Comparison of Classes by Name

Description

If the decision to trust the methods and data of an object is based on the name of a class, it is possible for malicious users to send objects of the same name as trusted classes and thereby gain the trust afforded to known classes and types.

Demonstrations

The following examples help to illustrate the nature of this weakness and describe methods or techniques which can be used to mitigate the risk.

Note that the examples here are by no means exhaustive and any given weakness may have many subtle varieties, each of which may require different detection methods or runtime controls.

Example One

In this example, the expression in the if statement compares the class of the inputClass object to a trusted class by comparing the class names.

if (inputClass.getClass().getName().equals("TrustedClassName")) {


  // Do something assuming you trust inputClass

  // ...


}

However, multiple classes can have the same name therefore comparing an object's class by name can allow untrusted classes of the same name as the trusted class to be use to execute unintended or incorrect code. To compare the class of an object to the intended class the getClass() method and the comparison operator "==" should be used to ensure the correct trusted class is used, as shown in the following example.

if (inputClass.getClass() == TrustedClass.class) {


  // Do something assuming you trust inputClass

  // ...


}

Example Two

In this example, the Java class, TrustedClass, overrides the equals method of the parent class Object to determine equivalence of objects of the class. The overridden equals method first determines if the object, obj, is the same class as the TrustedClass object and then compares the object's fields to determine if the objects are equivalent.

public class TrustedClass {

  ...

  @Override
  public boolean equals(Object obj) {

    boolean isEquals = false;

    // first check to see if the object is of the same class
    if (obj.getClass().getName().equals(this.getClass().getName())) {


      // then compare object fields
      ...
      if (...) {
        isEquals = true;
      }

    }

    return isEquals;

  }

  ...

}

However, the equals method compares the class names of the object, obj, and the TrustedClass object to determine if they are the same class. As with the previous example using the name of the class to compare the class of objects can lead to the execution of unintended or incorrect code if the object passed to the equals method is of another class with the same name. To compare the class of an object to the intended class, the getClass() method and the comparison operator "==" should be used to ensure the correct trusted class is used, as shown in the following example.

public boolean equals(Object obj) {

  ...

  // first check to see if the object is of the same class
  if (obj.getClass() == this.getClass()) {
    ...
  }

  ...

}

See Also