OBJ57-J. Do not rely on methods that can be overridden by untrusted code

Untrusted code can misuse APIs provided by trusted code to override methods such as Object.equals(), Object.hashCode(), and Thread.run(). These methods are valuable targets because they are commonly used behind the scenes and may interact with components in a way that is not easily discernible.

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-J. 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.

public class LicenseManager {
    Map<LicenseType, String> licenseMap = new HashMap<LicenseType, String>();
    
    public LicenseManager() {
        LicenseType type = new LicenseType();
        type.setType("demo-license-key");
        licenseMap.put(type, "ABC-DEF-PQR-XYZ");
    }
    public Object getLicenseKey(LicenseType licenseType) {
        return licenseMap.get(licenseType);
    }
    public void setLicenseKey(LicenseType licenseType, String licenseKey) {
        licenseMap.put(licenseType, licenseKey);
    }
}

class LicenseType {
    private String type;
    public String getType() {
        return type;
    }
    public void setType(String type) {
        this.type = type;
    }
    @Override
    public int hashCode() {
        int res = 17;
        res = res * 31 + type == null ? 0 : type.hashCode();
        return res;
    }
    @Override
    public boolean equals(Object arg) {
        if (arg == null || !(arg instanceof LicenseType)) {
            return false;
        }
        if (type.equals(((LicenseType) arg).getType())) {
            return true;
        }
        return false;
    }
}

The constructor for LicenseManager initializes licenseMap with a demo license key that must remain secret. The license key is hard-coded for illustrative purposes; it should ideally be read from an external configuration file that stores an encrypted version of the key. The LicenseType class provides overridden implementations of equals() and hashCode() methods.

This implementation is vulnerable to an attacker who extends the LicenseType class and overrides the equals() and hashCode() methods:

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;
    }
}

The following is the malicious client program:

public class DemoClient {
  public static void main(String[] args) {
    LicenseManager licenseManager = new LicenseManager();
    for (int i = 0; i <= Integer.MAX_VALUE; i++) {
      Object guessed = licenseManager.getLicenseKey(new CraftedLicenseType());
      if (guessed != null) {
        // prints ABC-DEF-PQR-XYZ
        System.out.println(guessed);
      }
    }
  }
}

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:

public class LicenseManager {
    Map<LicenseType, String> licenseMap = new IdentityHashMap<LicenseType, String>();

  // ...
}

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)).)

Consequently, the overridden methods cannot expose internal class details. The client program can continue to add license keys and can even retrieve the added key-value pairs, as demonstrated by the following client code.

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` 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:

public class Widget {
  private int noOfComponents;
    
  public Widget(int noOfComponents) {
    this.noOfComponents = noOfComponents;
  }
  public int getNoOfComponents() {
    return noOfComponents;
  }
  public final void setNoOfComponents(int noOfComponents) {
    this.noOfComponents = noOfComponents;
  }
  public boolean equals(Object o) {
    if (o == null || !(o instanceof Widget)) {
      return false;
    }
    Widget widget = (Widget) o;
    return this.noOfComponents == widget.getNoOfComponents();
  }
  @Override
  public int hashCode() {
    int res = 31;
    res = res * 17 + noOfComponents;
    return res;
  }
}  
public class LayoutManager {
  private Set<Widget> layouts = new HashSet<Widget>();
  public void addWidget(Widget widget) {
    if (!layouts.contains(widget)) {
      layouts.add(widget);
    }
  }
  public int getLayoutSize() {
    return layouts.size();
  }
}

An attacker can extend the Widget class as a Navigator widget and override the hashCode() method:

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

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

The set layouts is expected to contain just one item because the number of components for both the navigator and widget being added is 1. However, the getLayoutSize() method returns 2.

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` 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:

public class Worker implements Runnable {
  Worker() { }
  public void startThread(String name) {
    new Thread(this, name).start();
  }
  @Override
  public void run() {
    System.out.println("Parent");
  }
}

public class SubWorker extends Worker {
  @Override
  public void startThread(String name) {
    super.startThread(name);
	new Thread(this, name).start();        
  }
  @Override
  public void run() {
    System.out.println("Child");
  }
}

If a client runs the following code:

Worker w = new SubWorker();
w.startThread("thread");

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():

public class SubWorker extends Worker {
  @Override
  public void startThread(String name) {
    new Thread(this, name).start();
  }
 // ...
}

The client code is also modified to start the parent and child threads separately. This program produces the expected output:

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

Bibliography

[API 2013]	Class `IdentityHashMap`
[Hawtin 2006]	[drlvm][kernel_classes] ThreadLocal vulnerability

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Noncompliant Code Example (`hashCode`)

Compliant Solution (`IdentityHashMap`)

Compliant Solution (`final` Class)

Noncompliant Code Example

Compliant Solution (`final` Class)

Noncompliant Code Example (`run()`)

Compliant Solution

Bibliography

4 Comments

Dhruv Mohindra

Fred Long

Dean Sutherland

Dhruv Mohindra

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OBJ57-J. Do not rely on methods that can be overridden by untrusted code

Noncompliant Code Example (hashCode)

Compliant Solution (IdentityHashMap)

Compliant Solution (final Class)

Noncompliant Code Example

Compliant Solution (final Class)

Noncompliant Code Example (run())

Compliant Solution

Bibliography

4 Comments

Dhruv Mohindra

Fred Long

Dean Sutherland

Dhruv Mohindra

Noncompliant Code Example (`hashCode`)

Compliant Solution (`IdentityHashMap`)

Compliant Solution (`final` Class)

Compliant Solution (`final` Class)

Noncompliant Code Example (`run()`)