Integer Overflow or Wraparound

Description

An integer overflow or wraparound occurs when an integer value is incremented to a value that is too large to store in the associated representation. When this occurs, the value may wrap to become a very small or negative number. While this may be intended behavior in circumstances that rely on wrapping, it can have security consequences if the wrap is unexpected. This is especially the case if the integer overflow can be triggered using user-supplied inputs. This becomes security-critical when the result is used to control looping, make a security decision, or determine the offset or size in behaviors such as memory allocation, copying, concatenation, etc.

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

The following image processing code allocates a table for images.

img_t table_ptr; /*struct containing img data, 10kB each*/
int num_imgs;
...
num_imgs = get_num_imgs();
table_ptr = (img_t*)malloc(sizeof(img_t)*num_imgs);
...

This code intends to allocate a table of size num_imgs, however as num_imgs grows large, the calculation determining the size of the list will eventually overflow (CWE-190). This will result in a very small list to be allocated instead. If the subsequent code operates on the list as if it were num_imgs long, it may result in many types of out-of-bounds problems (CWE-119).

Example Two

The following code excerpt from OpenSSH 3.3 demonstrates a classic case of integer overflow:

nresp = packet_get_int();
if (nresp > 0) {
  response = xmalloc(nresp*sizeof(char*));
  for (i = 0; i < nresp; i++) response[i] = packet_get_string(NULL);
}

If nresp has the value 1073741824 and sizeof(char*) has its typical value of 4, then the result of the operation nresp*sizeof(char*) overflows, and the argument to xmalloc() will be 0. Most malloc() implementations will happily allocate a 0-byte buffer, causing the subsequent loop iterations to overflow the heap buffer response.

Example Three

Integer overflows can be complicated and difficult to detect. The following example is an attempt to show how an integer overflow may lead to undefined looping behavior:

short int bytesRec = 0;
char buf[SOMEBIGNUM];

while(bytesRec < MAXGET) {
  bytesRec += getFromInput(buf+bytesRec);
}

In the above case, it is entirely possible that bytesRec may overflow, continuously creating a lower number than MAXGET and also overwriting the first MAXGET-1 bytes of buf.

Example Four

In this example the method determineFirstQuarterRevenue is used to determine the first quarter revenue for an accounting/business application. The method retrieves the monthly sales totals for the first three months of the year, calculates the first quarter sales totals from the monthly sales totals, calculates the first quarter revenue based on the first quarter sales, and finally saves the first quarter revenue results to the database.

#define JAN 1
#define FEB 2
#define MAR 3

short getMonthlySales(int month) {...}

float calculateRevenueForQuarter(short quarterSold) {...}

int determineFirstQuarterRevenue() {


  // Variable for sales revenue for the quarter
  float quarterRevenue = 0.0f;

  short JanSold = getMonthlySales(JAN); /* Get sales in January */
  short FebSold = getMonthlySales(FEB); /* Get sales in February */
  short MarSold = getMonthlySales(MAR); /* Get sales in March */

  // Calculate quarterly total
  short quarterSold = JanSold + FebSold + MarSold;

  // Calculate the total revenue for the quarter
  quarterRevenue = calculateRevenueForQuarter(quarterSold);

  saveFirstQuarterRevenue(quarterRevenue);

  return 0;

}

However, in this example the primitive type short int is used for both the monthly and the quarterly sales variables. In C the short int primitive type has a maximum value of 32768. This creates a potential integer overflow if the value for the three monthly sales adds up to more than the maximum value for the short int primitive type. An integer overflow can lead to data corruption, unexpected behavior, infinite loops and system crashes. To correct the situation the appropriate primitive type should be used, as in the example below, and/or provide some validation mechanism to ensure that the maximum value for the primitive type is not exceeded.

...
float calculateRevenueForQuarter(long quarterSold) {...}

int determineFirstQuarterRevenue() {

  ...
  // Calculate quarterly total
  long quarterSold = JanSold + FebSold + MarSold;

  // Calculate the total revenue for the quarter
  quarterRevenue = calculateRevenueForQuarter(quarterSold);

  ...

}

Note that an integer overflow could also occur if the quarterSold variable has a primitive type long but the method calculateRevenueForQuarter has a parameter of type short.

See Also