OBJ05-J. Limit the extensibility of non-final classes and methods to trusted subclasses only

Some classes ("parent classes" hereafter) must permit extension by trusted subclasses while simultaneously preventing extension by untrusted code. Declaring such parent classes to be final is not an option, because it would prevent extension by trusted code. Such problems require careful design for inheritance.

Consider two classes belonging to different protection domains; one is malicious, and estends the other, which is a trusted parent class. Consider an object of the malicious class with a fully qualified invocation of a method defined by the trusted parent, and not overridden by the malicious class. In this case, the trusted parent class's permissions are examined to execute the method, with the consequence that the malicious object gets the method invoked inside the protection domain of the trusted parent class. [[Gong 2003]].

One commonly suggested solution is to place code at each point where the parent class can be instantiated to ensure that the instance being created has the same type as the parent class. When the type is found to be that of a subclass instead of the parent class's type, the checking code performs a security manager check to ensure that malicious classes cannot misuse the parent class. This approach is insecure because it allows a malicious class to add a finalizer and obtain a partially initialized instance of the parent class. This attack is detailed in guideline OBJ04-J. Do not allow 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.

Noncompliant Code Example

In this noncompliant code example, a malicious class can extend the public non-final parent class, NonFinal. Consequently, the attacker can invoke any of the parent class's accessible instance methods, can access the parent class's protected fields, and can even override any of the parent class's accessible non-final methods.

public class NonFinal {
  public NonFinal() {
    // ...   
  }
}

Noncompliant Code Example (Security Manager)

This noncompliant code example installs a security manager check in the constructor of the non-final parent class. The security manager denies access when it detects that a subclass without the requisite permissions is attempting to instantiate the superclass [[SCG 2007]]. It also compares class types, in compliance with [OBJ06-J. Compare classes and not class names].

public class NonFinal {
  public NonFinal() {
    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 OBJ04-J. Do not allow access to partially initialized objects).

Compliant Solution (Java SE 6+, public and 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 OBJ04-J. Do not allow 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 thwarts the finalizer attack. It is specific to Java SE 6 and onwards, where a finalizer is prevented from being executed when an exception is thrown before the java.lang.Object constructor exits [[SCG 2009]].

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

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

    return true;
  }
}

Risk Assessment

Permitting a non-final class or method to be inherited without checking the class instance allows a malicious subclass to misuse the privileges of the class.

Related Vulnerabilities

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

Related Vulnerabilities

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

Bibliography

[[Bloch 2008]] Item 1: "Consider static factory methods instead of constructors"
[[Gong 2003]] Chapter 6: "Enforcing Security Policy"
[[Lai 2008]]
[[McGraw 2000]] Chapter Seven Rule 3: "Make Everything Final, Unless There's a Good Reason Not To"[[SCG 2007]] Guideline 1-2 "Limit the extensibility of classes and methods"
[[SCG 2009]]

OBJ04-J. Do not allow access to partially initialized objects      04. Object Orientation (OBJ)      OBJ06-J. Compare classes and not class names