Tools

Format String Vulnerabilities

Theory

Format String Vulnerabilities:

Format string vulnerabilities are a type of security flaw that occurs when a program uses user-supplied input as a format string in functions like printf, sprintf, or fprintf without proper validation. These vulnerabilities can allow attackers to manipulate the format string to access or modify arbitrary memory locations, potentially leading to unauthorized data access, system crashes, or arbitrary code execution.

Understanding Format Strings:

In C and similar languages, functions such as printf use format strings to specify how data should be formatted for output. A format string includes placeholders (e.g., %d, %s, %x) that are replaced with the values of subsequent arguments. When an attacker can control the format string, they can exploit this by inserting format specifiers that allow them to read from or write to arbitrary locations in memory.

Common Format Specifiers:

  • %x: Displays the value at a memory address as a hexadecimal number.
  • %s: Treats the value at a memory address as a string and prints it.
  • %n: Writes the number of characters printed so far into a variable.

    • Preventing Format String Vulnerabilities:

  • Avoid Direct Use of User Input: Never use user-controlled input directly as a format string.
  • Input Validation: Sanitize and validate all user inputs to ensure they do not contain format specifiers or other harmful content.
  • Use Safe Functions: Use functions that do not accept format strings from user input, or use safe variants like snprintf that limit the amount of data processed.