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Sometimes null is returned intentionally to account for zero available instances. This practice can lead to vulnerabilities when the client code does not handle the null return case.

Noncompliant Code Example

The erroneous behavior is caused due to the server returning null while the client forgets to add in a check for such a value. This noncompliant example shows how the check item != null condition is missing from the if condition in class Client.

Code Block

bgColor	#FFCCCC

import java.util.Arrays;

class Inventory {
  private static int[] item;
    public Inventory() {
    item = new int[20];
  }

  public static int[] getStock() {
    if(item.length == 0)
      return null;
    else
      return item;
  }
}

  public class Client {
    public static void main(String[] args) {
      Inventory iv = new Inventory();
        int[] item = Inventory.getStock();
	  if (Arrays.asList(item[1]).contains(1)) {
	    System.out.println("Almost out of stock!" + item);
	  }
    }
}

Compliant Solution

Wiki Markup

This compliant solution eliminates the {{null}} return and simply returns the {{item}} array as is even if it is zero-length. The client can effectively handle this situation without exhibiting erroneous behavior. Be careful that the client does not try to access individual elements of a zero-length array such as {{item\[1\]}} while following this recommendation.

Code Block

bgColor	#ccccff

import java.util.Arrays;

class Inventory {
  private static int[] item;
    public Inventory() {
    item = new int[20];
    item[2] = 1;  //quantity of item 2 remaining is 1, almost out! 
  }

  public static int[] getStock() {
    return item;
  }
}

public class Client {
  public static void main(String[] args) {
    Inventory iv = new Inventory();
    int[] item = Inventory.getStock();
    if (Arrays.asList(item[1]).contains(1)) {
      System.out.println("Almost out of stock!" + item);
    }
  }
}

Risk Assessment

Returning null rather than a zero-length array may lead to vulnerabilities when the client code does not handle null properly.

Rule	Severity	Likelihood	Remediation Cost	Priority	Level
MET03-J	low	unlikely	high	P1	L3

Automated Detection

TODO

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

References

Wiki Markup
\[[Bloch 08\|AA. Java References#Bloch 08]\] Item 43: return empty arrays or collections, not nulls

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