Unrestricted resource consumption in Cuba-based applications typically manifests as Denial of Service (DoS) via memory exhaustion. Because Cuba is a thin wrapper around Rack, failing to constrain request body sizes or rate-limiting expensive operations allows an attacker to saturate server RAM or CPU by sending massive payloads that the application attempts to buffer in-memory.

The Vulnerable Pattern

require 'cuba'
Cuba.define do
on post, ‘process’ do
# VULNERABLE: req.body.read pulls the entire payload into memory
# An attacker can send a 2GB request to crash the Ruby process.
data = req.body.read
result = data.reverse # CPU/Memory intensive task
res.write “Processed: #{result}”
end
end

The Secure Implementation

The exploit leverages the default behavior of reading the entire input stream into a Ruby string object, which resides in the heap. To mitigate this, we implement a 'Fail Fast' strategy using Rack middleware to inspect the 'CONTENT_LENGTH' header before the route logic is executed. If the header exceeds our threshold (e.g., 1MB), we return a 413 Payload Too Large status. Furthermore, passing an integer argument to 'req.body.read(limit)' ensures the buffer never exceeds a specific size even if the middleware is bypassed or misconfigured.

require 'cuba'
require 'rack/attack'
1. Enforce global request body limits at the Rack level
Cuba.use Rack::Config do |env|
max_size = 1024 * 1024 # 1MB limit
if env[‘CONTENT_LENGTH’].to_i > max_size
[413, { ‘Content-Type’ => ‘text/plain’ }, [‘Payload Too Large’]]
end
end
Cuba.define do
on post, ‘process’ do
# 2. Use streaming or tempfiles if large data is necessary
# 3. Explicitly limit read size if not using middleware
data = req.body.read(1024 * 1024)
if data.nil? || data.empty?
  res.status = 400
  res.write "Empty Body"
else
  res.write "Processed safely"
end
end
end