>
 
int main(int argc, char **argv)
{
    char buffer[100];
    scanf("%s", buffer);
    printf(buffer);
    return 0;
}

The attacker can then pass the following format string as an argument to the program:

%x %x %x %x %x %x %x %x %x %x %x %x %n

This will cause the program to output a series of hexadecimal values, which can be used to overwrite the saved return address with 0x41414141.

Conclusion

Format string vulnerabilities are a serious security flaw in software applications. They can be exploited to gain control of the program and potentially execute arbitrary code. It is important to always sanitize user input before passing it to a vulnerable function, such as printf().