1099 - JavaRMI

Overview

Java Remote Method Invocation (Java RMI) is a set of Java APIs that allows Java objects running on separate Java Virtual Machines (JVM) to communicate. In some regards, it can be considered the Java object-oriented equivalent of Remote Procedure Calls (RPC), with the support of transfer of serialized Java classes and a distributed garbage collector.

As stated in the official Java documentation regarding Java RMI applications: "A typical server program creates some remote objects, makes references to these objects accessible, and waits for clients to invoke methods on these objects. A typical client program obtains a remote reference to one or more remote objects on a server and then invokes methods on them".

The original implementation of Java RMI relied on the Java Remote Method Protocol (JRMP) protocol (over TCP / IP). The JRMP protocol is specific to Java and can only be used to make calls from a JVM to another. An implementation based on the Common Object Request Broker Architecture (CORBA) standard was later implemented to support communications between non-JVM components. This implementation rely on the RMI over Internet Inter-Orb Protocol (RMI-IIOP) protocol. While older, the RMI-JRMP implementation is still actively maintained, more integrated into Java and easier to use than the more complicated RMI-IIOP implementation.

The official TCP port linked to the JRMP protocol (more precisely to the RMI registry component used by the protocol) is 1099 while the TCP port usually linked to the RMI-IIOP protocol is 1050.

While Java RMI shouldn't be used in modern applications (and replaced by REST or SOAP web services for inter-process communications), it can still be encountered in legacy or enterprise internal applications.

Stub and Skeleton classes

In Java RMI, the client side object, usually simply referred to as the client, communicate through Stub classes to server side objects on the Java RMI server. The Stub classes act as client-side gateway for all requests to remote objects. The remote object's stub instance is what the client will use to make remote method calls to the remote object.

Before Java Standard Edition (Java SE) 5, stub classes had to be pre-generated from the compiled code (.class) of the server-side class that would be called. This step is no longer required, as the stub classes can now be directly retrieved from the Java RMI server.

The Skeleton class used to be the server-side equivalent of the Stub classes to process all incoming clients requests. Skeletons are deprecated since J2SE 1.2 (1998).

RMI registry and invocation process

The Java RMI registry is a naming service that hold information about the remote objects registered by Java RMI servers.

The Java RMI servers call the Java RMI registry to register (remote) object(s) and associate a name with each registered object, an operation known as binding. When Java RMI clients request a reference to a named remote object, a lookup to the RMI registry is first performed by the clients to retrieve the remote object associated to the given name. The Java RMI registry returns a reference, which correspond to the remote object's stub instance, to the client.

The reference / remote object's stub instance is then used to call the methods of the remote object. More precisely, each stub contains an instance of the RemoteRef interface, used to carry out remote RMI calls on the remote object for which it is a reference. As stated in the official Java documentation: "the [methods of the RemoteRef interface] delegate method invocation to the stub's (object) remote reference and allows the reference to take care of setting up the connection to the remote host, marshaling some representation for the method and parameters then communicating the method invocation to the remote host".

The RMI calls are conducted using different methods depending of the Java version in use:

  • Since Java 2 SDK, Standard Edition, v1.2, using the (new) invoke(Remote obj, Method method, Object[] params, long opnum) method. The opnum parameter is a 64-bit (long) integer that represent a hash of the method signature.

  • using the (now deprecated) newCall(RemoteObject obj, Operation[] op, int opnum, long hash), invoke(RemoteCall call), and done(RemoteCall call) methods. The hash parameter is an equivalent to the opnum parameter of the new invoke method.

The method signatures correspond to a value calculated from the method prototypes (method names, return and parameters' types, and number of parameters).

In both cases, the signatures of the methods must be known to call the methods as they are not disclosed by the RMI registry.

Remote class loading

As stated, Java RMI supports the transfer of serialized objects over the network. In order to deserialize any serialized object received (that is transform back the serialized objects to object instances), the JVM must have access to the bytecode of the class of the object being deserialized.

Under certain circumstances, remote classes can be loaded by the JVM upon reception of a serialized object (as an argument or return value) to a Java RMI call, thus resulting in code execution from the sending JVM. As some methods of Java RMI servers can be called by default by unauthenticated users, remote class loading would allow unauthenticated remote code execution on the server hosting the Java RMI service, under the security context and privileges of the JVM.

The following conditions must be met for remote class loading to be enabled:

  • The class to load should not exist locally, that is should not be present in the CLASSPATH of the local JVM.

  • The SecurityManager should be enabled on the receiving JVM.

  • The receiving JVM's java.rmi.server.useCodebaseOnly property should be be set to false. Since JDK 7u21 (released in 2013), the java.rmi.server.useCodebaseOnly property is set to true by default (and was set to false in prior releases).

If the conditions for remote class loading are met, the loader will use, when marshalling objects, the codebase URL specified in the annotation of the object's class to download the definition of the class.

Network scan

nmap can be used to scan the network for Java RMI services:

# The rmi-dumpregistry and rmi-vuln-classloader NSE scripts are introduced below.
# Only rmi-dumpregistry is included in the default scripts.

nmap -v -p <1050,1098,1099 | PORT(S)> -sV [--script "rmi-dumpregistry or rmi-vuln-classloader"] -oA nmap_javarmi <IP | RANGE | CIDR>

Remote class loading

Detection

The nmap NSE script rmi-vuln-classloader and the Metasploit module auxiliary/scanner/misc/java_rmi_server can be used to check if Java RMI servers allow remote class loading.

Note however that the aforementioned tooling are, as of March 2021, prone to false-positives likely due to the original exploit code dating back to before the JDK 7u21 default configuration hardening.

nmap -v -p <PORT> -sV --script rmi-vuln-classloader <IP | RANGE | CIDR>

msf > use auxiliary/scanner/misc/java_rmi_server

Exploitation

The Metasploit module exploit/multi/misc/java_rmi_server can be used to exploit Java RMI server allowing remote class loading to execute system commands.

In order to be successful, the exploitation requires:

  • that the attacking machine can be reached by the Java RMI server (to retrieve the class on a webserver hosted by Metasploit)

  • the Runtime.getRuntime().exec() method can be called

Note that Runtime.getRuntime().exec() does make use of a shell (such as /bin/sh) to deport arguments parsing. Instead it splits the command line in an array of words, with the first word being executed and the others words used as arguments. In result, shell metacharacters (| ; & > < etc.) are not supported by Runtime.getRuntime().exec().

msf > use exploit/multi/misc/java_rmi_server

Enumeration of Java RMI registry bound objects

The remote objects bound in a Java RMI registry may be enumerated using the list() method of the (deprecated) java/rmi/registry/RegistryImpl_Stub class (as implemented by Metasploit and nmap) or java.rmi.registry.LocateRegistry class (as implemented by rmiscout and BaRMIe).

Note that restrictions may be implemented by the Java RMI server to limit the listing of the bound objects (for example to limit listing to calls originating from the local host).

The nmap NSE script rmi-dumpregistry, the Metasploit module auxiliary/gather/java_rmi_registry, rmiscout, and BaRMIe can be used to attempt to list the objects bound in a Java RMI registry.

The aforementioned tools may return different level of information, with nmap and BaRMIe attempting to retrieve more data about the remote objects.

nmap -v -p <PORT> -sV --script rmi-dumpregistry <IP | RANGE | CIDR>

msf > use auxiliary/gather/java_rmi_registry

java -jar rmiscout.jar <IP> <PORT>

java -jar BaRMIe_v1.01.jar -enum <IP> <PORT>

The following Java code snippet can be used as a template to implement a very simple enumerator of Java RMI registry bound objects. Usage of the tools above is recommended for a better implementation of error handling.

import java.rmi.registry.LocateRegistry;
import java.rmi.registry.Registry;
import java.rmi.RemoteException;
import static java.lang.System.out;

public class SimpleRMIClient {
    public static void main(String[] args) {
        Registry registry;
        try {
            registry = LocateRegistry.getRegistry("<HOST | IP>", 1000);
        } catch(RemoteException re) {
            throw new RuntimeException("Could not connect to remote Java RMI server.");
        }

        System.out.println("Retrieved the registry...");

        try {
            String[] objNames = registry.list();

            for (String objName: objNames) {
                System.out.println(objName);
            }
        } catch(Exception re) {
            throw new RuntimeException("An error occurred will attempting to list the bound objects.");
        }
    }
}

Enumeration of available methods

Publicly documented classes

If publicly documented classes are enumerated, the documentation could contain information about dangerous methods that, for example, could lead to filesystem access or system command execution.

BaRMIe_v1.01.jar's enum checks for the presence of some AxiomSL's methods allowing access to the underlying filesystem.

Method's prototypes or signatures bruteforce

rmiscout can be used to bruteforce methods, either by using a wordlist of method prototypes or a permutation of possible method names, return and parameters' types, and number of parameters. The computing method signatures are automatically calculated from the given method prototype.

These bruteforce techniques, while not covering all possible method signatures (2^64 possibilities), will still give a good probability of findings methods (according to statistical analysis of 15,000+ method signatures by rmiscout's author (@theBumbleSec).

In order to check if a method signature is valid with out actually invoking the method, rmiscout will deliberately mismatch parameters types to trigger RemoteExceptions. The original supplied parameter types will be used but the parameters will have for value a serialized instance of a non existing class (random name). This technique cannot be used for methods which do not take parameters.

In RMI-JRMP, it is thus possible to safely bruteforce methods that take parameters without invoking the methods. On the contrary, bruteforcing methods that do not take input parameters requires to actually invoke the methods, which may induce undesirable effects.

Enumeration of method signatures for RMI-IIOP services while following the same overall principle works a bit differently. More information on the bruteforcing process and differences can be found in the following blog post: https://labs.bishopfox.com/tech-blog/lessons-learned-on-brute-forcing-rmi-iiop-with-rmiscout.

# Method names and prototypes can be found in the rmiscout GitHub repository: https://github.com/BishopFox/rmiscout/tree/master/lists
# --allow-unsafe: bruteforce methods that do not take parameters by invoking them.

# Bruteforce using the specified method prototypes wordlist.
java -jar rmiscout.jar wordlist [--allow-unsafe] [-n <REGISTRY_NAME>] -i <METHOD_PROTOTYPES_WORDLIST> <IP> <PORT>

# Bruteforce using the permutations of the given parameters.
# RETURN_TYPES / PARAMETER_TYPES example: String,void,int,long,boolean
# PARAMETER_LENGTH example: 1,5
java -jar rmiscout.jar bruteforce [--allow-unsafe] [-n <REGISTRY_NAME>] -i <METHOD_NAMES_WORDLIST> -r <RETURN_TYPES> -p <PARAMETER_TYPES> -l <PARAMETER_LENGTH_MIN,PARAMETER_LENGTH_MAX> <IP> <PORT>

Java RMI method invocation

With knowledge of their prototypes, methods of remote objects can be invoked. If a SecurityManager / (deprecated) RMISecurityManager is implemented, the client must have the necessary permissions, as dicted by the security policy to conduct the call. Otherwise a SecurityException is thrown by the service.

rmiscout can be used as a Java RMI client to invoke arbitrary methods:

# METHOD_PROTOTYPE example: int add(int a, int b)
# Parameters example: -p 2 -p 2
# Parameters array example: -p "a,b,c"

java -jar rmiscout.jar invoke [-n <REGISTRY_NAME>] -s '<METHOD_PROTOTYPE>' [-p <PARAM1_VALUE> [-p <PARAM2_VALUE> ...]] <IP> <PORT>

The following Java code snippet can be used as a template to implement a very simple Java RMI client to invoke methods of remote objects. While the code below illustrate a remote object method invocation, the use of rmiscout should generally be preferred.

import java.rmi.registry.LocateRegistry;
import java.rmi.registry.Registry;
import java.rmi.RemoteException;
import static java.lang.System.out;

public class SimpleRMIClient {
    public static void main(String[] args) {
        Registry registry;
        try {
            registry = LocateRegistry.getRegistry("<HOST | IP>", 1000);
        } catch(RemoteException re) {
            throw new RuntimeException("Could not connect to remote Java RMI server.");
        }

        System.out.println("Retrieved the registry...");

        try {
            <CLASS> objInstance = (<CLASS>) registry.lookup("<REMOTE_OBJECT_NAME>");
            System.out.println(objInstance.<METHOD>(<METHOD_PARAMETERS));
        } catch(Exception re) {
            throw new RuntimeException("An error occurred will attempting to invoke the method.");
        }
    }
}

Probe

Java deserialization

https://mogwailabs.de/en/blog/2019/03/attacking-java-rmi-services-after-jep-290/

References

https://docs.oracle.com/javase/tutorial/rmi/overview.html https://docs.oracle.com/javase/8/docs/technotes/guides/rmi/codebase.html https://docs.oracle.com/javase/7/docs/technotes/guides/rmi/enhancements-7.html https://docs.oracle.com/javase/7/docs/api/java/rmi/server/RMIClassLoader.html https://en.wikipedia.org/wiki/Java_remote_method_invocation https://apiacoa.org/publications/teaching/distributed/rmi.pdf http://www2.ift.ulaval.ca/IFT-Stage/ateliers/old/RMI/atelierRMI.pdf https://www.jmdoudoux.fr/java/dej/chap-rmi.htm https://www.clear.rice.edu/comp310/course/rmi/stub_passing.html https://book.hacktricks.xyz/pentesting/1099-pentesting-java-rmi https://itnext.io/java-rmi-for-pentesters-part-two-reconnaissance-attack-against-non-jmx-registries-187a6561314d https://null-byte.wonderhowto.com/how-to/exploit-java-remote-method-invocation-get-root-0187685/ https://docs.oracle.com/javase/7/docs/api/java/rmi/registry/Registry.html https://docs.oracle.com/javase/7/docs/api/java/rmi/registry/LocateRegistry.html http://www.docjar.com/docs/api/sun/rmi/registry/RegistryImpl.html https://github.com/BishopFox/rmiscout https://github.com/NickstaDB/BaRMIe https://labs.bishopfox.com/tech-blog/rmiscout https://labs.bishopfox.com/tech-blog/lessons-learned-on-brute-forcing-rmi-iiop-with-rmiscout https://ctftime.org/writeup/6953 https://github.com/allesctf/writeups/tree/master/2018/RealWorldCTF2018_Finals/RMI

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