Why is this code vulnerable to buffer overflow attacks?

Question

int func(char* str)
{
   char buffer[100];
   unsigned short len = strlen(str);

   if(len >= 100)
   {
        return (-1);
   }

   strncpy(buffer,str,strlen(str));
   return 0;
}

This code is vulnerable to a buffer overflow attack, and I'm trying to figure out why. I'm thinking it has to do with len being declared a short instead of an int, but I'm not really sure.

Any ideas?

Answer 1

On most compilers the maximum value of an unsigned short is 65535.

Any value above that gets wrapped around, so 65536 becomes 0, and 65600 becomes 65.

This means that long strings of the right length (e.g. 65600) will pass the check, and overflow the buffer.

---

Use size_t to store the result of strlen(), not unsigned short, and compare len to an expression that directly encodes the size of buffer. So for example:

char buffer[100];
size_t len = strlen(str);
if (len >= sizeof(buffer) / sizeof(buffer[0]))  return -1;
memcpy(buffer, str, len + 1);