Expression is Always False

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 Java example the updateUserAccountOrder() method used within an e-business product ordering/inventory application will validate the product number that was ordered and the user account number. If they are valid, the method will update the product inventory, the user account, and the user order appropriately.

public void updateUserAccountOrder(String productNumber, String accountNumber) {

  boolean isValidProduct = false;
  boolean isValidAccount = false;

  if (validProductNumber(productNumber)) {
    isValidProduct = true;
    updateInventory(productNumber);
  }
  else {
    return;
  }

  if (validAccountNumber(accountNumber)) {
    isValidProduct = true;
    updateAccount(accountNumber, productNumber);
  }

  if (isValidProduct && isValidAccount) {
    updateAccountOrder(accountNumber, productNumber);
  }

}

However, the method never sets the isValidAccount variable after initializing it to false so the isValidProduct is mistakenly used twice. The result is that the expression "isValidProduct && isValidAccount" will always evaluate to false, so the updateAccountOrder() method will never be invoked. This will create serious problems with the product ordering application since the user account and inventory databases will be updated but the order will not be updated.

This can be easily corrected by updating the appropriate variable.

...
if (validAccountNumber(accountNumber)) {
  isValidAccount = true;
  updateAccount(accountNumber, productNumber);
}
...

Example Two

In the following example, the hasReadWriteAccess method uses bit masks and bit operators to determine if a user has read and write privileges for a particular process. The variable mask is defined as a bit mask from the BIT_READ and BIT_WRITE constants that have been defined. The variable mask is used within the predicate of the hasReadWriteAccess method to determine if the userMask input parameter has the read and write bits set.

#define BIT_READ 0x0001 // 00000001
#define BIT_WRITE 0x0010 // 00010000

unsigned int mask = BIT_READ & BIT_WRITE; /* intended to use "|" */

// using "&", mask = 00000000
// using "|", mask = 00010001

// determine if user has read and write access
int hasReadWriteAccess(unsigned int userMask) {

  // if the userMask has read and write bits set
  // then return 1 (true)
  if (userMask & mask) {
    return 1;
  }

  // otherwise return 0 (false)
  return 0;

}

However the bit operator used to initialize the mask variable is the AND operator rather than the intended OR operator (CWE-480), this resulted in the variable mask being set to 0. As a result, the if statement will always evaluate to false and never get executed.

The use of bit masks, bit operators and bitwise operations on variables can be difficult. If possible, try to use frameworks or libraries that provide appropriate functionality and abstract the implementation.

Example Three

In the following example, the updateInventory method used within an e-business inventory application will update the inventory for a particular product. This method includes an if statement with an expression that will always evaluate to false. This is a common practice in C/C++ to introduce debugging statements quickly by simply changing the expression to evaluate to true and then removing those debugging statements by changing expression to evaluate to false. This is also a common practice for disabling features no longer needed.

int updateInventory(char* productNumber, int numberOfItems) {


  int initCount = getProductCount(productNumber);

  int updatedCount = initCount + numberOfItems;

  int updated = updateProductCount(updatedCount);

  // if statement for debugging purposes only
  if (1 == 0) {


    char productName[128];
    productName = getProductName(productNumber);

    printf("product %s initially has %d items in inventory \n", productName, initCount);
    printf("adding %d items to inventory for %s \n", numberOfItems, productName);

    if (updated == 0) {
      printf("Inventory updated for product %s to %d items \n", productName, updatedCount);
    }

    else {
      printf("Inventory not updated for product: %s \n", productName);
    }


  }

  return updated;

}

Using this practice for introducing debugging statements or disabling features creates dead code that can cause problems during code maintenance and potentially introduce vulnerabilities. To avoid using expressions that evaluate to false for debugging purposes a logging API or debugging API should be used for the output of debugging messages.

See Also