Challenge Description

In this challenge, we need to exploit a web application called Spookifier. The application is a Single Page Application (SPA), featuring a form in the center of the page where users can submit their new Halloween name. After submitting the form with the input Dracula, the application displays the result with different fonts, as shown below:

Code Review

Upon inspecting the source code of the application, we discovered that the backend is built using Flask in Python. When a user submits the form, a GET request is sent to the root endpoint (/) with the submitted Halloween name passed as a query parameter (text).

The server processes this input by calling a function named spookify, which then passes the input to another function responsible for generating the formatted result. The function generate_render generates the final output, and its implementation is as follows:

def generate_render(converted_fonts):
    result = '''
        <tr>
            <td>{0}</td>
        </tr>
        
        <tr>
            <td>{1}</td>
        </tr>
        
        <tr>
            <td>{2}</td>
        </tr>
        
        <tr>
            <td>{3}</td>
        </tr>
    '''.format(*converted_fonts)
    
    return Template(result).render()

The generate_render function uses the Template class from the Jinja2 templating engine to render the final output. This indicates a potential vulnerability, as improper input sanitization can lead to a Server-Side Template Injection (SSTI) attack.

Proof of Concept (PoC)

To verify the SSTI vulnerability, we can inject a basic payload like ${7*7} into the text parameter. If the application is vulnerable, it will evaluate the expression and return the result of the calculation.

After injecting the payload, the server responded with the following output:

The result 49 confirms that the application is vulnerable to SSTI.

Exploitation

Since we have access to the source code, we know that the flag is stored in a file named flag.txt. To exploit the SSTI vulnerability and read the flag, we first need to determine the current working directory by executing the pwd command. We can use the following payload:

${__import__('os').popen('pwd').read()}

The output revealed that the current directory is /app, meaning we need to go up one level to access the flag.txt file. The following payload reads the content of the file:

${__import__('os').popen('cat ../flag.txt').read()}

The result shows the flag:

HTB{f4k3_fl4g_f0r_t3st1ng}

We successfully retrieved the flag! You can now try this on the actual Hack The Box (HTB) server to obtain the real flag.

References

• HackTricks - Jinja2 SSTI