Identify Buffer Overflow

 

a) Identify and Describe the Vulnerability

The vulnerability in the provided C source code is a buffer overflow. Here’s a detailed description:

Code Segment with Vulnerability:

void dumbFunction (char *str) {
    char buffer[16];
    strcpy(buffer, str);
}

void main() {
    char large_string[256];
    int i;

    for (i = 0; i < 255; i++)
        large_string[i] = 'A';
    large_string[255] = '\0'; // Null-terminate the string
    dumbFunction(large_string);
}

Description:

• Buffer Overflow: In dumbFunction, the strcpy function copies the content of str into buffer, which is a fixed-size array of 16 bytes. The strcpy function does not perform any bounds checking, so if str is longer than 15 characters (plus 1 for the null terminator), it will overflow buffer, leading to potential overwriting of adjacent memory.

Why This Vulnerability Can Happen:

• Lack of Bounds Checking: The strcpy function does not check if the source string fits within the destination buffer. It simply copies data until it encounters a null terminator, which can cause it to write beyond the allocated buffer if the source string is too long.
• Fixed-Size Buffer: The buffer buffer in dumbFunction is only 16 bytes in size, while the input string large_string in main is 256 bytes long. This mismatch makes the overflow possible.
• Assumptions in main: The main function creates a string that is exactly 256 bytes long and passes it to dumbFunction. Since dumbFunction is not equipped to handle such large inputs, this results in overflow.

How Buffer Overflow Can Be Exploited:

• Memory Corruption: The overflow can overwrite important data or control structures in memory, potentially leading to unexpected behavior, crashes, or security vulnerabilities.
• Arbitrary Code Execution: An attacker could exploit a buffer overflow to overwrite a return address or function pointer, allowing them to execute arbitrary code with the same privileges as the vulnerable program.

Prevention:

• Use Safer Functions: Replace strcpy with strncpy or similar functions that allow you to specify the maximum number of characters to copy, preventing overflow.
• Bounds Checking: Always validate the size of input data before copying it into buffers.
• Dynamic Allocation: Consider using dynamic memory allocation (malloc/free) instead of fixed-size arrays when dealing with variable-length data.

In summary, the buffer overflow vulnerability occurs because strcpy copies more data than the fixed-size buffer can handle, leading to potential memory corruption and security risks.