If you’re running a Ruby web application (especially with Rails), this vulnerability should catch your attention. Discovered in February 2024, CVE-2024-26141 exposes a nasty bug in the popular Ruby web server library, Rack. It all centers on the Range HTTP header—a feature meant to let clients download parts of files. But carefully crafted requests can make your server try to send *massive* responses, eventually choking resources or even shutting down.

References for verifying and fixing your code

Let’s keep it in straight talk and see clear examples.

What Is Rack and Who’s Vulnerable?

Rack is the backbone for Ruby web servers. It sits between your app (like Rails or Sinatra) and the outside world. This vulnerability in particular affects apps that:

Use the Rack::File middleware (for serving static files)

- Directly or indirectly use Rack::Utils.byte_ranges—which includes most Rails setups

Bottom line: Pretty much any classic Rails app serving files (static or generated) is at risk until patched.

The Heart Of The Problem: Malicious Range Headers

The Range header lets users ask for part of a file. For example, Range: bytes=-499 asks for the first 500 bytes. But if a client is clever, they can abuse this by making weird or overly broad requests.

Think of it this way:
What if someone asked for an impossible range, like, thousands of bytes beyond the real file end—or even overlapping/illegal ranges? Before the fix, Rack would try to honor it and build the huge (non-existent) response, wasting memory and CPU.

Vulnerable Code Behavior

Below is a simplified version of how a vulnerable app might respond, with comments to show what goes wrong:

# Vulnerable: Rails or any Rack use before 3..9.1 or 2.2.8.1
require 'rack'
# Typical file serving via Rack::File (simplified)
app = Rack::File.new('/path/to/public')

# Rack parses the 'Range' header:
# BAD: Attackers send something like "Range: bytes=-18446744073709551615"
env = { 'HTTP_RANGE' => 'bytes=-18446744073709551615' } # excessive range!
status, headers, body = app.call(env)

# Server will try to create a huge response, choking itself.

The dangerous line:
When Rack::File or Rack::Utils.byte_ranges tries to honor a maliciously large requested range, its response can balloon.

Here’s how you (or an attacker) can exploit this in practice, using curl

curl -v -H "Range: bytes=-999999999999" http://your.rack.server/path/to/file

If unpatched, your server will attempt to generate a crazy-big response, quickly eating through memory and possibly leading to a Denial of Service.

Another more advanced example is requesting multiple ranges at once

curl -v -H "Range: bytes=-1,2-999999999999" http://your.rack.server/path/to/file

Older versions of Rack didn’t check these carefully, so several weird combinations cause big trouble.

If you use Ruby Bundler or Gem

bundle update rack

_(check your Gemfile.lock to confirm)_

If your app is using Rails, updating Rails should also bring in the fixed Rack gem, but always double check.

Relevant fixed versions

- Rack 3..9.1 release notes
- Rack 2.2.8.1 release notes

2. Block or Limit Range headers (Temporary Workaround)

If you can’t upgrade Rack right away, block suspicious Range headers in your web server configuration *(like NGINX or Apache)*.

Example for NGINX

# In your server block:
if ($http_range ~* "[^-9,.-]|\d{10,}") {
    return 416;
}

This refuses ranges with crazy large numbers or weird characters.

Or, as a last-resort Ruby filter

# Middleware example: reject overly large Range headers
class RangeGuard
  def initialize(app)
    @app = app
  end

  def call(env)
    range = env['HTTP_RANGE']
    if range && range.match(/\d{10,}/)
      return [416, {}, ["Range too large"]]
    else
      @app.call(env)
    end
  end
end

# Insert early in the Rack middleware stack
use RangeGuard

Official Advisory:

- Rack Security Announcement
- Mitre/NVD
- CVE-2024-26141 | NVD listing

GitHub Advisory Database:

- GHSA-xqhc-36q8-c34v (GitHub advisory)

Upstream code fixes:

- Fix commit for Rack

Conclusion

CVE-2024-26141 is a textbook modern web security flaw—simple, sneaky, and capable of taking down big Ruby apps with one weird HTTP header. Upgrading Rack (and by extension, Rails) is the safest solution. Until then, block wild Range headers on your web layer. Protect your servers before someone else finds them!

Stay Safe. Patch Early. Monitor Your Logs.

Let us know if you have questions on upgrading or patching.

Timeline

Published on: 02/29/2024 00:15:51 UTC
Last modified on: 02/29/2024 13:49:47 UTC