OBJ33-J. Limit the extensibility of non-final classes and methods to only trusted subclasses

A non-final class or method that is not meant to be inherited can be overridden by an attacker if it is not declared final. Sometimes, it is desired that only trusted implementations be allowed to extend the class. Declaring the class final proves to be a very stringent alternative in this case. In such cases, the class must be carefully designed for inheritance.

Another related pitfall occurs when malicious classes are allowed to extend from a non-final class. Consider two classes belonging to different protection domains. One of them is malicious whereas the other is trusted. If the malicious class extends the trusted public non-final class and inherits without overriding a method of the trusted class, the fully qualified invocation of the malicious class's version of the method uses the protection domain of the trusted class. In this case, the trusted class's permissions are examined to execute the method. [[Gong 03]])

At all points that the class can be instantiated, there must be checks to ensure that the instance being created has the same type as the class. If the type is found to be that of a subclass, instead of the non-final public superclass', a security manager check must be performed to ensure that malicious classes cannot misuse the class.

The use of reflection is necessary if the non-final class has members that are declared private or are otherwise inaccessible to the attacker. Declaring the 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 class, NonFinal. As a result, it can call any of its instance methods and access its protected fields.

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

Compliant Solution

This compliant solution installs a security manager check in the constructor of the non-final class. Access is denied if it is detected that a subclass is trying to instantiate the superclass. Only if the subclass has the requisite permissions, is it allowed to proceed. [[SCG 07]]

public class NonFinal {
  public NonFinal() {
    // invoke java.lang.Object.getClass to get class instance
    Class c = getClass();
    // confirm class type
    if (c != NonFinal.class) {
      // check the permission needed to subclass NonFinal
      securityManagerCheck();
    }
  // ...   
  }
}

It is critical to compare the class types and not the class names (OBJ34-J. Compare classes and not class names).

Risk Assessment

Allowing 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 rule on the CERT website

Related Vulnerabilities

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

References

[[McGraw 00]] Chapter Seven Rule 3: "Make Everything Final, Unless There's a Good Reason Not To"
[[Lai 08]]
[[SCG 07]] Guideline 1-2 "Limit the extensibility of classes and methods"
[[Gong 03]] Chapter 6: "Enforcing Security Policy"
[[Bloch 08]] Item 1: "Consider static factory methods instead of constructors"

OBJ32-J. Do not allow partially initialized objects to be accessed      07. Object Orientation (OBJ)      OBJ34-J. Compare classes and not class names