ABSTRACT

The program can dereference a null pointer because it does not check the return value of a function that might return null.

EXPLANATION

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.



Example 1: 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)) {
...
}
...


Example 2:. The following code shows a system property that is set to null and later dereferenced by a programmer who mistakenly assumes it will always be defined.


System.clearProperty("os.name");
...
String os = System.getProperty("os.name");
if (os.equalsIgnoreCase("Windows 95") )
System.out.println("Not supported");


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.

REFERENCES

[1] Standards Mapping - OWASP Top 10 2004 - (OWASP 2004) A9 Application Denial of Service

[2] Standards Mapping - Security Technical Implementation Guide Version 3 - (STIG 3) APP3120 CAT II, APP6080 CAT II

[3] Standards Mapping - Common Weakness Enumeration - (CWE) CWE ID 253, CWE ID 690

[4] Standards Mapping - Payment Card Industry Data Security Standard Version 1.1 - (PCI 1.1) Requirement 6.5.9