Abstract

Ignoring a method's return value can cause the program to overlook unexpected states and conditions.

Description

Just about every serious attack on a software system begins with the violation of a programmer's assumptions. After the attack, the programmer's assumptions seem flimsy and poorly founded, but before an attack many programmers would defend their assumptions well past the end of their lunch break.

Two dubious assumptions that are easy to spot in code are "this function call can never fail" and "it doesn't matter if this function call fails". When a programmer ignores the return value from a function, they implicitly state that they are operating under one of these assumptions.

Examples

The following code does not check to see if the string returned by getParameter() is null before calling the member function compareTo(), potentially causing a null dereference.

	String itemName = request.getParameter(ITEM_NAME);
	if (itemName.compareTo(IMPORTANT_ITEM)) {
		...
	}
	...

The traditional defense of this coding error is:

"I know the requested value will always exist because ... If it does not exist, the program
cannot perform the desired behavior so it doesn't matter whether I handle the error or simply 
allow the program to die dereferencing a null value."

But attackers are skilled at finding unexpected paths through programs, particularly when exceptions are involved.