Difference between non- final public static and non- final public instance fields in terms of security?

Question

I am going through this link , OBJ10-J. Do not use public static nonfinal fields and it says that ,

Client code can trivially access public static fields because access to such fields are not checked by a security manager.

what do they actually mean by that? i.e what do they mean by escaping from security manager?

If they simply meant it because field being non-final and public , then how come non-final , public instance fields different than their static counterparts? ( as far as code security is concerned )

I have been through this question and have not seen any mention in terms of security , Why are static variables considered evil

public class's public static fields would be accessible from anywhere and so public instance fields too, so where is the difference? Why non-final public instance fields not a security issue but being static is?

Answer 1

Why non-final public instance fields not a security issue but being static is?

If you want to access an instance field you need the reference to that object instance. If you don't have a reference you can't access it.

So your code can control to which objects a refernce is passed. If malicious code tries to hijack one of your objects using reflection to get a reference you can install a security manager to prevent this.

On the other side a public static field can be accessed from everybody that has access to the class, because the Class object is accessible. So malicious code might only use

YourClass.PUBLIC_INSTANCE_FIELD = someValue;

or the reflection way

Class clazz = Class.forName("YourClass");
Field publicStaticField = clazz.getDeclaredField("PUBLIC_INSTANCE_FIELD");
publicStaticField.set(null, someValue);

Answer 2

That's because the case of non-static fields is already covered by OBJ01-J. Limit accessibility of fields

public instance field are covered by OBJ01-J for slightly different reasons. For one, you need to have a reference to an instance before you can change public instance fields, while public static fields can be accessed directly at class level. But both are against the CERT rules.