Vulnerable Client-Server Application (VuCSA)

Vulnerable client-server application (VuCSA) is made for learning/presenting how to perform penetration tests of non-http client-server applications. It is written in Java (with JavaFX graphical user interface).

Currently, the vulnerable application contains the following challenges:

1. Buffer Over-read (simulated)
2. Command Execution
3. SQL Injection
4. Enumeration
5. XML
6. Horizontal Access Control
7. Vertical Access Control
8. RCE Deserialization

If you want to know how to solve these challenges, take a look at the PETEP website, which describes how to use the open-source tool PETEP to exploit them.

Tip 1: Before you start hacking, do not forget to check the data structure of messages bellow. When modifying the network traffic, you will probably have to consider the structure, especially payload length bytes.

Tip 2: Most of the challenges can be exploited through modification of network traffic. Therefore, it is recommended to use TCP proxy or process hooks for the testing.

Tip 3: Some challenges have input validation and restrictions in place, which is common in thick clients, but it does not mean that the server uses the same validation.

Buffer Over-read (simulated) Vulnerability

In this challenge, your goal is to manipulate the network traffic between the client and the server in a way that leads to buffer over-read.

Command Execution Vulnerability

Command execution challenge represents a very simple command execution vulnerability. The goal is to execute malicious command on the server.

SQL Injection Vulnerability

SQL Injection challenge contains a search input vulnerable to SQL injection, but as you will soon notice, the input does not allow you to input the characters you need.

Enumeration Vulnerability

Enumeration challenge is based on simulated login form that is not protected from enumeration. Will you be able to find all 5 users and guess their passwords?

XML Vulnerabilities

In this challenge, you can find multiple XML vulnerabilities:

• XML External Entity Attack (XXE Injection)
• XInclude Attack
• XML Bomb Attack

Horizontal Access Control Vulnerability

Horizontal Access Control challenge represents document reader that allows the user to see own documents and read their content. The goal is to find 5 documents of other users.

Vertical Access Control Vulnerability

Vertical Access Control challenge is based on simulated user panel, which shows basic user information. The goal is to find a hidden admin functionality and check if it is possible to use it as a Guest user.

RCE Deserialization Vulnerability

RCE Deserialization vulnerability uses Java deserialization/serialization for transmitting data through the network. The application contains two paths that you can use to achieve remote code execution through the vulnerable Java deserialization.

You can find both paths by examining the server's JAR file or by looking into the source code.

The goal is to create exploits for both paths and execute malicious command on the server.

Tip: You can use the server JAR as library to make the exploit creation easier.

Requirements

You need Java 11 or newer version to run VuCSA.

Note: For Mac with ARM64 architecture (M1, M2 chips), use special build for Java 17.

How to Run

In order to run the vulnerable server and client, you can use one of releases on GitHub or run gradle assemble, which creates distribution packages (for both Windows and Unix). These packages contain sh/bat scripts that will run the server and client using JVM:

# Linux / Mac
chmod +x client.sh server.sh
./client.sh
./server.sh

# Windows
client.bat
server.bat

Note: These run scripts contain useful variables, including path to Java executable. You might need to change it if you do not have it in PATH or you use multiple Java versions.

Run Configuration

Server configuration is automatically created if it does not exist and after that it is loaded from server.json in the same directory where the server is running:

{
  "network": {
    "serverHost": "0.0.0.0",
    "serverPort": 8765
  }
}

Client configuration can be specified in the running application.

Project Structure

Project is divided into three modules:

• vucsa-common - common functionality for both client and server (including protocol processing utilities)
• vucsa-client - vulnerable client with JavaFX GUI
• vucsa-server - vulnerable server for terminal use

Data Structure

Messages transmitted between server and client have the following simple format:

[type][target][length][payload]
  32b    32b     32b     ???

These four parts have the following meaning:

• type - type of the message (used for serialization/deserialization)
• target - target handler that will receive the message (identifier)
• length - length of the payload
• payload - data serialized into bytes

In order to send custom payloads, you might have to update the payload length. Otherwise, it will not work properly. In the tutorial, automatic script is developed to auto-fix the payload length bytes.

Tutorial (Solutions)

Vulnerable client-server application (VuCSA) contains multiple vulnerabilities, which can be exploited in various ways. Official guide for exploiting these vulnerabilities uses open-source PEnetration TEsting Proxy (see PETEP Methodology).

In the PETEP methodology, the whole process of exploiting the challenges is explained, including useful payloads.