Page History

The prevalence of null pointer dereferencing bugs is so widespread that it is not uncommon to find errors in security contexts. For instance, Java Webstart applications and applets particular to JDK version 1.6, prior to update 4, were affected by a bug that had some noteworthy security consequences. A {{NullPointerException}} resulted in some isolated cases when the application or applet attempted to establish an https connection with the server \[[SDN 08|AA. Java References#SDN 08]\]. This caused a denial of service issue as clients were temporarily left with no choice but to provide an insecure channel for data exchange.

This noncompliant example shows a bug in Tomcat version 4.1.24 initially discovered by Reasoning \[[Reasoning 03|AA. Java References#Reasoning 03]\]. The {{cardinality}} method was designed to return the number of occurrences of object {{obj}} in collection {{col}}. A valid use case would be to determine how many objects in the collection are {{null}}, given a {{null}} object parameter. However, when coupled with the {{obj.equals(elt)}} expression, this leads to a guaranteed null pointer dereference whenever {{obj}} is {{null}}. Such ambiguity can also result from the short-circuit behavior of the conditional AND and OR operators (See [EXP06-J. Be aware of the short-circuit behavior of the conditional AND and OR operators]). 

public static int cardinality(Object obj, final Collection col) {
  int count = 0;
  Iterator it = col.iterator();
  while(it.hasNext()) {
    Object elt = it.next();
    if((null == obj && null == elt) || obj.equals(elt)) {  // null pointer dereference
      count++;
    }
  }
  return count;
}

if (obj == null) {
  for(Iterator it = col.iterator();it.hasNext();) { // col is currently named coll
    if (it.next() == null) {
      count++;
    }
  }
} else {
  for (Iterator it = col.iterator();it.hasNext();) { // col is currently named coll
    if (obj.equals(it.next())) {
      count++;
    }
  }
}

Dereferences of null pointers can happen in many path dependent ways. Due to the limitations of automatic detection tools, code review and manual inspection of code are inevitable \[[Hovemeyer 07|AA. Java References#Hovemeyer 07]\]. Annotations for method parameters that must be non-null can also alleviate the occurrences to a certain extent by aiding automatic detection.  

\[[API 06|AA. Java References#API 06]\] [method doPrivileged()|http://java.sun.com/javase/6/docs/api/java/security/AccessController.html#doPrivileged(java.security.PrivilegedAction)]
\[[Reasoning 03|AA. Java References#Reasoning 03]\] Defect ID 00-0001, Null Pointer Dereference
\[[SDN 08|AA. Java References#SDN 08]\] [Bug ID 6514454|http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=6514454]
\[[Hovemeyer 07|AA. Java References#Hovemeyer 07]\]
\[[MITRE 09|AA. Java References#MITRE 09]\] [CWE ID 479|http://cwe.mitre.org/data/definitions/476.html]