SEC02-A. Beware of standard APIs that may bypass Security Manager checks

SecurityManager checks may be allowed to get bypassed depending on the immediate caller's class loader. When an API (See table below) is invoked on a class object, a comparison is run between the immediate caller's class loader and that of the class object. The class object is the object on which an API is invoked. For instance, in the presence of a SecurityManager, the getSystemClassLoader and getParent methods succeed only if the caller's class loader is the delegation ancestor of the current class loader or if the caller's class loader is the same as the current one or if the code in the current execution context has the RunTimePermission, namely "getClassLoader".

Non-Compliant Code Example

The createInstance method is the immediate caller of java.lang.Class.newInstance <TO BE CONFIRMED> in this non-compliant example. The newInstance method is being invoked on the dateClass class object. The issue is that the untrustedCode method can trigger the instantiation of a new class even though it should not have the permission to do so. This behavior is not caught by the SecurityManager.

import java.util.Date;

public class exception
{
    public static void untrustedCode()
    {
    	 Date now = new Date();
         Class<?> dateClass = now.getClass();
         createInstance(dateClass);
    }
    public static void createInstance(Class<?> dateClass)
    {
        try
        {   // Create another Date object using the Date Class
            Object o = dateClass.newInstance();
            if (o instanceof Date)
            {
                Date d = (Date)o;
                System.out.println("The time is: " + d.toString());
            }
        }
        catch (InstantiationException ie) { System.out.println(ie.toString()); }
        catch (IllegalAccessException lae) { System.out.println(iae.toString()); }    	
    }
}

Compliant Solution

Do not accept Class, ClassLoader or Thread instances from untrusted code. If inevitable, safely acquire these instances by ensuring they come from trusted sources. Additionally, make sure to discard tainted inputs from untrusted code. Likewise, objects returned by the affected methods should not be propagated back to the untrusted code. In essence, never perform any operation on behalf of untrusted code because its class loader instance may have insufficient permissions.

References

Inside Java 2 Platform Security, 4.3.2 Class Loader Delegation Hierarchy
Sun Secure Coding Guidelines