Numeric Truncation Error

Description

When a primitive is cast to a smaller primitive, the high order bits of the large value are lost in the conversion, potentially resulting in an unexpected value that is not equal to the original value. This value may be required as an index into a buffer, a loop iterator, or simply necessary state data. In any case, the value cannot be trusted and the system will be in an undefined state. While this method may be employed viably to isolate the low bits of a value, this usage is rare, and truncation usually implies that an implementation error has occurred.

Demonstrations

The following examples help to illustrate the nature of this weakness and describe methods or techniques which can be used to mitigate the risk.

Note that the examples here are by no means exhaustive and any given weakness may have many subtle varieties, each of which may require different detection methods or runtime controls.

Example One

This example, while not exploitable, shows the possible mangling of values associated with truncation errors:

int intPrimitive;
short shortPrimitive;
intPrimitive = (int)(~((int)0) ^ (1 << (sizeof(int)*8-1)));
shortPrimitive = intPrimitive;
printf("Int MAXINT: %d\nShort MAXINT: %d\n", intPrimitive, shortPrimitive);

The above code, when compiled and run on certain systems, returns the following output:

Int MAXINT: 2147483647
Short MAXINT: -1

This problem may be exploitable when the truncated value is used as an array index, which can happen implicitly when 64-bit values are used as indexes, as they are truncated to 32 bits.

Example Two

In the following Java example, the method updateSalesForProduct is part of a business application class that updates the sales information for a particular product. The method receives as arguments the product ID and the integer amount sold. The product ID is used to retrieve the total product count from an inventory object which returns the count as an integer. Before calling the method of the sales object to update the sales count the integer values are converted to The primitive type short since the method requires short type for the method arguments.

...
// update sales database for number of product sold with product ID
public void updateSalesForProduct(String productID, int amountSold) {


  // get the total number of products in inventory database
  int productCount = inventory.getProductCount(productID);
  // convert integer values to short, the method for the

  // sales object requires the parameters to be of type short
  short count = (short) productCount;
  short sold = (short) amountSold;
  // update sales database for product
  sales.updateSalesCount(productID, count, sold);

}
...

However, a numeric truncation error can occur if the integer values are higher than the maximum value allowed for the primitive type short. This can cause unexpected results or loss or corruption of data. In this case the sales database may be corrupted with incorrect data. Explicit casting from a from a larger size primitive type to a smaller size primitive type should be prevented. The following example an if statement is added to validate that the integer values less than the maximum value for the primitive type short before the explicit cast and the call to the sales method.

...
// update sales database for number of product sold with product ID
public void updateSalesForProduct(String productID, int amountSold) {


  // get the total number of products in inventory database
  int productCount = inventory.getProductCount(productID);
  // make sure that integer numbers are not greater than

  // maximum value for type short before converting
  if ((productCount < Short.MAX_VALUE) && (amountSold < Short.MAX_VALUE)) {


    // convert integer values to short, the method for the

    // sales object requires the parameters to be of type short
    short count = (short) productCount;
    short sold = (short) amountSold;
    // update sales database for product
    sales.updateSalesCount(productID, count, sold);


  else {
  // throw exception or perform other processing

    ...
  }

}
...

See Also