I have a requirement to restrict access to loading of a particular class of a JAVA application, at runtime. Just wondering whether this is possible using JAVA Security Manager with a security policy. Also, if this is a possibility, I'd like to know how the permission configuration would look like in a JAVA security policy. I'd done some researching on this but couldn't really find any useful solution yet.
These two:
java.security.Permission
java.io.FilePermission
and providing you need to continue the application operating in some way without it you need to load classes using java.lang.ClassLoader and loadClass method to be able to throw it into an if-else to bypass potentially if simple instantiation is not involved.
I managed to come up with a custom permission implementation extending java.security.RuntimePermission class and get my requirement fulfilled. The only overhead (which is unavoidable) associated with the approach of introducing a custom permission is that, you need to hack into all your classloading implementations and add an additional check to evaluate your custom permission.
Related
I have developed an android library and I want to prevent reflection to access my class fields and methods. I remember something about SecurityManager but I don't know if it can help.
How can I achieve such functionality ?
The JVM has security mechanisms that allows you to define restrictions to code through a Java security policy file.
It will use the default one unless you specify otherwise.
Run your application using a SecurityManager and a sufficiently restrictive security policy, policy can be found here:
http://docs.oracle.com/javase/7/docs/technotes/guides/security/PolicyFiles.html
You may find this tutorial useful:
http://docs.oracle.com/javase/tutorial/essential/environment/security.html
To set the value of a private Field, it needs to be marked as accessible:
field.setAccessible(true);
When will the SecurityManager not allow this? How portable is it to include this in a library? Will it fail when imported into certain contexts?
If you know your library won't be used inside a JVM with the Security Manager enabled, like an applet or a secured application server, then it's fine. But I would try to avoid it if possible.
There are others answers like this link that suggest there's no problem using it. So if you think it's the best approach, and the other options are too cumbersome or directly don't exist, then go ahead.
When will the SecurityManager not allow this?
The javadoc says:
First, if there is a security manager, its checkPermission method is called with a ReflectPermission("suppressAccessChecks") permission.
A SecurityException is raised if flag is true but accessibility of this object may not be changed (for example, if this element object is a Constructor object for the class Class).
As to your other question
How portable is it to include this in a library? Will it fail when imported into certain contexts?
It is portable across JVM implementations because Field is defined in the core library with these semantics. It is not portable across instances because different JVM instances may have differently configured security policies.
I am trying to design a plugin framework that will sandbox loaded plugins so that they can only perform risky actions through my plugin interfaces.
I need to figure out how to use AccessControllerContext to see what code is calling the methods.
The documentation shows no way to recover the code source or originating class. How can this be done?
Is it possible to set the context for all classes loaded from a particular class loader?
Rather than using an AccessControlContext directly, you should use AccessController.checkPermission, which will check the active security context for you. To control what permissions are granted a class, use ClassLoader.defineClass, and pass a ProtectionDomain containing a PermissionCollection with the appropriate set of permissions. It might be useful to review an Overview of Basic Concepts in the Java Security Architecture document.
Suppose I have a class in my package org.jake and it has a method with default access (no modifier). Then the method is visible inside the package only.
However, when someone receives the jar of my framework, what is to stop them from writing a new class, declaring its package as org.jake, and using my supposedly invisible method?
In other words, is there anything I can do to prevent this?
You could seal the package in your jar file. It's not bullet-proof though.
The main thing is not to rely on access modifiers etc from a security point of view to start with, really. If someone is running the code with unrestricted permissions, they're going to have access to all kinds of things. Access modifiers really just help to stop people from accidentally shooting themselves in the foot.
If someone is willing to put classes in your package to circumvent your encapsulation, they're clearly ignoring your best intentions - I say let 'em get on with it, but don't provide support for that scenario.
There is nothing you can do to prevent this. Even private members can be accessed via reflection. You should consider the access modifiers in java to be merely suggestive.
First off, this is the “DRM” scenario: ultimately, someone determined enough can defeat any protections you put in place by supplying a funky modified runtime or other such things. The reverse scenario – where the runtime is trusted but some of the packages are not – is tackled properly by Java through the use of suitable ClassLoader restrictions, but that can only work where there's something that can enforce the restrictions in a trusted fashion; that's why your scenario is basically doomed.
However, if we assume that the runtime itself is trustable then you could try, in your super-secret method, getting the stack trace of the currently executing stack (see stackoverflow.com/questions/1069066/… for how) and testing to see whether the caller of the current method is one that you trust to get access. A security manager would be even more suitable, but you can't trust the environment to have one of those installed that you like (it's much more clearly under the control of the attacker). Note that I have not tried the options in this paragraph!
The other alternative is to put your secrets on a service you control and only offer remote access to them. Or stop worrying about using technical mechanisms to deal with a problem that is fundamentally about business and legal issues (e.g., why are you dealing with people you can't trust?)
I'd say simply do not allow them to run code where it can call yours, i.e. in the same JVM. You could instead consider offering only a (web)service they can call externally. I'm not very up to date on the best ways to implement this though.
I need to call some semi-trustworthy Java code and want to disable the ability to use reflection for the duration of that code's execution.
try{
// disable reflection somehow
someObject.method();
}
finally{
// enable reflection again
}
Can this be done with a SecurityManager, and if so, how?
Clarification/Context: This is a follow-up to another question about restricting the packages that can be called from JavaScript/Rhino. The accepted answer references a blog entry on how to do that, and it requires two steps, the first one using a Rhino API (ClassShutter), the second one turning off reflection and Class.forName(). I was thinking I can do that second step more cleanly using a SecurityManager (learning about SecurityManager, which as has been pointed out, is a complex beast, along the way).
To sum up, I want (from code, not setting file) to turn off Class.forName() and any access to the whole reflection package.
It depends on what you are trying to restrict.
In general, publicly accessible API is not restricted. However, as long as you don't grant the untrustworthy code the ReflectPermission("suppressAccessChecks") permission, it won't be able to get access to non-public API in another package.
If you have a list of packages to which you want to restrict all access, there are two steps. First, in the Security properties, include the restricted package in the package.access list. Then give your trusted code RuntimePermission("accessClassInPackage." + pkg).
A common way to distinguish your untrusted code is to load it from a different location, and refer to the different codebases in your policy file when granting permissions.
The Java security architecture is very powerful, but I know it is also complicated; if you would like a more concrete example, please describe exactly what calls you want to restrict and I'll try to be more explicit.
To do what you want without modifying the java.policy file and/or the java.security file would be very difficult, maybe impossible. The java.security.Policy represents the information in java.policy, but it doesn't offer write access. You could create your own Policy implementation and install it at runtime as long as any existing SecurityManager permits it.
On the other hand, you can specify a custom java.policy file as a command-line option. If you are providing a complete application with some sort of launcher, that might be easily accomplished. It also provides some transparency to your users. A sophisticated user can review the permissions you'd like to have granted to the application.
Well, you can override SecurityManager.checkMemberAccess and give a stricter definition. However, it doesn't really work like that. What happens for instance if the code defines a finaliser?
On the clarification: Other APIs use reflection and other APIs. For instance, java.beans, LiveConnect and Rhino. An adversary could from within a script, say, create a new Rhino context without the shutter and thereby bootstrap into the full JRE. With an open system, a blacklist can never be finished.
In summary: to use the Java security model you need to work with it, not against it.
I wrote a replacement of ClassShutter that allows fine grained access control, per instance, per method, per field:
http://riven8192.blogspot.com/2010/07/java-rhino-fine-grained-classshutter.html