Sinatra, the popular Ruby web application framework, has a security vulnerability in versions before 2.2.. The vulnerability, identified as CVE-2022-29970, is a potential risk for web applications using Sinatra to serve static files. This post will delve into the details of the vulnerability, including code snippets, links to original references, and exploit details, making it easier for developers using Sinatra to understand and address the issue.

Background on CVE-2022-29970

Sinatra, an open-source project, is widely used in web applications, especially for serving static files, such as images, stylesheets, and JavaScript files. In versions before 2.2., Sinatra does not validate that the expanded path matches the public_dir configuration when serving static files. This means that an attacker could potentially exploit this vulnerability to access unauthorized files on the webserver, leading to data leaks or other security risks.

For more information, you can refer to the official CVE (Common Vulnerabilities and Exposures) record, CVE-2022-29970, which provides details on the vulnerability and its impact.

Code Snippet: The Vulnerable Code

Sinatra's source code for serving static files did not check if the expanded path matched the public_dir configuration set by the developers. This is the vulnerable code snippet from Sinatra:

def static_file?(path_info)
  return unless public_dir?
  public_file = File.join(public_dir, Utils::unescape(request.path_info))
  return if File.directory?(public_file)
  return unless File.file?(public_file) # <- This does not check if it matches public_dir
  content_type File.extname(path_info), :default => 'application/octet-stream'
  return File.expand_path(public_file)  # <- Returns the expanded path
end

Exploiting CVE-2022-29970

An attacker who is aware of the vulnerability can attempt to manipulate the path using escape characters to access unintended files hosted on the server. For instance, consider the following URL:

http://example.com/../secret_file.txt

In this example, an attacker could navigate to the URL, and Sinatra, failing to validate that the expanded path matches the public_dir, might serve the secret_file.txt located outside the intended public_dir, exposing unauthorized files.

Fixing Sinatra: Upgrading to 2.2. or Higher

The simplest and most effective way to address the CVE-2022-29970 vulnerability is to upgrade the Sinatra gem to the latest version. As of version 2.2., the vulnerability has been patched. If you're currently using a version prior to 2.2., you should update immediately to protect your web application from potential exploits. To upgrade Sinatra, modify your Gemfile and update the Sinatra version:

gem 'sinatra', '~> 2.2.'

Then, execute $ bundle update sinatra to apply the update.

Conclusion

CVE-2022-29970 is a critical security vulnerability impacting Sinatra versions before 2.2.. It's crucial for developers using Sinatra to serve static files to understand the vulnerability and take the necessary steps to protect their web applications from potential exploits. Make sure to upgrade Sinatra to at least 2.2., validate the public_dir configuration, and ensure that all static files are served as intended.

For more information, you can refer to the official resources

- Sinatra Repository on GitHub
- CVE-2022-29970 Official Record
- Sinatra 2.2. Release Notes

Timeline

Published on: 05/02/2022 05:15:00 UTC
Last modified on: 05/09/2022 17:27:00 UTC