Need

Secure resource allocation and prevent buffer overflow vulnerabilities

Context

• Usage of C# for building robust and scalable applications

Description

Non compliant code

        public void VulnerableMethod(string userString) 
{
    char[] buffer = new char[100];
    int i = 0;
    while (i < userString.Length) 
    {
        buffer[i] = userString[i];
        i++;
    }
}
        
        

The above code is a classic example of a buffer overflow vulnerability due to improper resource allocation.

A buffer is a temporary area for data storage. When the amount of data exceeds the buffer's capacity, the excess data will overflow into adjacent memory space. This can cause data corruption, unpredictable application behavior, and can even allow attackers to execute arbitrary code.

In the code snippet, we have a buffer with a fixed size of 100 characters ( char[] buffer = new char[100];). The VulnerableMethod takes a string as input ( string userString) and copies each character from the input string to the buffer using a while loop.

The vulnerability lies in the fact that there is no check to ensure the input string's length does not exceed the buffer's capacity. If userString has more than 100 characters, it will overflow the buffer, leading to a buffer overflow.

This vulnerability can lead to serious security risks as an attacker can use this overflow to overwrite critical program data or inject malicious code.

To avoid this, always ensure that you are not writing more data to a buffer than it can hold.

Steps

• Replace the use of char array with a safer alternative, such as StringBuilder or a dynamically resizing collection.
• Ensure that the buffer size is dynamically allocated based on the length of the user input to prevent buffer overflow.
• Implement proper input validation to ensure that the user input does not exceed the allocated buffer size.
• Consider using built-in security features or libraries provided by the framework to handle user input securely.

Compliant code

        public void SafeMethod(string userString) 
{
    StringBuilder buffer = new StringBuilder(userString.Length);
    for (int i = 0; i < userString.Length; i++) 
    {
        buffer.Append(userString[i]);
    }
}
        
        

The above code fixes the buffer overflow vulnerability by replacing the use of a fixed-size char array with a StringBuilder.

StringBuilder is a safer alternative because it automatically resizes as needed, which prevents buffer overflow vulnerabilities. The initial capacity of the StringBuilder is set to the length of the user input string, userString.Length, to optimize performance.

The for loop iterates over each character in the user input string and appends it to the StringBuilder using the Append method. This method is safer than directly accessing the buffer by index, as it automatically handles resizing if the StringBuilder capacity is exceeded.

This code does not implement input validation or use any built-in security features or libraries to handle user input securely. These are important considerations for further improving the security of the code. For example, you could add checks to ensure that the user input string is not excessively long before processing it.

References

• 316. Improper resource allocation - Buffer overflow