EXP01-J. Ensure a null pointer is not dereferenced

Null pointer dereferencing refers to treating a null variable as if it were a valid object or field and proceeding to use it without checking its state. Typically, this condition results in a NullPointerException which may sometimes result in denial of service. While other runtime exceptions can produce similar effects, NullPointerException is often found to be the most frequent show-stopper.

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 was generated in some isolated cases when the application or applet attempted to establish an https connection with a server [[SDN 08]]. The failure to establish a secure https connection with the server caused a denial of service issue as clients were temporarily forced to use an insecure http channel for data exchange.

Noncompliant Code Example

This noncompliant example shows a bug in Tomcat version 4.1.24 initially discovered by Reasoning [[Reasoning 03]]. The cardinality method was designed to return the number of occurrences of object obj in collection col. A valid use of the cardinality method is to determine how many objects in the collection are null. However, because membership in the collection is checked with the expression obj.equals(elt), a null pointer dereference is guaranteed 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;
}

Compliant Solution

This compliant solution eliminates the null pointer dereference.

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) || 
        (null != obj && obj.equals(elt))) {
      count++;
    }
  }
  return count;
}

Null pointer dereferences can happen in many path dependent ways. Because of the limitations of automatic detection tools, it is required to manually inspect code [[Hovemeyer 07]] to detect instances of null pointer dereferences. Annotations for method parameters that must be non-null can also alleviate the problem to a certain extent by aiding automatic null pointer dereference detection.

Risk Assessment

Dereferencing a null pointer can lead to denial of Service. In multithreaded programs, this can violate cache coherency policies and cause resource leaks.

Automated Detection

TODO

Related Vulnerabilities

GERONIMO-4467

References

[[API 06]] method doPrivileged()
[[Reasoning 03]] Defect ID 00-0001, Null Pointer Dereference
[[SDN 08]] Bug ID 6514454
[[Hovemeyer 07]]
[[MITRE 09]] CWE ID 479

EXP00-J. Use the same type for the second and third operands in conditional expressions      04. Expressions (EXP)      EXP02-J. Do not ignore values returned by methods