01 Бер, 2024

Insufficient protection against format string vulnerabilities

Insufficient protection against format string vulnerabilities refers to a security weakness where a software application does not adequately validate or sanitize user-provided input that is used in format string functions. Format string vulnerabilities arise when untrusted input can influence the format string parameter in functions like printf, sprintf, fprintf , or similar functions in programming languages like C and C++. Attackers can exploit these vulnerabilities to read or write arbitrary memory, leading to potential unauthorized disclosure of sensitive information, denial of service, or remote code execution.


Format String Specifiers:

Attack Vector: Insufficient input validation allows attackers to include format string specifiers (%s, %x, etc.) in user-controlled input.

Приклад: An attacker may input %s to read data from the stack or %n to write data to a memory location.

Memory Access:

Attack Vector: Lack of proper validation enables attackers to access and manipulate the process memory.

Приклад: An attacker might use %p to leak memory addresses or %n to write arbitrary values to memory locations.

Відмова в обслуговуванні (DoS):

Attack Vector: Format string vulnerabilities can be exploited to cause a crash or system instability.

Приклад: Crafting input with excessive format specifiers can lead to crashes or resource exhaustion.


Розкриття інформації: Attackers can leak sensitive information from the process memory, including addresses, stack contents, or other secrets.

Відмова в обслуговуванні: Exploitation can lead to crashes or system instability, causing a denial of service.

Віддалене виконання коду: In severe cases, attackers may achieve remote code execution by manipulating memory and control flow.

					#include <stdio.h>

void vulnerableFunction(char *userInput) {
    // Vulnerable code: user-controlled input is used in a format string function

int main() {
    char userInput[100];

    // Get user input (for the sake of example)
    printf("Enter your name: ");
    scanf("%s", userInput);

    // Call the vulnerable function with user-controlled input

    return 0;

In this example:

В vulnerableFunction takes user input as a parameter and directly passes it to the printf function without proper validation or formatting.

An attacker could provide a malicious input containing format specifiers to exploit the vulnerability.

For instance, if an attacker inputs %x %x %x %x %x , the printf function will interpret these as format specifiers, attempting to read values from the stack. This could lead to unintended information disclosure.

To mitigate this vulnerability, proper validation and formatting should be applied to user-controlled input before using it in format string functions. Here’s an improved version of the code:

					#include <stdio.h>

void saferFunction(char *userInput) {
    // Mitigated code: use a format string with a fixed format and validate user input
    printf("Hello, %s!\n", userInput);

int main() {
    char userInput[100];

    // Get user input
    printf("Enter your name: ");
    scanf("%99s", userInput);  // Limit input length to avoid buffer overflow

    // Call the safer function with validated user input

    return 0;

In this improved version:

В saferFunction uses a fixed format string (Hello, %s!\n) and incorporates the user input using a format specifier, eliminating the risk of arbitrary format string injection.

В scanf function is used with a length specifier to limit the input length, preventing buffer overflow vulnerabilities.

Scanners that detect vulnerability


Опис: Examines C/C++ code for уразливості безпеки, including format string vulnerabilities.

Приклад: flawfinder source_code.c


Опис: A security linter for Python source code that can identify common security issues.

Приклад: bandit -r source_code_directory


Опис: A static analysis tool for C/C++ code that checks for a wide range of issues, including format string vulnerabilities.

Приклад: cppcheck source_code.cpp


Опис: A static analysis security scanner for Ruby on Rails applications.

Приклад: brakeman source_code_directory

AFL (American Fuzzy Lop):

Опис: A fuzzer that uses genetic algorithms to discover vulnerabilities, including format string issues.

Приклад: afl-fuzz -i input_dir -o output_dir — ./target_binary

OWTF (Offensive Web Testing Framework):

Опис: A comprehensive tool for penetration testing web applications, including detection of web-related vulnerabilities.

Приклад: OWTF is used interactively through its web interface.


Опис: A versatile network scanning tool that can be used to identify potential vulnerabilities.

Example: nmap -p 80 target_host


Опис:Пентест framework that includes various modules for identifying and exploiting vulnerabilities.

Приклад: Metasploit is used through its command-line interface.


Опис: A static application security testing (SAST) tool that identifies security vulnerabilities in source code.

Приклад: Checkmarx is typically used through its web-based interface.

Average CVSS score

Assigning a specific average Common Vulnerability Scoring System (CVSS) score for “Insufficient Protection Against Format String Vulnerabilities” can be challenging, as CVSS scores are generally determined on a case-by-case basis, taking into account various factors specific to each vulnerability.

CWE information

CWE-134: Use of Externally-Controlled Format String:

Опис: This weakness occurs when a program uses user-provided data to construct the format string argument for a function, leading to format string vulnerabilities.

Приклад: Using user-controlled input in a format string function like printf.

CWE-252: Unchecked Return Value:

Опис: This weakness involves the failure to check the return value of a function, potentially leading to issues such as format string vulnerabilities.

Приклад: Failing to check the return value of a function that constructs format strings.

CWE-456: Missing Initialization of a Variable:

Опис: This weakness involves using a variable before it has been properly initialized, leading to unpredictable behavior and potentially format string vulnerabilities.

Приклад: Using an uninitialized variable as part of a format string.

CWE-805: Buffer Access with Incorrect Length Value:

Опис: This weakness involves accessing a buffer using an incorrect length value, potentially leading to buffer overflows and format string vulnerabilities.

Приклад: Using an incorrect length value in a function that takes a format string and buffer.

CWE-120: Buffer Copy without Checking Size of Input (‘Classic Buffer Overflow’):

Опис: This weakness involves copying data to a buffer without proper bounds checking, potentially leading to buffer overflows and format string vulnerabilities.

Приклад: Copying user-controlled input to a buffer without validating its length.

Conclusion and Mitigation

Key Points:

Risk of Exploitation:

Format string vulnerabilities can be exploited by attackers to manipulate memory, disclose sensitive information, or disrupt the normal operation of the application.

Diverse Impact:

The impact of format string vulnerabilities can range from information disclosure and denial of service to remote code execution, depending on the context and severity of the vulnerability.

Mitigation Strategies:

Перевірка вхідних даних:

Реалізація: Validate and sanitize user input before using it in format string functions to prevent malicious input.

Приклад: Ensure that user input does not contain format specifiers or control characters.

Limited User Control:

Реалізація: Minimize user control over format strings, avoiding scenarios where user input directly influences the format specifier.

Приклад: Use predefined and controlled format strings whenever possible.

Use Safer Alternatives:

Реалізація: Prefer safer alternatives to format string functions, such as those that do not interpret format specifiers.

Приклад: Utilize functions like snprintf with a specified buffer size to prevent buffer overflows.

Static Code Analysis:

Реалізація: Use static code analysis tools to identify and address potential format string vulnerabilities during the development process.

Приклад: Regularly scan code for insecure usage of format string functions.

Compiler Flags:

Реалізація: Compile code with compiler flags that provide additional warnings and protections against format string vulnerabilities.

Приклад: Use compiler flags like -Wformat in GCC to enable additional format string checks.

Регулярні перевірки безпеки:

Реалізація: Conduct regular security audits and code reviews to identify and remediate format string vulnerabilities.

Приклад: Include format string vulnerability checks in AMAZON WEB SERVICES processes.

Security Training:

Реалізація: Provide security training to developers to raise awareness of the risks associated with format string functions.

Приклад: Include secure coding practices for input validation in developer training programs.

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