The program prompts for a number of values that should be stored. The program fills the internal array with a countdown of integers from the provided number to 0. Then, it prompts for an index of a stored value to be retrieved. If the user provides a valid index (one less than the initial number), the corresponding integer from the countdown is displayed. Otherwise, the program displays a 0 or, if the index is 10 or higher, throws an exception. If the user gives an integer over 10 (such as 12 or 20) to the first prompt, the program crashes. It also crashes if the user requests an index over 9 in the second prompt.
Security Checklist: Finding arrays
Loops that modify index variables
Bounds Checking
Bounds checking should occur anywhere user input is requested. In this program’s case, invalid user input can request values that result in trying to access areas of memory that have not been assigned yet. Thus, the program should check for bounds when processing the user input for the number of values to be stored and retrieving a value from the array. Since the array is size 10 and indexes are numbers between 0 and (size – 1), its valid range is (0-9). Any other number will cause an out of bounds exception: -1, 95, 11, -215.
Buffer Overflow
Buffer overflow is an error which occurs when a program tries to write or read data outside of the allocated bounds an object, such as a buffer or array. In java, arrays are fixed-size, which means they can only store a limited amount of objects. Trying to read or write an object outside this range will cause the program to throw an ArrayIndexOutOfBoundsException and crash. This behavior is not robust because it allows a user to crash the program with a simple input error, and it can be exploited by an attacker.
Buffer overflow errors often happen when processing user input. For instance, the provided program prompts the user for an index in the array. If the user does not know the size of the array or makes a mistake, he or she can can input a number that will result in a buffer overflow error. Buffer overflow errors can also happen in loops, especially if the loop conditional is derived from user input or another part of the program. Such a loop can continue iterating beyond the allocated bounds of an array, causing the error. Finally, attempting to read or assign values directly outside of an array’s bounds can cause this error.
To prevent buffer overflow errors in user input, a programmer must take additional steps to verify this input. For instance, if the user interacts with a known array, his or her input should first be checked for whether it is a valid index — one between 0 and the array’s size – 1. Furthermore, when looping over an array, the conditional can be checked to ensure that it is not higher than the array’s size.