Loop with Unreachable Exit Condition ('Infinite Loop')

Description

If the loop can be influenced by an attacker, this weakness could allow attackers to consume excessive resources such as CPU or memory.

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

In the following code the method processMessagesFromServer attempts to establish a connection to a server and read and process messages from the server. The method uses a do/while loop to continue trying to establish the connection to the server when an attempt fails.

int processMessagesFromServer(char *hostaddr, int port) {

  ...
  int servsock;
  int connected;
  struct sockaddr_in servaddr;

  // create socket to connect to server
  servsock = socket( AF_INET, SOCK_STREAM, 0);
  memset( &servaddr, 0, sizeof(servaddr));
  servaddr.sin_family = AF_INET;
  servaddr.sin_port = htons(port);
  servaddr.sin_addr.s_addr = inet_addr(hostaddr);

  do {


    // establish connection to server
    connected = connect(servsock, (struct sockaddr *)&servaddr, sizeof(servaddr));

    // if connected then read and process messages from server
    if (connected > -1) {


      // read and process messages
      ...

    }



  // keep trying to establish connection to the server
  } while (connected < 0);

  // close socket and return success or failure
  ...

}

However, this will create an infinite loop if the server does not respond. This infinite loop will consume system resources and can be used to create a denial of service attack. To resolve this a counter should be used to limit the number of attempts to establish a connection to the server, as in the following code.

int processMessagesFromServer(char *hostaddr, int port) {

  ...
  // initialize number of attempts counter
  int count = 0;
  do {


    // establish connection to server
    connected = connect(servsock, (struct sockaddr *)&servaddr, sizeof(servaddr));

    // increment counter
    count++;

    // if connected then read and process messages from server
    if (connected > -1) {


      // read and process messages
      ...

    }



  // keep trying to establish connection to the server

  // up to a maximum number of attempts
  } while (connected < 0 && count < MAX_ATTEMPTS);

  // close socket and return success or failure
  ...

}

Example Two

For this example the method isReorderNeeded as part of a bookstore application that determines if a particular book needs to be reordered based on the current inventory count and the rate at which the book is being sold.

public boolean isReorderNeeded(String bookISBN, int rateSold) {


  boolean isReorder = false;

  int minimumCount = 10;
  int days = 0;

  // get inventory count for book
  int inventoryCount = inventory.getIventoryCount(bookISBN);

  // find number of days until inventory count reaches minimum
  while (inventoryCount > minimumCount) {


    inventoryCount = inventoryCount - rateSold;
    days++;


  }

  // if number of days within reorder timeframe

  // set reorder return boolean to true
  if (days > 0 && days < 5) {
    isReorder = true;
  }

  return isReorder;

}

However, the while loop will become an infinite loop if the rateSold input parameter has a value of zero since the inventoryCount will never fall below the minimumCount. In this case the input parameter should be validated to ensure that a value of zero does not cause an infinite loop,as in the following code.

public boolean isReorderNeeded(String bookISBN, int rateSold) {

  ...

  // validate rateSold variable
  if (rateSold < 1) {
    return isReorder;
  }

  ...

}

See Also