Integer Coercion Error

Description

Several flaws fall under the category of integer coercion errors. For the most part, these errors in and of themselves result only in availability and data integrity issues. However, in some circumstances, they may result in other, more complicated security related flaws, such as buffer overflow conditions.

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 code is intended to read an incoming packet from a socket and extract one or more headers.

DataPacket *packet;
int numHeaders;
PacketHeader *headers;

sock=AcceptSocketConnection();
ReadPacket(packet, sock);
numHeaders =packet->headers;

if (numHeaders > 100) {
  ExitError("too many headers!");
}
headers = malloc(numHeaders * sizeof(PacketHeader);
ParsePacketHeaders(packet, headers);

The code performs a check to make sure that the packet does not contain too many headers. However, numHeaders is defined as a signed int, so it could be negative. If the incoming packet specifies a value such as -3, then the malloc calculation will generate a negative number (say, -300 if each header can be a maximum of 100 bytes). When this result is provided to malloc(), it is first converted to a size_t type. This conversion then produces a large value such as 4294966996, which may cause malloc() to fail or to allocate an extremely large amount of memory (CWE-195). With the appropriate negative numbers, an attacker could trick malloc() into using a very small positive number, which then allocates a buffer that is much smaller than expected, potentially leading to a buffer overflow.

Example Two

The following code reads a maximum size and performs validation on that size. It then performs a strncpy, assuming it will not exceed the boundaries of the array. While the use of "short s" is forced in this particular example, short int's are frequently used within real-world code, such as code that processes structured data.

int GetUntrustedInt () {
  return(0x0000FFFF);
}

void main (int argc, char **argv) {

  char path[256];
  char *input;
  int i;
  short s;
  unsigned int sz;

  i = GetUntrustedInt();
  s = i;
  /* s is -1 so it passes the safety check - CWE-697 */
  if (s > 256) {
    DiePainfully("go away!\n");
  }

  /* s is sign-extended and saved in sz */
  sz = s;

  /* output: i=65535, s=-1, sz=4294967295 - your mileage may vary */
  printf("i=%d, s=%d, sz=%u\n", i, s, sz);

  input = GetUserInput("Enter pathname:");

  /* strncpy interprets s as unsigned int, so it's treated as MAX_INT
  (CWE-195), enabling buffer overflow (CWE-119) */
  strncpy(path, input, s);
  path[255] = '\0'; /* don't want CWE-170 */
  printf("Path is: %s\n", path);

}

This code first exhibits an example of CWE-839, allowing "s" to be a negative number. When the negative short "s" is converted to an unsigned integer, it becomes an extremely large positive integer. When this converted integer is used by strncpy() it will lead to a buffer overflow (CWE-119).

See Also