CWE 119 - Buffer Overflow

Disclaimer

  • The code provided is for **educational purposes** only and should not be used in a **production environment** without proper review and testing.

  • The provided code should be regarded as **best practice**. There are also several ways to remediate the Vulnerabilities.

  • The code provided **may contain vulnerabilities** that could be exploited by attackers, and is intended to be used as a learning tool to help improve security awareness and best practices.

  • The mitigations provided are intended to address **specific vulnerabilities** in the code, but may not be effective against all potential attack vectors or scenarios.

  • The code provided is not **guaranteed to be secure or free** from all vulnerabilities, and should be reviewed and tested thoroughly before being used in a production environment.

  • The code provided is **not a substitute for professional security** advice or guidance, and users should consult with a qualified security professional before implementing any security measures.

  • The authors and contributors of the code provided **cannot be held responsible** for any damages or losses resulting from the use of this code.

  • The code provided is provided "as is" without any warranties, express or implied, including but not limited to the implied warranties of merchantability and fitness for a particular purpose.

  • We do not have any sponsors or financial interests in any specific products or services, and the information provided is based solely on our own knowledge and experience.

  • The vulnerable and mitigated code samples provided on our platform are generated with the help of AI and sourced from the internet. We have made every effort to ensure that the code is accurate and up-to-date, but we cannot guarantee its correctness or completeness. If you notice that any of the code samples belong to you and you wish for it to be removed, please contact us and we will take appropriate action. We also apologize for any inconvenience caused and are committed to giving appropriate credit to the respective authors.

About CWE 119

Improper Restriction of Operations within the Bounds of a Memory Buffer

This Vulnerability occurs when a program does not properly validate or control the size or boundaries of data that it reads from or writes to a memory buffer.

Impact

  • Buffer Overflow

  • Arbitrary code Execution

  • Denial of Service (DoS)

  • Privilege Escalation

  • Remote code Execution

Example with Code Explanation

C

Vulnerable Code

#include <stdio.h>
#include <string.h>

void copyData(const char* input) {
    char buffer[10];
    strcpy(buffer, input); // Vulnerable line - no bounds checking on input size
    printf("Buffer contents: %s\n", buffer);
}

int main() {
    char userInput[20];
    printf("Enter your input: ");
    scanf("%s", userInput);
    copyData(userInput);
    return 0;
}

In this code, we have a function called copyData that takes a string as input and copies it into a local buffer named buffer. The buffer size is set to 10 characters. However, there is no check to ensure that the input provided by the user will fit within this buffer size. If the user enters a string that is longer than 10 characters, it will cause a buffer overflow.

  • Some of the ways the Vulnerable code can be mitigated is:

    • Use Safe Functions: Replace unsafe functions like strcpy with safer alternatives such as strncpy. Safer functions perform bounds checking to prevent buffer overflows.

    • Ensure Null Termination: When using functions like strncpy, ensure that the destination buffer is null-terminated after copying data to it. This ensures that the resulting string is valid and properly terminated.

    • Limit Input Size: When reading user input, limit the number of characters read to the size of the buffer to prevent buffer overflows.

    • Use Functions with Return Value Checking: Check the return values of functions like strncpy and scanf to detect potential errors. If these functions fail, handle the errors gracefully, and avoid processing data from uninitialized or improperly copied buffers.

    • Sanitize and Validate Input: Always sanitize and validate user input before processing or copying it. Reject or handle input that exceeds the expected size or contains unexpected characters.

    • Use Modern Programming Languages: If possible, use modern programming languages that have built-in memory safety features, such as Rust or Java, which can help prevent buffer overflow vulnerabilities.

Mitigated Code

void copyData(const char* input) {
    char buffer[10];
    if (strncpy(buffer, input, sizeof(buffer) - 1) == NULL) {
        fprintf(stderr, "Error copying input to buffer\n");
        return;
    }
    buffer[sizeof(buffer) - 1] = '\0'; // Ensure null-terminated string
    printf("Buffer contents: %s\n", buffer);
}

int main() {
    char userInput[20];
    printf("Enter your input: ");
    if (scanf("%19s", userInput) != 1) {
        fprintf(stderr, "Error reading input from stdin\n");
        return 1;
    }
    copyData(userInput);
    return 0;
}

  • The Mitigated code does the following:

    • Use Safe Function: The code uses strncpy instead of strcpy to copy the input string to the buffer. strncpy takes an additional argument, which is the maximum number of characters to copy, preventing buffer overflows.

    • Null Termination: After using strncpy to copy data to the buffer, the code explicitly adds a null terminator ('\0') at the end of the copied string. This ensures that the buffer contains a valid null-terminated string, regardless of the input length.

    • Limit Input Size: The scanf function is used to read user input, but it includes a format specifier %19s, which limits the number of characters read to 19 (the size of the userInput buffer minus 1 for the null terminator). This prevents buffer overflows in the userInput array.

C++

Vulnerable Code

#include <iostream>
#include <cstring>

int main() {
    const int bufferSize = 5; // Small buffer size for demonstration purposes
    char buffer[bufferSize];
    char* userInput = nullptr;

    std::cout << "Enter a string: ";
    std::cin >> userInput;

    // Copy the user input into the buffer without proper bound checking
    // This can lead to buffer overflow if the user input is longer than bufferSize
    strcpy(buffer, userInput);

    std::cout << "Buffer content: " << buffer << std::endl;

    return 0;
}

In this example, we have a buffer with a size of 5 characters (bufferSize = 5). The program asks the user for input, but it does not check whether the input length exceeds the buffer size. If the user enters a string longer than 5 characters, it will overwrite adjacent memory locations beyond the buffer, causing undefined behavior and potentially opening up security vulnerabilities.

  • Some of the ways the Vulnerable code can be mitigated is:

    • Use Safe Functions: Replace unsafe functions like strcpy, strcat, and sprintf with their safer counterparts. For example, use strncpy, strncat, and snprintf, which take the buffer size as an additional parameter to prevent buffer overflows.

    • Limit Input Size: Always validate and limit user input to ensure it fits within the allocated buffer. Truncate or reject input that exceeds the buffer size.

    • Bound Checking: Perform explicit bound checking when copying or manipulating data into buffers. Ensure that the copy operations do not exceed the bounds of the destination buffer.

    • Use C++ Standard Library: If you're working with C++, prefer using std::string or C++ Standard Library containers over raw C-style arrays. These containers handle memory management automatically and prevent many buffer overflow vulnerabilities.

    • Avoid Mixing Data and Control: Buffer overflows can sometimes be introduced by mixing data with control instructions. Ensure that data is properly validated before using it to control the program flow.

    • Minimize Buffer Sizes: Allocate buffer sizes based on the actual data requirements and not arbitrary maximum values. Avoid unnecessarily large buffers that can be tempting targets for attackers.

Mitigated Code

#include <iostream>
#include <string>

int main() {
    const int bufferSize = 5;
    std::string buffer;
    std::string userInput;

    std::cout << "Enter a string: ";
    std::getline(std::cin, userInput);

    // Safely copy user input into the buffer
    if (userInput.size() > bufferSize - 1) {
        // Option 1: Truncate user input
        buffer = userInput.substr(0, bufferSize - 1);
        // Option 2: Reject user input
        // std::cerr << "Error: User input is too long\n";
        // return 1;
    } else {
        // User input fits in buffer
        buffer = userInput;
    }

    // Safely access individual characters of the buffer
    char c = buffer.at(0); // Safe, will throw std::out_of_range exception if buffer is empty

    std::cout << "Buffer content: " << buffer << std::endl;

    return 0;
}

  • The Mitigated code does the following:

    • Safe Data Structure: The code uses std::string instead of C-style arrays, which handles memory allocation and ensures bounds checking for you, reducing the risk of buffer overflows.

    • Input Validation: The code checks the size of the userInput before copying it into the buffer. If the userInput exceeds the bufferSize - 1, it either truncates the input (Option 1) or rejects it with an error message (Option 2). This prevents buffer overflows by ensuring the input fits within the allocated buffer.

    • Safely Access Individual Characters: The code uses buffer.at(0) to access the first character of the buffer. The std::string::at() function performs bounds checking and throws a std::out_of_range exception if the index is out of bounds, ensuring safe access to individual characters.

JAVA

Vulnerable Code

public class BufferOverflowExample {
    public static void main(String[] args) {
        int bufferSize = 10;
        int[] array = new int[bufferSize];

        // Simulate user input or data from an untrusted source
        int userInput = 20;

        // Unsafe copying of userInput into the array without proper bounds checking
        for (int i = 0; i <= bufferSize; i++) {
            array[i] = userInput;
        }

        System.out.println("Data copied successfully!");
    }
}

  • In this example, we have an array array with a fixed size of 10 elements, and we attempt to copy the userInput value into it using a loop. The loop runs from 0 to bufferSize, inclusive. However, this is incorrect because arrays in Java are 0-indexed, so the valid indices are from 0 to bufferSize - 1. Accessing the element at array[bufferSize] will cause a buffer overflow as it accesses memory beyond the bounds of the array.

  • Some of the ways the Vulnerable code can be mitigated is:

    • Use Safe Data Copying Functions: Instead of using a manual loop for data copying, prefer using built-in functions like System.arraycopy in Java, which automatically handle bounds checking.

    • Validate User Input: Ensure that any user input or data from external sources is properly validated and sanitized before using it in the code. Verify that the input does not exceed the expected bounds of the buffer.

    • Use Data Structures with Built-in Bounds Checking: Use data structures like ArrayList or other collections that handle dynamic resizing and bounds checking automatically. This way, you won't have to manage the buffer size manually.

    • Use for-each Loop or Stream API: If you want to copy elements from one collection to another, you can use the for-each loop or Stream API, which handles the bounds automatically.

Mitigated Code

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.ArrayList;

public class BufferOverflowExample {
    public static void main(String[] args) {
        try {
            // Use a BufferedReader to read input from stdin
            BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));

            // Use an ArrayList to store the input as integers
            ArrayList<Integer> array = new ArrayList<>();

            // Simulate user input or data from an untrusted source
            System.out.println("Enter your input: ");
            String line = reader.readLine();

            // Split the input by whitespace and parse each token as an integer
            String[] tokens = line.split("\\s+");
            for (String token : tokens) {
                int userInput = Integer.parseInt(token);
                // Add the userInput to the array
                array.add(userInput);
            }

            // Copy the array to a fixed-size buffer of 10 integers
            int bufferSize = 10;
            int[] buffer = new int[bufferSize];

            // Check if the array size is less than or equal to the buffer size
            if (array.size() <= bufferSize) {
                // Use System.arraycopy to copy the array to the buffer
                System.arraycopy(array.stream().mapToInt(i -> i).toArray(), 0, buffer, 0, array.size());
                System.out.println("Data copied successfully!");
            } else {
                // Throw an exception if the array size is larger than the buffer size
                throw new IndexOutOfBoundsException("Input is too large for the buffer");
            }

            // Print the buffer contents
            System.out.print("Buffer contents: ");
            for (int i = 0; i < bufferSize; i++) {
                System.out.print(buffer[i] + " ");
            }
            System.out.println();

        } catch (IOException e) {
            // Handle IOException
            e.printStackTrace();
        } catch (NumberFormatException e) {
            // Handle NumberFormatException
            e.printStackTrace();
        } catch (IndexOutOfBoundsException e) {
            // Handle IndexOutOfBoundsException
            e.printStackTrace();
        }
    }
}

  • The Mitigated code does the following:

    • Dynamic Buffer Size: Instead of using a fixed-size array, the code uses an ArrayList<Integer> to store user input. Since an ArrayList automatically resizes itself to accommodate the data, there is no risk of a buffer overflow when adding elements to it.

    • Bounds Checking: Before copying the data from the ArrayList to a fixed-size buffer, the code checks if the size of the ArrayList is less than or equal to the buffer size. If the ArrayList size exceeds the buffer size, it throws an IndexOutOfBoundsException. This check ensures that no data is copied beyond the allocated buffer, preventing buffer overflow.

    • Proper Data Parsing: The code uses Integer.parseInt(token) to parse the individual tokens from user input as integers. This ensures that the data is correctly interpreted as integers and helps avoid possible buffer overflow or other numeric-related vulnerabilities.

    • Input Validation: The code uses exception handling to catch potential errors during input reading and parsing, such as IOException, NumberFormatException, and IndexOutOfBoundsException. Although exception handling doesn't directly prevent buffer overflows, it provides a way to handle errors gracefully and avoid unexpected program termination.

Mitigation

Some Common Mitigation techniques include:

  • Bounds Checking: Always validate the size of buffers and ensure that operations on them stay within their defined bounds. Use built-in functions or libraries that provide bounds checking when working with arrays or strings.

  • Safe String Handling: Use functions that automatically handle string termination, such as null-terminated strings in C or the String class in Java or C++, to prevent buffer overflow when dealing with strings.

  • Memory-safe Functions: Favor safer memory manipulation functions like memcpy_s, strcpy_s, strncpy_s, sprintf_s, and their equivalents in other languages that automatically check the size of the destination buffer.

  • Language Features: Use programming languages or frameworks that offer built-in memory safety, like Rust or C# with safe arrays, to prevent direct manipulation of memory pointers.

  • Avoid Unsafe Functions: Refrain from using functions like gets in C or strcpy in C/C++, which lack bounds checking and are susceptible to buffer overflows. Instead, use their safer alternatives, such as fgets or strncpy.

  • Use Safe Data Types: Choose appropriate data types that can handle bounds checking automatically, such as std::vector in C++ or ArrayList in Java, instead of raw arrays.

  • Input Validation: Always validate and sanitize user input to ensure it does not exceed expected buffer sizes.

References

LogoCWE - CWE-119: Improper Restriction of Operations within the Bounds of a Memory Buffer (4.18)cwe.mitre.org

What is a Buffer Overflow, Attack Examples and Prevention Methods | Sternum IoT

Buffer Overflow | OWASP Foundation

Last updated

Was this helpful?