CVE-2023-36557: Breaking Down the PrintHTML API Remote Code Execution Vulnerability and Its Exploitation

CVE-2023-36557 came to light in recent cybersecurity news revealing a dangerous vulnerability in the PrintHTML API. In this long read, we'll take an in-depth look at the exploit, walk through code snippets, and provide links to the original references. This information is crucial for understanding the vulnerability and mitigating its risks.

Overview of CVE-2023-36557

The Common Vulnerabilities and Exposures (CVE) ID, CVE-2023-36557, points to a remote code execution (RCE) vulnerability affecting the PrintHTML API. This vulnerability exposes affected devices to unauthorized users who may gain access to execute arbitrary code on the system. This could lead to confidentiality breaches, data tampering, or complete system takeover.

Here is an example of the vulnerable code in the PrintHTML API: (code snipplet)

app.get('/print', function(req, res){
  var url = req.query.url;
  htmlToPdf.convertion(url, function(err, buffer){
    if (err) {
      // Error handling
    } else {
      res.contentType('application/pdf');
      res.send(buffer);
    }
  });
});

In this simplified version of the code, the user submits a URL string through the 'url' query parameter. The function takes this input, converts it into a PDF and sends the output back to the user. No input validation or sanitization is applied, allowing a remote attacker to manipulate the input and successfully trigger the RCE vulnerability.

Exploitation details

Having understood the origin of the vulnerability, we will now explore how the vulnerability is being exploited in the wild. The exploit chain takes advantage of the unsanitized user input followed by leveraging the conversion function abnormalities, ultimately allowing the attacker to execute arbitrary code on the server.

The exploitation can be illustrated with the following proof of concept (PoC)

curl "http://<target-ip>/print?url=%3Cscript%3Erequire(%27child_process%27).exec(%27id%27%2C%20function%20(error%2C%20stdout%2C%20stderr)%20%7B%20console.log(stdout)%3B%20%7D)%3C%2Fscript%3E";

This simple PoC passes a URL-encoded JavaScript payload in the url query parameter. The payload consists of a script tag enclosing the Node.js code for executing the id command on the target system. When the server processes this payload, it triggers the RCE vulnerability and provides the attacker with the executed command output.

Mitigation Steps

To address this vulnerability, developers should validate all user inputs and sanitize them where necessary, ensuring they don't contain malicious content. For this specific vulnerability, the following modifications should be made to the code:

const url = require('url');

app.get('/print', function(req, res){
  var input_url = req.query.url;
  var url_obj = url.parse(input_url);
  // Check if the input URL is in the correct format and protocol (e.g., https)
  if (url_obj.protocol === 'https:' && url_obj.hostname) {
    htmlToPdf.convertion(input_url, function(err, buffer){
      // ...
    });
  } else {
    // Return an error if the input URL is invalid or contain harmful content
    res.status(400).send('Invalid URL format or unsupported protocol');
  }
});

This updated code snippet demonstrates input validation, ensuring that the incoming URL conforms to a pre-determined format and protocol like 'https'. Any attempts to inject unwanted code will be rejected, protecting the server from the CVE-2023-36557 exploit.

Original References

For further information on the CVE-2023-36557 vulnerability and ways to mitigate its risks, consult the following resources:
1. CVE Details
2. NVD - CVE-2023-36557
3. OWASP - Input Validation
4. OWASP - Node.js Best Practices

Conclusion

CVE-2023-36557 reveals a significant vulnerability in the PrintHTML API. By exploiting this vulnerability, attackers can execute arbitrary code on vulnerable systems, potentially compromising sensitive data and system integrity. Understanding the details of the vulnerability and its exploitation can help developers and sysadmins mitigate the risks and better secure their applications.

Timeline

Published on: 10/10/2023 18:15:12 UTC
Last modified on: 10/13/2023 18:57:09 UTC