CVE-2021-3838: A deep dive into a PHAR Deserialization vulnerability in DomPDF before version 2..

In this post, we are going to explore a cybersecurity vulnerability identified as CVE-2021-3838. The vulnerability exists in DomPDF, which is a popular library widely used for generating PDFs from HTML content. This security issue affects DomPDF versions before 2.. and can be exploited by a malicious actor who can upload files to a web server. The exploitation of this vulnerability can lead to remote code execution. This is especially concerning when DomPDF is used in conjunction with frameworks with known POP (Property-Oriented Programming) chains like Laravel or when vulnerable developer code exists in the application.

How the vulnerability works

The core of CVE-2021-3838 lies in DomPDF's lack of proper checking on the protocol before passing it into the file_get_contents() function. Generally, the webhook (an HTTP callback) functions in a way that it accepts URLs as input, fetches the content from the URL, and then generates a PDF from that content. However, due to insufficient validation, an attacker can abuse this function by uploading a file with a PHAR deserialization payload and passing a phar:// protocol URL as input to the webhook. As a result, the malicious PHP objects contained within the uploaded PHAR file are unserialized and instantiated, potentially leading to remote code execution.

Here's a code snippet that demonstrates the vulnerable code in DomPDF

function generatePdfFromUrl($url) {
    // No protocol validation here, 'phar://' can be passed
    $html = file_get_contents($url);
    
    // ...
    
    $dompdf = new Dompdf();
    $dompdf->loadHtml($html);
    $dompdf->render();
    $pdf = $dompdf->output();
    
    // ...
}

To exploit this vulnerability, an attacker can follow these steps

1. Create a malicious PHAR file containing a deserialization payload targeting the application or its underlying framework, like Laravel.
2. Upload the PHAR file to a publicly accessible location, such as an image upload or file-sharing platform.
3. Call the vulnerable webhook with the phar:// protocol URL pointing to the uploaded file.

An example payload for Laravel-based applications could look like this

// Laravel payload 
class ExampleLaravelExploit {
    public function __construct() {
        /* attack-specific code here */
    }
}

When the payload is instantiated, the attacker's code within the constructor will be executed.

Mitigation

To address this vulnerability, developers should update DomPDF to version 2.. or later if they are using an older version. Additionally, developers should ensure that their PHP applications validate URLs before passing them to functions like file_get_contents(). This can be achieved by using proper URL validation libraries or by implementing checks on the accepted protocols, ensuring that malicious protocols like phar:// are not allowed.

Original references

- DomPDF GitHub Repository: https://github.com/dompdf/dompdf
- CVE-2021-3838 Vulnerability Details: https://nvd.nist.gov/vuln/detail/CVE-2021-3838

Conclusion

CVE-2021-3838 highlights the importance of validating inputs in web applications to prevent the exploitation of vulnerabilities such as PHAR deserialization. By understanding how this vulnerability works and how it can be exploited, developers can take the necessary steps to protect their applications and their users from security threats.

Timeline

Published on: 11/15/2024 11:15:05 UTC
Last modified on: 11/15/2024 13:58:08 UTC