C++ Format String Vulnerability

G5 Cyber Security

2 weeks ago

TL;DR

A format string vulnerability happens when a program uses user-supplied input directly as the format string in functions like printf, fprintf, or sprintf. This allows attackers to read from and write to arbitrary memory locations, potentially taking control of the application.

How it Works

Format Strings: Functions like printf use a format string to determine how to display data. Format specifiers (e.g., %d for integers, %s for strings) tell the function what type of data is being passed and how to interpret it.
```
printf("%d %s", 10, "hello"); // Prints: 10 hello
```

The Vulnerability: If the format string comes from user input, an attacker can inject malicious format specifiers. For example:

char user_input[] = "%x %x %x %x"; // Example of potentially dangerous input

printf(user_input); // Vulnerable!  Prints values from the stack.

Stack Exploration: The %x specifier reads an integer value from the stack and prints it as hexadecimal. By using multiple %x specifiers, an attacker can dump parts of the program’s stack.
The stack contains return addresses, local variables, and other important data.
Arbitrary Memory Read: An attacker can use format specifiers like %s to read strings from arbitrary memory locations. This requires knowing the address of the desired string on the stack.
```
char user_input[] = "%s %s %s %s"; // Attempting to read strings from the stack
```
Arbitrary Memory Write: The %n specifier writes the number of bytes written so far to a memory address specified by an argument. This is the most dangerous part, as it allows attackers to modify program data.
```
char user_input[] = "AAAA%n"; // Attempts to write 'AAAA' (4 bytes) to the address pointed to by the next argument
```

Example Code

Consider this vulnerable C++ code:

#include <iostream>
#include <string.h>

int main() {
    char buffer[64];
    std::cout << "Enter some text: ";
    std::cin.getline(buffer, 64);
    printf(buffer); // Vulnerable line!
    return 0;
}

If you enter a string like "%x %x %x %x", the program will print values from the stack.

How to Prevent It

Never use user input directly as a format string: Always use a fixed format string and pass user data as arguments.
```
printf("User input: %s", buffer); // Safe!
```
Use safer alternatives: Consider using std::cout or other I/O streams, which are less prone to format string vulnerabilities.
```
std::cout << "User input: " << buffer << std::endl; // Safer!
```
Input Validation: If you absolutely must use printf with user-supplied data, carefully validate the input to ensure it doesn't contain any format specifiers.
However, this is difficult to do reliably.
Compiler and Static Analysis Tools: Use compilers with built-in security features and static analysis tools to detect potential vulnerabilities.