Unchecked Input for Loop Condition

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 example demonstrates the weakness.

void iterate(int n){
  int i;
  for (i = 0; i < n; i++){
    foo();
  }
}
void iterateFoo()
{
  unsigned int num;
  scanf("%u",&num);
  iterate(num);
}

Example Two

In the following C/C++ example the method processMessageFromSocket() will get a message from a socket, placed into a buffer, and will parse the contents of the buffer into a structure that contains the message length and the message body. A for loop is used to copy the message body into a local character string which will be passed to another method for processing.

int processMessageFromSocket(int socket) {

  int success;

  char buffer[BUFFER_SIZE];
  char message[MESSAGE_SIZE];

  // get message from socket and store into buffer

  //Ignoring possibliity that buffer > BUFFER_SIZE
  if (getMessage(socket, buffer, BUFFER_SIZE) > 0) {


    // place contents of the buffer into message structure
    ExMessage *msg = recastBuffer(buffer);

    // copy message body into string for processing
    int index;
    for (index = 0; index < msg->msgLength; index++) {
      message[index] = msg->msgBody[index];
    }
    message[index] = '\0';

    // process message
    success = processMessage(message);

  }
  return success;

}

However, the message length variable from the structure is used as the condition for ending the for loop without validating that the message length variable accurately reflects the length of the message body (CWE-606). This can result in a buffer over-read (CWE-125) by reading from memory beyond the bounds of the buffer if the message length variable indicates a length that is longer than the size of a message body (CWE-130).

See Also