Chapter 14: Basic Security

Security plays an important role in distributed systems. The basic Jini security model is based on the JDK 1.2 security system. This chapter deals with handling permissions granted to downloaded code and applies to Jini 1.1 and Jini 2.0. The advanced security for Jini 2.0 is considered in a later chapter.

Basic security for Jini is based on the JDK 1.2 security model. This makes use of a SecurityManager to grant or deny access to resources. A few of the examples may work fine without a security manager. Most will require an appropriate security manager in place or RMI will be unable to download class files. Installing a suitable manager may be done by

System.setSecurityManager(new RMISecurityManager());

The security manager will need to make use of a security policy. This is typically given in policy files which are in default locations or are specified to the Java runtime. If policy.all is a policy file in the current directory, then invoking the runtime by

java -Djava.security.policy="policy.all" ...

A totally permissive policy file can contain

grant {
    permission java.security.AllPermission "", "";
};

Granting all permissions to everyone is a very trusting act, in the potentially hostile world of the Internet. Not everyone is "mister nice guy". The client is vulnerable to attack because it is downloading code that satisfies a request for a service, and then executing that code. Without security checks, the client can download code from a hostile service. While the code has to implement the requested interface, and maybe satisfy conditions on associated Entry objects, without security it is otherwise unconstrained as to what it does.

As an example of a deliberately hostile service, a client asking for a simple file classifier could end up getting this object:

package hostile;

import common.MIMEType;
import common.FileClassifier;

/**
 * HostileFileClassifier1.java
 */

public class HostileFileClassifier1 implements FileClassifier,
              java.io.Serializable {

    public MIMEType getMIMEType(String fileName) {
	if (java.io.File.pathSeparator.equals("/")) {
	    // Unix - don't uncomment the next line!
	    // Runtime.getRuntime().exec("/bin/rm -rf /");
	} else {
	    // DOS - don't uncomment the next line!
	    // Runtime.getRuntime().exec("format c: /u");
	}
	return null;
    }


    public HostileFileClassifier1() {
	// empty
    }
    
} // HostileFileClassifier1

Mind you, if the attacker was stupid enough to implement the service using RMI, that exports a proxy stub, then the method would run in the service's JVM and attempt to trash the attacker's system instead!

It is not necessary to actually call a method on the downloaded object - the mere act of downloading can do the damage, if the object overrides the deserialization method:

package hostile;

import common.MIMEType;
import common.FileClassifier;

/**
 * HostileFileClassifier2.java
 */

public class HostileFileClassifier2 implements FileClassifier,
    java.io.Externalizable {

    public MIMEType getMIMEType(String fileName) {
	return null;
    }

    public void readExternal(java.io.ObjectInput in) {
	if (java.io.File.pathSeparator.equals("/")) {
	    // Unix - don't uncomment the next line!
	    // Runtime.getRuntime().exec("/bin/rm -rf /");
	} else {
	    // DOS - don't uncomment the next line!
	    // Runtime.getRuntime().exec("format c: /u");
	}
    }

    public void writeExternal(java.io.ObjectOutput out) 
	throws java.io.IOException{
	out.writeObject(this);
    }

    public HostileFileClassifier2() {
	// empty
    }
    
} // HostileFileClassifier2

The two classes above assume that clients will make requests for the implementation of a particular interface, and this means that the attacker would need to know this interface. It would require some knowledge of the clients it is attacking (that they will ask for this interface). At the moment, there are no standard interfaces, so this may not be a feasible way of attacking many clients. As interfaces such as those for a printer become specified and widely used, attacks based on hostile implementations of services may become more common. Even without well-known interfaces, clients such as service browsers which attempt to find all possible services can be attacked, simply because they lookup subclasses of Object.

Setting the security access to AllPermission is easy and removes all possible security issues that may hinder development of a Jini application. But it leaves your system open, so that you must start using a more rigorous security policy at some stage - hopefully before others have damaged your system. The problem with moving away from this policy is that permissions are additive rather than subtractive. That is, you can't take permissions away from AllPermission, but have to start with an empty permission set and add to that.

Not giving enough permission can result in at least three cases:

1. A security-related exception can be thrown. This is comparatively easy to deal with, because the exception will tell you what permission is being denied. You can then decide if you should be granting this permission or not. Of course, this should be caught during testing, not when the application is deployed!
2. A security-related exception can be thrown but caught by some library object which attempts to handle it. This happens within the multicast lookup methods, which make multicast requests. If this permission is denied it will be retried several times before giving up. This leads to a cumulative time delay before anything else can happen. The application may be able to continue, and will just suffer this time delay
3. A security-related exception can be thrown but caught by some library object and ignored. The application may be unable to continue in any rational way after this, and may just appear to hang. This may happen if network access is requested but denied, and then a thread waits for messages which can never arrive. Or it may just get stuck in a loop...

There is a system property java.security.debug that can be set to print information about various types of access to the security mechanisms. This can be used with a slack security policy to find out exactly what permissions are being granted. Then, with the screws tightened, you can see where permission is being denied. An appropriate value for this property is access, as in

java -Djava.security.debug=access ...

...
access: access allowed (java.util.PropertyPermission socksProxyHost read)
access: access allowed (java.net.SocketPermission 127.0.0.1:1174 accept,resolve)
access: access denied (java.net.SocketPermission 130.102.176.249:1024 accept,resolve)
access: access denied (java.net.SocketPermission 130.102.176.249:1025 accept,resolve)
access: access denied (java.net.SocketPermission 130.102.176.249:1027 accept,resolve)
...

The safest way for a Jini client or service to be part of a Jini federation is through abstinence: that is, refuse to take part. This doesn't get you very far in populating a federation. The JDK 1.2 security model allows a number of ways in which more permissive activity may take place:

1. Grant permission only for certain activities, such as socket access at various levels on particular ports, or access to certain files for reading, writing or execution

   
   grant {
       permission java.net.SocketPermission "224.0.1.85", "connect,accept";
       permission java.net.SocketPermission "*.edu.au:80", "connect";
   }
   

2. Grant access only to particular hosts, subdomains or domains

   
   grant codebase "http://sunshade.dstc.edu.au/classes/" {
       permission java.security.AllPermission "", "";
   }
   

3. Require digital signatures attached to code

   
   grant signedBy "sysadmin" {
       permission java.security.AllPermission "", "";
   }
   

In order to partake in a Jini federation, a service must become "sufficiently" visible. A service needs to find a service locator before it can advertise its services. Once found, it registers the service and then waits for calls to come in. Where is the security risk in this? Well firstly, as a result of finding a service locator, the service gets a ServiceRegistrar. This runs in the server's JVM. If the lookup discovery service was hostile, then it could atack the server. Secondly, if the service has event listeners then clients will be loading listener objects into the JVM and these could attack too.

Lookup locator discovery can be done by unicast to particular locations or by multicast. Sufficient permissions to do this must be granted.

Unicast discovery does not need any particular permissions to be set. The discovery can be done without any policy file needed.

For the multicast case, the service must have DiscoveryPermission for each group that it is trying to join. For all groups, the wildcard "*" can be used. So to join all groups, the permission granted should be

permission net.jini.discovery.DiscoveryPermission "*";

permission net.jini.discovery.DiscoveryPermission,
           "printers, toasters";

permission java.net.SocketPermission "224.0.1.84", "connect,accept";
permission java.net.SocketPermission "224.0.1.85", "connect,accept";

The service may export a UnicastRemoteObject. This will require listening on a port for requests. The default constructor will assign a random port (above 1024) for this. If desired, this port may be specified by other constructors. This will require further socket permissions, such as

permission java.net.SocketPermission "localhost:1024-", "connect,accept";
permission java.net.SocketPermission "*.dstc.edu.au:1024-", "connect,accept";   

A number of parameters may be set by preferences, such as net.jini.discovery.ttl. It does no harm to allow the Jini system to look for these parameters, and this may be allowed by

permission java.util.PropertyPermission "net.jini.discovery.*", "read";

A fairly minimal policy file suitable for a service exporting an RMI object could then be

grant {
    permission net.jini.discovery.DiscoveryPermission "*";
    // multicast request address
    permission java.net.SocketPermission "224.0.1.85", "connect,accept";
    // multicast announcement address
    permission java.net.SocketPermission "224.0.1.84", "connect,accept";

    // RMI connections
    permission java.net.SocketPermission "*.canberra.edu.au:1024-", "connect,accept";
    permission java.net.SocketPermission "130.102.176.249:1024-", "connect,accept";
    permission java.net.SocketPermission "127.0.0.1:1024-", "connect,accept";

    // reading parameters
    // like net.jini.discovery.debug!
    permission java.util.PropertyPermission "net.jini.discovery.*", "read";
};

The client is most at risk in the Jini environment. The service exports objects (and only imports relatively trusted service registrars); the lookup locator stores objects, but does not "bring them to life" and execute any of their methods; but the client brings external objects into its address space and runs it using all of the permissions that it has as a process running in an operating system. So it will run under the permissions of a particular user, in a particular directory, with user accesses to the local file system and network. It could destroy files, make network connections to undesirable sites (or desirable, depending on your tastes!) and download images from them, start processes to send obnoxious mail to anyone in your address book, and generally make a mess of your electronic identity!

A client using multicast search to find service locators will need to grant discovery permission and multicast announcement permission, just like the service

permission net.jini.discovery.DiscoveryPermission "*";
permission java.net.SocketPermission "224.0.1.84", "connect,accept";
permission java.net.SocketPermission "224.0.1.85", "connect,accept";

RMI connections on random ports may also be needed

permission java.net.SocketPermission "*.dstc.edu.au:1024-", "connect,accept"

In addition to these, class definitions will probably need to be uploaded so that services can actually run in the client. This is the most serious risk area for the client, as the code contained in these class definitions will be run in the client, and any errors or malicious code will have their effect because of this. The client view of the different levels of trust are shown in figure 14.1.

Many services will just make use of whatever HTTP server is running on their system, and this will probably be on port 80. Permission to connect on this port can be granted by

permission java.net.SocketPermission "127.0.0.1:80", "connect,accept";
permission java.net.SocketPermission "*.dstc.edu.au:80", "connect,accept";

However, while this will allow code to be downloaded on port 80, it may not block some malicious attempts. Any user can start an HTTP server on any port (Windows) or above 1024 (Unix). A service can then set its codebase to whatever port the HTTP server is using. Perhaps these other ports should be blocked. Unfortunately, RMI uses random ports, so these ports need to be open. So it is not probably possible to close all holes for hostile code to be downloaded to a client. What you do is a second stage defence: given that hostile code may reach you, set the JDK security so that hostile (or just buggy) code cannot perform harmful actions in the client.

A fairly minimal policy file suitable for a client could then be

grant {
    permission net.jini.discovery.DiscoveryPermission "*";

    // multicast request address
    permission java.net.SocketPermission "224.0.1.85", "connect,accept";
    // multicast announcement address
    permission java.net.SocketPermission "224.0.1.84", "connect,accept";

    // RMI connections
    // DANGER
    // HTTP connections - this is where external code may come in - careful!!!
    permission java.net.SocketPermission "127.0.0.1:1024-", "connect,accept";
    permission java.net.SocketPermission "*.canberra.edu.au:1024-", "connect,accept";
    permission java.net.SocketPermission "130.102.176.249:1024-", "connect,accept";

    // DANGER
    // HTTP connections - this is where external code may come in - careful!!!
    permission java.net.SocketPermission "127.0.0.1:80", "connect,accept";
    permission java.net.SocketPermission "*.dstc.edu.au:80", "connect,accept";

    // reading parameters
    // like net.jini.discovery.debug!
    permission java.util.PropertyPermission "net.jini.discovery.*", "read";

};

A service is specified by an interface. In many cases, an RMI proxy will be delivered to the client which implements this interface. Depending on the interface, this can be used by the client to attack the service. The FileClassifier interface is safe. But in a later chapter we look at how a client can upload a new MIME type to a service, and this extended interface exposes a service to attack.

The relevant method from the MutableFileClassifier interface is

    public void addType(String suffix, MIMEType type)
        throws java.rmi.RemoteException;                                        

This particular attack can be avoided by ensuring that the parameters to any method call in an interface are all final classes. If the class MIMEType was defined by

public final class MIMEType {...}

Services will transfer objects to be run within clients. This chapter has been concerned about the security policies that will allow this, and the restrictions that may need to be in place. The major protection to clients at the moment is that there are no standardised service interfaces, so attackers do not yet know what hostile objects to write.

A lookup service exports an object that implements ServiceRegistrar. It does not use the same mechanism as a service would to get its code into a client. Instead, the lookup service replies directly to unicast connections with a registrar object, or responds to multicast requests by establishing a unicast connection to the requester and again sending a registrar. The mechanism is different, but it is clearly documented in the Jini specifications and it is quite easy to write an application that performs at least this much of the discovery protocols.

The end result of lookup discovery is that the lookup service will have downloaded registrar objects. The registrar objects run in both clients and services - they both need to find lookup services. The ServiceRegistrar interface is standardised by the Jini specification, so it is fairly easy to write a hostile lookup service that can attack both clients and services.

While it is unlikely that anyone will knowingly make a unicast connection to a hostile lookup service, someone might get tricked into it. There are already some quite unscrupulous Web sites that will offer "free" services on production of a credit card (to the user's later cost). There is every probability that they will try to entice Jini clients if they see a profit in doing so. Also, anyone with access to the network within broadcast range of clients and services (i.e. on your local network) can start lookup services which will be found by multicast discovery.

The only real counter to this attack is to require that all connections that can result in downloaded code should be covered by digital certificates, so that all downloaded code must be signed. This covers all possible ports, since an HTTP server can be started on any port on a Windows machine. The objects that are downloaded in the Sun implementation of the lookup service, reggie, are all in reggie-dl.jar. This is not signed by any certificates. If you are worried about an attack through this route, you should sign this file, as well as the jar files of any services you wish to use.

Jini applications download and execute code from other sources. These include

1. Both clients and services download ServiceRegistrar objects from lookup services. They then call methods such as lookup() and register().
2. A client will download services and execute whatever methods are defined in the interface.
3. A remote listener will call the notify() method of foreign code

In a "safe" environment where all code can be trusted, no safeguards need to be employed. However, most environments carry some kind of risk from hostile agents. An attack will consist of a hostile agent implementing one of the known interfaces (of ServiceRegistrar, of a well-known service such as the transaction manager, or of RemoteEventListener) with code that does not implement the implied "contract" of the interface but instead tries to perform malicious acts. These acts may not even be deliberately hostile: most programmers make at least some errors, and these errors may result in risky behaviour.

There are all sorts of malicious acts that can be performed. Hostile code can simply terminate the application, but it can read sensitive files, alter sensitive files, forge messages to other applications, perform denial of service attacks such as filling the screen with useless windows, and so on.

It doesn't take much reading about security issues to instill a strong sense of paranoia, and possible over-reaction to security threats. If you can trust everyone on your local network (which you are already doing if you run a number of common network services such as NFS), then the techniques discussed in this section are possibly overkill. If you can't, then paranoia may be a good frame of mind to be in!

The Java 1.2 security model is based on the traditional idea of "protection domains". In Java, a protection domain is associated to classes based on their CodeSource, which consists of the URL from which the class file was loaded, plus a set of digital certificates used to sign the class files. For example, the class files for the LookupLocator class are in the file jini-core.jar (in the lib directory of the Jini distribution). This class has a protection domain associated with the CodeSource for jin-core.jar. (So do all the other classes defined in this file.)

Information about protection domains and code sources can be found by code such as

    java.security.ProtectionDomain domain = registrar.
                                            getClass().getProtectionDomain();
    java.security.CodeSource codeSource = domain.getCodeSource();

    Object [] signers = registrar.getClass().getSigners();
    if (signers == null) {
        System.out.println("No signers");
    } else {
        System.out.println("Signers");
        for (int m = 0; m < signers.length; m++)
            System.out.println(signers[m].toString());
    }                                                                    

By default, no class files or jar files have digital signatures attached. Digital signatures can be created using keytool (part of the standard Java distribution). These signatures are stored in a keystore. From there, they can be used to sign classes and jar files using jarsigner, exported to other keystores and generally spread around. Certificates don't mean anything unless you believe that they really do guarantee that they refer to the "real" person, and certificate authorities such as Verisign provide validation techniques for this.

The above description is horribly brief, and is mainly intended as reminders to those who already understand this stuff. For this section, I just created certificates as needed using keytool, although there was no independant authority to verify them. A good explanation of this area is given by Bill Venners at http://www.artima.com/insidejvm/ed2/ch03Security1.html

None of the Java file in the standard distribution are signed. None of the files in the Jini distribution are signed either. For most of these it probably won't matter, since they are local files.

However, all of the Jini jar files ending in -dl.jar are downloaded to clients and services across the network, and are Sun implementations of "well-known" interfaces. For example, the ServiceRegistrar object that you get from discovery has its class files defined in reggie-dl.jar, as a com.sun.jini.reggie.RegistrarImpl_Stub object. Hostile code implementing the ServiceRegistrar interface can be written quite easily. If there is the possibility that hostile versions of lookup services (or other Sun-supplied services) may be set running on your network, then you should only accept implementations of ServiceRegistrar if they are signed by an authority you trust.

Interfaces to services such as printers will eventually be decided, and will become "well known". There should be no need to sign these interface files for security reasons, but an authority may wish to sign them for, say, copyright reasons. Any implementations of these interfaces are a different matter. Just like the cases above, these implementation class files will come to client machines from other machines on the local or even remote networks. These are the files that can have malicious implementations. If this is a possibility, you should only accept implementations of the interfaces if they are signed by an authority you trust.

Permissions are granted to protection domains based on their codesource. In the Sun implementation, this is done in the policy files, by grant blocks

grant codeBase "url" signedBy "signer" {
    ...
}

When code executes, it belongs to the protection domains of all classes on the call stack above it. So for example, when the ServiceRegistration object in the complete.FileClassifierServer is executing the register() method, the following classes are on the call stack:

1. The com.sun.jini.reggie.RegistrarImpl_Stub class from reggie-dl.jar
2. The complete.FileClassifierServer class, from the call discovered()
3. Classes from core Jini classes, that have called the discovered() method
4. Classes from the Java system core, that are running the application

The permissions for a executing code are generally the intersection of all the permissions of the protection domains it is running in. Classes in the Java system core grant all permissions, but if you restrict the permissions granted to your own application code, to core Jini classes, or to code that comes across the network, you restrict what an executing method can do. For example, if multicast request permission is not granted to the Jini core classes then discovery cannot take place. This permission needs to be granted to the application code and also to the Jini core classes.

It may not be immediately apparent what protection domains are active at any point. For example, in the call above of

    registrar.getClass().getProtectionDomain()

There are two exceptions to the intersection rule: the first is that the RMI security manager grants SocketPermission to connect back to their codebase host. The second is that methods may call the AccessController.doPrivileged() method. This essentially prunes the class call stack, discarding all classes below this one for the duration of the call. This is to allow permissions based on this class's methods, even though the permissions may not be granted by classes earlier in the call chain. This allows some methods to continue to work even though the application has not granted the permission, and means that the application does not have to generally grant permissions required only by a small subset of code. For example, the Socket class needs access to file permissions in order to allow methods such as getOutputStream() to function. By using doPrivileged(), the class can limit the "security breakout" to particular methods in a controlled manner. If you are running with security access debugging turned on, this explains how a large number of accesses are granted even though the application has not given many of the permissions.

Adding all these bits of information together leads to security policy files that restrict possible attacks

1. Grant permissions to application code based on the codesource. If you suspect these might get tampered with, sign them as well.
2. Grant permission to Jini core classes based on the codesource. These may be signed if need be.
3. Grant permission to downloaded code only if it is signed by an authority you trust. Even then, grant only the minimum permission that is needed to perform the service's task.
4. Don't grant any other permissions to other code.

A policy file based on this might be

keystore "file:/home/jan/.keystore", "JKS";

// Permissions for downloaded classes
grant signedBy "Jan" {
    permission java.net.SocketPermission "137.92.11.117:1024-", "connect,accept,resolve";
};

// Permissions for the Jini classes
grant codeBase "file:/home/jan/tmpdir/jini1_1/lib/-" signedBy "Jini" {
    // The Jini classes shouldn't require more than these

    permission java.util.PropertyPermission "net.jini.discovery.*", "read";     
    permission net.jini.discovery.DiscoveryPermission "*"; 
    // multicast request address
    permission java.net.SocketPermission "224.0.1.85", "connect,accept";
    // multicast announcement address
    permission java.net.SocketPermission "224.0.1.84", "connect,accept";        

    // RMI and HTTP
    permission java.net.SocketPermission "127.0.0.1:1024-", "connect,accept";
    permission java.net.SocketPermission "*.canberra.edu.au:1024-", "connect,accept";
    permission java.net.SocketPermission "137.92.11.*:1024-", "connect,accept,resolve";
    permission java.net.SocketPermission "130.102.176.*:1024-", "connect,accept,resolve";
    permission java.net.SocketPermission "130.102.176.249:1024-", "connect,accept,resolve";
    // permission java.net.SocketPermission "137.92.11.117:1024-", "connect,accept,resolve";

    // debugging
    permission java.lang.RuntimePermission "getProtectionDomain"; 
};

// Permissions for the application classes
grant codeBase "file:/home/jan/projects/jini/doc/-" {
    permission java.util.PropertyPermission "net.jini.discovery.*", "read";     
    permission net.jini.discovery.DiscoveryPermission "*"; 
    // multicast request address
    permission java.net.SocketPermission "224.0.1.85", "connect,accept";
    // multicast announcement address
    permission java.net.SocketPermission "224.0.1.84", "connect,accept";        

    // RMI and HTTP
    permission java.net.SocketPermission "127.0.0.1:1024-", "connect,accept";
    permission java.net.SocketPermission "*.canberra.edu.au:1024-", "connect,accept";
    permission java.net.SocketPermission "137.92.11.*:1024-", "connect,accept,resolve";
    permission java.net.SocketPermission "130.102.176.*:1024-", "connect,accept,resolve";
    permission java.net.SocketPermission "130.102.176.249:1024-", "connect,accept,resolve";
    // permission java.net.SocketPermission "137.92.11.117:1024-", "connect,accept,resolve";

    // debugging
    permission java.lang.RuntimePermission "getProtectionDomain"; 

    // Add in any file, etc, permissions needed by the application classes
};

If you found this chapter of value, the full book "Foundations of Jini 2 Programming" is available from APress or Amazon .

This file is Copyright (©) 1999, 2000, 2001, 2003, 2004, 2005 by Jan Newmarch (http://jan.newmarch.name) jan@newmarch.name.

This work is licensed under a Creative Commons License, the replacement for the earlier Open Content License.