Abstract

An attacker may be able to create unexpected control flow paths through the application, potentially bypassing security checks.

Description

If an attacker can supply values that the application then uses to determine which class to instantiate or which method to invoke, the potential exists for the attacker to create control flow paths through the application that were not intended by the application developers. This attack vector may allow the attacker to bypass authentication or access control checks or otherwise cause the application to behave in an unexpected manner.

This situation becomes a doomsday scenario if the attacker can upload files into a location that appears on the application's classpath or add new entries to the application's classpath. Under either of these conditions, the attacker can use reflection to introduce new, presumably malicious, behavior into the application.

Examples

A common reason that programmers use the reflection API is to implement their own command dispatcher. The following example shows a command dispatcher that does not use reflection:

	String ctl = request.getParameter("ctl");
	Worker ao = null;
	if (ctl.equals("Add")) {
	  ao = new AddCommand();
	} else if (ctl.equals("Modify")) {
	  ao = new ModifyCommand();
	} else {
	  throw new UnknownActionError();
	}
	ao.doAction(request);

A programmer might refactor this code to use reflection as follows:

	String ctl = request.getParameter("ctl");
	Class cmdClass = Class.forName(ctl + "Command");
	Worker ao = (Worker) cmdClass.newInstance();
	ao.doAction(request);

The refactoring initially appears to offer a number of advantages. There are fewer lines of code, the if/else blocks have been entirely eliminated, and it is now possible to add new command types without modifying the command dispatcher.

However, the refactoring allows an attacker to instantiate any object that implements the Worker interface. If the command dispatcher is still responsible for access control, then whenever programmers create a new class that implements the Worker interface, they must remember to modify the dispatcher's access control code. If they fail to modify the access control code, then some Worker classes will not have any access control.

One way to address this access control problem is to make the Worker object responsible for performing the access control check. An example of the re-refactored code follows:

	String ctl = request.getParameter("ctl");
	Class cmdClass = Class.forName(ctl + "Command");
	Worker ao = (Worker) cmdClass.newInstance();
	ao.checkAccessControl(request);
	ao.doAction(request);

Although this is an improvement, it encourages a decentralized approach to access control, which makes it easier for programmers to make access control mistakes.

This code also highlights another security problem with using reflection to build a command dispatcher. An attacker can invoke the default constructor for any kind of object. In fact, the attacker is not even constrained to objects that implement the Worker interface; the default constructor for any object in the system can be invoked. If the object does not implement the Worker interface, a ClassCastException will be thrown before the assignment to ao, but if the constructor performs operations that work in the attacker's favor, the damage will already have been done. Although this scenario is relatively benign in simple applications, in larger applications where complexity grows exponentially it is not unreasonable that an attacker could find a constructor to leverage as part of an attack.