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For instance, an immutable class that lacks the final qualifier can be extended by a malicious subclass that can modify the state of the supposedly-immutable object. Further, the malicious subclass can impersonate the immutable object while actually remaining mutable. Such malicious subclasses can then violate program invariants on which clients depend, consequently introducing security vulnerabilities.

As a result, classes with invariants on which other code depends should be declared final, thereby preventing malicious subclasses from subverting their invariants. Furthermore, immutable classes must be declared final.

Superclasses must permit extension by trusted subclasses while simultaneously preventing extension by untrusted code. Consequently, declaring such superclasses to be final is infeasible because it would prevent the required extension by trusted code. Such problems require careful design for inheritance.

Consider two classes belonging to different protection domains: one is malicious and extends the other, which is trusted.  Consider an object of the malicious subclass with a fully qualified invocation of a method defined by the trusted superclass, not overridden by the malicious class. In this case, the trusted superclass's permissions are examined to execute the method, and, as a result, the malicious object gets the method invoked inside the protection domain of the trusted superclass \[[Gong 2003|AA. Bibliography#Gong 03]\].

One commonly suggested solution is to place code at each point where the superclass can be instantiated to ensure that the instance being created has the same type as the superclass. When the type is found to be that of a subclass rather than the superclass's type, the checking code performs a security manager check to ensure that malicious classes cannot misuse the superclass. This approach is insecure because it allows a malicious class to add a finalizer and obtain a partially - initialized instance of the superclass. This attack is detailed in guideline "OBJ05-J. Prevent access to partially initialized objects."

For non-final classes, the method that performs the security manager check must be invoked as an argument to a private constructor to ensure that the security check is performed before any superclass's constructor can exit.

When the parent class has members that are declared private or are otherwise inaccessible to the attacker, the attacker must use reflection to exploit those members of the parent class. Declaring the parent class or its methods final prohibits this level of access.

A method which receives an untrusted, non-final input argument must beware that other methods or threads might modify the input object. Some methods attempt to prevent modification by making a local copy of the input object. This is insufficient because a shallow copy of an object can still allow it to refer to mutable sub-objects, which can still be modified by other methods or threads. Some methods go farther further and perform a deep copy of the input object. This mitigates the problem of modifiable sub-objects, but the method could still receive as an argument a mutable object that extends the input object class.

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BigInteger msg = new BigInteger("123");
msg = msg.modPow(exp, m);  // Always returns 1  

// Malicious subclassing of java.math.BigInteger
class BigInteger extends java.math.BigInteger {
  private int value;

  public BigInteger(String str) {
    super(str);
    value = Integer.parseInt( str);
  }

  public setValue(int value) {
    this.value = value;
  }

  @Override public java.math.BigInteger modPow(java.math.BigInteger exponent, java.math.BigInteger m) {
    this.value = ((int) (Math.pow( this.value, exponent))) % m;
    return this;
  }
}

This malicious BigInteger class is clearly mutable because of the setValue() method. Furthermore, the modPow() method is subject to precision loss. (See guidelines "NUM00-J. Detect or prevent integer overflow," "NUM11-J. Check floating-point inputs for exceptional values," "NUM15-J. Ensure conversions of numeric types to narrower types do not result in lost or misinterpreted data," and "NUM17-J. Beware of precision loss when converting primitive integers to floating-point" for more information.) Any code that receives an object of this class , and assumes that the object is immutable will have unexpected behavior. This is particularly important because the BigInteger.modPow() method has several useful cryptographic applications.

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This noncompliant code example installs a security manager check in the constructor of the {{BigInteger}} class. The security manager denies access when it detects that a subclass without the requisite permissions is attempting to instantiate the superclass \[[SCG 2007|AA. Bibliography#SCG 07]\]. It also compares class types, in compliance with guideline "[OBJ06-J. Compare classes and not class names|OBJ06-J. Compare classes and not class names]."

public class BigInteger {
  public BigInteger(String str) {
    Class c = getClass();  // java.lang.Object.getClass(), which is final
    // Confirm class type
    if (c != NonFinal.class) {
      // Check the permission needed to subclass NonFinal
      securityManagerCheck(); // throws a security exception if not allowed
    }
    // ...   
  }
}

Unfortunately, throwing an exception from the constructor of a non-final class is insecure because it allows a finalizer attack. (See guideline "OBJ05-J. Prevent access to partially initialized objects.".)

Compliant Solution (

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Final)

This compliant solution prevents creation of malicious subclasses by declaring the immutable BigInteger class to be final. Although this solution would be appropriate for locally maintained code, it cannot be used in the case of java.math.BigInteger because it would require changing the Java SE API, which has already been published and must remain compatible with previous versions.

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public static BigInteger safeInstance(BigInteger val) {
  // create a defensive copy if it is not java.math.BigInteger
  if (val.getClass() != java.math.BigInteger.class)  
    return new BigInteger(val.toByteArray());
  
  return val;
}

The guidelines "FIO00-J. Defensively copy mutable inputs and mutable internal components" and "OBJ08-J. Provide mutable classes with copy functionality to allow passing instances to untrusted code safely" discuss defensive copying in great depth.

Compliant Solution (Java SE 6,

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Public and

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Private Constructors)

This compliant solution invokes a security manager check as a side-effect of computing the boolean value passed to a private constructor (as seen in guideline "OBJ05-J. Prevent access to partially initialized objects"). The rules for order of evaluation require that the security manager check must execute before invocation of the private constructor. Consequently, the security manager check also executes before invocation of any superclass's constructor. Note that the security manager check is made without regard to whether the object under construction has the type of the parent class or the type of a subclass (whether trusted or not).

This solution prevents the finalizer attack; it appilesapplies to Java SE 6 and later versions, where throwing an exception before the {{java.lang.Object}} constructor exits prevents execution of finalizers \[[SCG 2009|AA. Bibliography#SCG 09]\].

public class BigInteger {
  public BigInteger(String str) {
    this( str, check( this.getClass())); // throws a security exception if not allowed
  }
  
  private BigInteger(String str, boolean securityManagerCheck) {
    // regular construction goes here
  }

  private static boolean check(Class c) {
    // Confirm class type
    if (c != BigInteger.class) {
      // Check the permission needed to subclass BigInteger
      securityManagerCheck(); // throws a security exception if not allowed
    }

    return true;
  }
}

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Code in privileged blocks should be as simple as possible, both to improve reliability and also to ease security audits. Invocation of overridable methods permits modification of the code that is executed in the privileged context without modification of previously - audited classes. Furthermore, calling overridable methods disperses the code over multiple classes, making it harder to determine which code must be audited. Malicious subclasses cannot directly exploit this issue because privileges are dropped as soon as unprivileged code is executed. Nevertheless, maintainers of the subclasses might unintentionally violate the requirements of the base class. For example, even when the base class's overridable method is thread-safe, a subclass might provide an implementation that lacks this property, leading to security vulnerabilities.

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public class MethodInvoker {
  public void invokeMethod() {
    AccessController.doPrivileged(new PrivilegedAction<Object>() {
      public Object run() {
        try {
          Class<?> thisClass = MethodInvoker.class;
          String methodName = getMethodName();
          Method method = thisClass.getMethod(methodName, null);
          method.invoke(new MethodInvoker(), null);
        } catch (Throwable t) {
          // Forward to handler   
        }
        return null;
      }
    });
  }

  String getMethodName() {
    return "someMethod";
  }
  
  public void someMethod() {
    // ...    
  }

  // Other methods
}

A subclass can override getMethodName() to return a string other than someMethod. If an object of such a subclass runs invokeMethod(), control flow will divert to some mothod method other than someMethod.

Compliant Solution (

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Final)

This compliant solution declares the getMethodName() method final so that it cannot be overridden.

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Alternative approaches that prevent overriding of the getMethodName() method include declaring it as private or declaring the enclosing class as final.

Compliant Solution (Disallow

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Polymorphism)

This compliant solution specifically invokes the correct getMethodName(), preventing diversion of control flow.

  public void invokeMethod() {
  AccessController.doPrivileged(new PrivilegedAction<Object>() {
    public Object run() {
      try {
        Class<?> thisClass = MethodInvoker.class;
        String methodName = MethodInvoker.this.getMethodName();
        Method method = thisClass.getMethod(methodName, null);
        method.invoke(new MethodInvoker(), null);
      } catch (Throwable t) {
        // Forward to handler   
      }
      return null;
    }
  });
}

Risk Assessment

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