CVE-2025-27219 - How a Cookie Parsing Bug in Ruby’s CGI Gem Threatens Your App

Introduction
In early 2025, a security vulnerability was discovered in the CGI gem for Ruby—specifically versions before .4.2. Labeled CVE-2025-27219, this bug is a Denial of Service (DoS) issue found in the way the CGI::Cookie.parse method processes incoming cookies. If unchecked, attackers can easily consume all your server’s resources with just a few oversized HTTP cookies.

This post gives a simple explanation of the bug, shows you how an attacker could exploit it (including Ruby code), and ends with tips on protecting your app right now.

What is CVE-2025-27219?

Ruby’s CGI gem is widely used, especially in legacy Rails and Sinatra apps. Its CGI::Cookie.parse method takes raw cookie strings and turns them into Ruby objects you can inspect.

But here’s the problem: there’s no limit on how big those cookie values can be. When a cookie with a _huge_ value comes in, the parser tries processing it all—which eats up system memory and CPU. If an attacker sends enough gigantic cookies at once, your server chokes, slows down, or even dies, making it an open door for DoS attacks.

Exploiting the Vulnerability

Suppose you run a Sinatra app (or any rack-based Ruby app) that depends on the CGI gem, without the patched version.

`ruby

require 'net/http'

payload = "A" * 100_000_000 # 100 MB of 'A'

uri = URI("http://target-app.com/")

The vulnerable app receives the request

When its middleware or controllers call CGI::Cookie.parse, Ruby tries to process the massive cookie.

Common in old Rails/Sinatra projects

require 'cgi'

# Imagine incoming_cookie is big and untrusted
def cookies_hash(incoming_cookie)
  CGI::Cookie.parse(incoming_cookie)
end

If incoming_cookie holds massive data, this call hogs memory until the server bogs down.

Original References

- GitHub security advisory: cgi gem Arbitrary Memory Exhaustion Vulnerability *(example advisory, replace with actual ID when available)*
- NVD entry for CVE-2025-27219
- Ruby CGI Gem Changelog

Real-World Impact

- Any Rails/Sinatra app using old CGI gems can be hit.

How Can You Fix or Reduce the Risk?

1. Upgrade immediately

Update to CGI gem version .4.2 or later

gem update cgi

or in your Gemfile

gem 'cgi', '>= .4.2'

2. Add middleware to limit cookie size
If you can’t upgrade, add a security layer to block cookies that are simply too large.

class LimitCookieSize
  MAX_COOKIE_SIZE = 4096 # 4KB

  def initialize(app)
    @app = app
  end

  def call(env)
    cookies = env['HTTP_COOKIE'] || ""
    if cookies.bytesize > MAX_COOKIE_SIZE
      return [400, { "Content-Type" => "text/plain" }, ["Cookie too large"]]
    end
    @app.call(env)
  end
end

And use it in your stack.

Conclusion

CVE-2025-27219 is a classic example of a small oversight leading to a big problem. Attackers don’t need fancy tools—just a single big cookie to crash your Ruby backend.

Don’t let your app be an easy target. Update your CGI gem, or patch your stack to defend against oversized cookies.

If your app touches cookies in any way with Ruby’s CGI gem, act now.

Timeline

Published on: 03/04/2025 00:15:31 UTC
Last modified on: 03/05/2025 14:08:20 UTC

CVE-2025-27219 - How a Cookie Parsing Bug in Ruby’s CGI Gem Threatens Your App – Exploit Details & Guide