A Denial-of-Service (DoS) vulnerability exists in Rack versions prior to v3..4.2, v2.2.6.3, v2.1.4.3, and v2..9.3. This DoS vulnerability lies within the Multipart MIME parsing code, which could allow potential attackers to craft requests that abuse this vulnerability and cause multipart parsing to take much longer than expected, significantly impacting server response time and efficiency. In this post, we'll unravel the details of this vulnerability, the affected Rack versions, and how to fix it. We'll also share some code snippets and links with additional details for better comprehension.

Exploit Details

The vulnerability is attributed to the multipart MIME parsing code, which handles how Rack processes HTTP requests with the "multipart/form-data" content type. Such requests are commonly used when a user submits a form with file uploads. When an attacker crafts a request designed to cause the parser to take longer than expected, it can lead to the server becoming unresponsive or drastically slow down response times for legitimate users. This behavior can consequently cause a DoS attack on the affected server.

Here's a code snippet illustrating the issue within the multipart MIME parsing code

def parse_multipart(env, params)
  Parser.new(env, params).parse
end

class Parser
  def parse
    ...
    # This loop iterates through each part of the request to process the
    # form fields and file uploads. An attacker can manipulate the request,
    # causing this loop to take much longer than expected.
    loop do
      ...
    end
  end
end

The exploit affects the following Rack versions

* Rack < v3..4.2
* Rack < v2.2.6.3
* Rack < v2.1.4.3
* Rack < v2..9.3

Solution

Developers and system administrators should update their Rack installations to the following unaffected versions:
* Rack v3..4.2 or later
* Rack v2.2.6.3 or later
* Rack v2.1.4.3 or later
* Rack v2..9.3 or later

To update your Rack version, add the following line to your Gemfile and run bundle update rack

gem 'rack', '>= 3..4.2'

Remember to replace '3..4.2' with the specific minimum version required for your project.

For a more in-depth analysis of this vulnerability, review the following references

1. Rack GitHub Repository Issue: A detailed discussion on the issue, including the solution and the released Rack versions containing the fixed code.
2. Rack Vulnerability Advisory: The official advisory from the Rack team discussing the impact, the fix, and the strategy for staying protected against this vulnerability.

Conclusion

In conclusion, we've discussed the details of the DoS vulnerability found in Rack versions prior to v3..4.2, v2.2.6.3, v2.1.4.3, and v2..9.3. Any developers or administrators utilizing the mentioned vulnerable versions should update their installations as soon as possible to mitigate any potential security risks.

Timeline

Published on: 03/10/2023 22:15:00 UTC
Last modified on: 03/16/2023 16:18:00 UTC