CarrierWave has a denylisted_content_type bypass via Unescaped Regex Metacharacters

Summary

CarrierWave's content_type_denylist check fails to escape regex metacharacters in string entries, causing the denylist to silently not match the content types it is intended to block.

Note: CarrierWave is aware #content_type_denylist is deprecated for the security reason, but it still used by developers, and the problem here isn't denylist allows any filetype, and thats not a vulnerability in carrierwave, its an implementation problem in developers using CarrierWave, the problem is its denylist entries are interpolated directly into a regex without Regexp.quote or anchoring. The denylist is still useful when developers want to ban specific content types but allow everything else.

Details

In lib/carrierwave/uploader/content_type_denylist.rb:57, string denylist entries are interpolated directly into a regex without Regexp.quote or anchoring:

def denylisted_content_type?(denylist, content_type)
  Array(denylist).any? { |item| content_type =~ /#{item}/ }
end
The entry "image/svg+xml" becomes the regex /image\/svg+xml/ where + is a quantifier meaning "one or more g", not a literal +. This regex never matches the real MIME type "image/svg+xml" which contains a literal +.
This is inconsistent with the allowlist implementation at lib/carrierwave/uploader/content_type_allowlist.rb:53-57, which correctly applies both Regexp.quote and a \A anchor:
rubydef allowlisted_content_type?(allowlist, content_type)
  Array(allowlist).any? do |item|
    item = Regexp.quote(item) if item.class != Regexp
    content_type =~ /\A#{item}/
  end
end

Other affected MIME types include application/xhtml+xml and any type containing regex metacharacters.

Fix: Apply Regexp.quote for string entries and anchor with \A, matching the existing allowlist implementation:

rubydef denylisted_content_type?(denylist, content_type)
  Array(denylist).any? do |item|
    item = Regexp.quote(item) if item.class != Regexp
    content_type =~ /\A#{item}/
  end
end

PoC

 app.rb
require "sinatra"
require "carrierwave"
require "fileutils"

FileUtils.mkdir_p("uploads/files")

CarrierWave.configure do |config|
  config.root      = File.expand_path("uploads")
  config.store_dir = "files"
end

class VaultUploader < CarrierWave::Uploader::Base
  storage :file
  def store_dir = "files"
  def content_type_denylist = %w[image/svg+xml]
end

post "/upload" do
  content_type :json
  san = CarrierWave::SanitizedFile.new(
    tempfile:     params[:file][:tempfile],
    filename:     params[:file][:filename],
    content_type: params[:file][:type]
  )
  uploader = VaultUploader.new
  begin
    uploader.store!(san)
    { result: "VULNERABLE", message: "SVG bypassed denylist", path: uploader.path }.to_json
  rescue CarrierWave::IntegrityError => e
    { result: "blocked", message: e.message }.to_json
  end
end