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By providing overridden implementations, an attacker can use untrusted code to glean sensitive information, run arbitrary code, or launch a denial of service attack.

See MET52-JGJ. Do not use the clone() method to copy untrusted method parameters for more specific details regarding overriding the Object.clone() method.

Noncompliant Code Example (hashCode)

This noncompliant code example shows a LicenseManager class that maintains a licenseMap. The map stores a LicenseType and license value pair.

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public class CraftedLicenseType extends LicenseType {
    private static int guessedHashCode = 0;
    @Override
    public int hashCode() {
        // Returns a new hashCode to test every time get() is called
        guessedHashCode++;
        return guessedHashCode;
    }
    @Override
    public boolean equals(Object arg) {
        // Always returns true
        return true;
    }
}

Following The following is the malicious client program:

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The client program runs through the sequence of all possible hash codes using CraftedLicenseType until it successfully matches the hash code of the demo license key object stored in the LicenseManager class. Consequently, the attacker can discover the sensitive data present within the licenseMap in only a few minutes. The attack operates by discovering at least one hash collision with respect to the key of the map.

Compliant Solution (IdentityHashMap)

This compliant solution uses an IdentityHashMap rather than a HashMap to store the license information:

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According to the Java API class IdentityHashMap documentation [API 2006],

This class implements the Map interface with a hash table, using reference-equality in place of object-equality when comparing keys (and values). In other words, in an IdentityHashMap, two keys k1 and k2 are considered equal if and only if (k1==k2). (In normal Map implementations (like HashMap) two keys k1 and k2 are considered equal if and only if (k1==null ? k2==null : k1.equals(k2)).)

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public class DemoClient {
    public static void main(String[] args) {
        LicenseManager licenseManager = new LicenseManager();
        LicenseType type = new LicenseType();
        type.setType("custom-license-key");
        licenseManager.setLicenseKey(type, "CUS-TOM-LIC-KEY");
        Object licenseKeyValue = licenseManager.getLicenseKey(type);
        // Prints CUS-TOM-LIC-KEY
        System.out.println(licenseKeyValue);
    }
}

Compliant Solution (final

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Class)

This compliant solution declares the LicenseType class final so that its methods cannot be overridden:

final class LicenseType {
  // ...
} 

Noncompliant Code Example

This noncompliant code example consists of a Widget class and a LayoutManager class containing a set of widgets:

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public class Navigator extends Widget {
  public Navigator(int noOfComponents) {
    super(noOfComponents);
  }
  @Override
  public int hashCode() {
    int res = 31;
    res = res * 17;
    return res;
  }
}

Client The client code follows:

Widget nav = new Navigator(1);
Widget widget = new Widget(1);
LayoutManager manager = new LayoutManager();
manager.addWidget(nav);
manager.addWidget(widget);
System.out.println(manager.getLayoutSize()); // Prints 2

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The reason for this discrepancy is that the hashCode() method of Widget is used only once when the widget is added to the set. When the navigator is added, the hashCode() method provided by the Navigator class is used.  Consequently, the set contains two different object instances.

Compliant Solution (final

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Class)

This compliant solution declares the Widget class final so that its methods cannot be overridden:

public final class Widget {
  // ...
}

Noncompliant Code Example (run())

In this noncompliant code example, class Worker and its subclass SubWorker each contain a startThread()method intended to start a thread:

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the client may expect Parent and Child to be printed. However, Child is printed twice because the overridden method run() is invoked both times that a new thread is started.

Compliant Solution

This compliant solution modifies the SubWorker class and removes the call to super.startThread():

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Worker w1 = new Worker();
w1.startThread("parent-thread");
Worker w2 = new SubWorker();
w2.startThread("child-thread");

Bibliography

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