Step-by-Step Guide to Building a Vulnerable App in 2026

What You'll Learn

• How to create a simple vulnerable application using React Native and FastAPI.
• Insights from testing with various large language models (LLMs) to exploit the app.
• Understanding common vulnerabilities in app development, especially with Firebase.
• How to analyze and interpret results from LLMs in security contexts.
• Practical tips for future-proofing your applications against similar exploits.

Prerequisites

Before you begin building a vulnerable app, it is essential to have a basic understanding of programming and web development. Familiarity with React Native and Python will be particularly beneficial, as the application will be built using these technologies. You should also have access to a development environment, ideally with tools like Expo for React Native and FastAPI for the backend. Additionally, having some knowledge of Firebase will help you understand the data layer and its potential vulnerabilities. Lastly, a willingness to experiment and learn from failures is crucial when engaging in security testing.

Step 1: Setting Up Your Development Environment

The first step in building your vulnerable application is to set up the development environment. Start by installing Node.js and Expo CLI, which allows you to create React Native applications efficiently. You can download Node.js from the official website, and once installed, you can install Expo CLI using npm with the command npm install -g expo-cli. This setup provides a robust platform for developing mobile apps quickly.

Next, install Python and FastAPI. Python can be downloaded from its official site, and FastAPI can be installed using pip with pip install fastapi. FastAPI is a modern, fast (high-performance) web framework for building APIs with Python 3.6+ based on standard Python type hints. This setup will enable you to create a backend service that interacts with your app.

For database management, you will also need Firebase. Create a Firebase project in the Firebase Console, and set up Firestore, which will be used to store user data. Make sure to download the google-services.json file, as this will contain the necessary configuration for your app to connect to Firebase. This initial setup is crucial for the next steps in your app development.

Step 2: Building the App

Now that your environment is set up, it's time to build the actual app. Begin by creating a new React Native project using Expo with the command expo init VulnerableApp. Choose a template that suits your needs, such as the blank template. This will create a foundational structure for your app.

In the project folder, create components for user authentication and review submissions. The app should allow users to sign up and submit book reviews. Use Firebase Authentication for user login and Firestore to store reviews. The goal is to ensure that the app appears functional but has underlying vulnerabilities that can be exploited.

One common vulnerability in such apps is improper access controls. For example, if the API is secure but the Firestore database allows anyone to read the data without proper authorization, this could lead to severe data breaches. Make sure the google-services.json file is included in your app structure correctly, as it will be crucial for Firebase interactions.

Step 3: Testing with LLMs

With your vulnerable app up and running, the next step is to test it using various large language models (LLMs). In this phase, you will spend approximately $1,500 (around ₹124,500) on API calls to different models to see if they can successfully exploit the vulnerabilities you've built into the app.

Set up a testing environment where you can input various prompts to the LLMs, directing them to find the flag in the user’s private reviews stored in Firestore. Each model will need to be tested for a predefined number of attempts to ensure consistent results. For example, in a recent experiment, a model named GPT-5.5 was able to solve the challenge in 7 out of 10 runs, focusing primarily on Firebase to uncover the vulnerabilities.

As you conduct these tests, make sure to log the outcomes, costs, and the strategies each model used to attempt to find exploits. This data will be invaluable for analyzing the effectiveness of the models in identifying vulnerabilities.

Model Performance Comparison

Common Mistakes and How to Avoid Them

• Ignoring Security Best Practices: When building your app, always apply security best practices even if the goal is to create a vulnerable app. This will help you understand what pitfalls to avoid in real-world applications.
• Rushing the Testing Phase: Take your time during the testing phase with LLMs. Ensure you have enough data points to analyze trends effectively.
• Neglecting Documentation: Document all your findings and the process thoroughly. This will help you in future projects and in understanding each step you've taken.
• Overlooking Firebase Security Rules: Make sure to review Firebase security rules. Even a vulnerable app should have some level of control over data access.

India-Specific Tips

When developing a vulnerable application in India, consider leveraging local development tools and communities. For instance, utilize Indian cloud service providers for hosting your backend services. These providers often offer competitive pricing and localized support, which can be beneficial for your project. Additionally, the costs associated with developing and testing your app can vary significantly. Spending ₹1,500 (approximately $18,500 INR) on testing can be manageable within a local context compared to other regions.

Moreover, explore local tech communities and forums such as Stack Overflow India or local Meetups, where you can share insights and gather feedback from fellow developers. Engaging with these communities can enhance your learning experience and provide invaluable support during your development journey.