Merge pull request from GHSA-9928-3cp5-93fm

* Fix attachments getting processed despite failing content-type validation

* Add a restrictive ImageMagick security policy tailored for Mastodon

* Fix misdetection of MP3 files with large cover art

* Reject unprocessable audio/video files instead of keeping them unchanged

app/models/concerns/attachmentable.rb

@@ -22,15 +22,14 @@ module Attachmentable
 
   included do
     def self.has_attached_file(name, options = {}) # rubocop:disable Naming/PredicateName
-      options = { validate_media_type: false }.merge(options)
       super(name, options)
-      send(:"before_#{name}_post_process") do
+
+      send(:"before_#{name}_validate") do
         attachment = send(name)
         check_image_dimension(attachment)
         set_file_content_type(attachment)
         obfuscate_file_name(attachment)
         set_file_extension(attachment)
-        Paperclip::Validators::MediaTypeSpoofDetectionValidator.new(attributes: [name]).validate(self)
       end
     end
   end

config/application.rb

@@ -28,6 +28,7 @@ require_relative '../lib/paperclip/url_generator_extensions'
 require_relative '../lib/paperclip/attachment_extensions'
 require_relative '../lib/paperclip/lazy_thumbnail'
 require_relative '../lib/paperclip/gif_transcoder'
+require_relative '../lib/paperclip/media_type_spoof_detector_extensions'
 require_relative '../lib/paperclip/transcoder'
 require_relative '../lib/paperclip/type_corrector'
 require_relative '../lib/paperclip/response_with_limit_adapter'

config/imagemagick/policy.xml

@@ -0,0 +1,27 @@
+<policymap>
+  <!-- Set some basic system resource limits -->
+  <policy domain="resource" name="time" value="60" />
+
+  <policy domain="module" rights="none" pattern="URL" />
+
+  <policy domain="filter" rights="none" pattern="*" />
+
+  <!--
+    Ideally, we would restrict ImageMagick to only accessing its own
+    disk-backed pixel cache as well as Mastodon-created Tempfiles.
+
+    However, those paths depend on the operating system and environment
+    variables, so they can only be known at runtime.
+
+    Furthermore, those paths are not necessarily shared across Mastodon
+    processes, so even creating a policy.xml at runtime is impractical.
+
+    For the time being, only disable indirect reads.
+  -->
+  <policy domain="path" rights="none" pattern="@*" />
+
+  <!-- Disallow any coder by default, and only enable ones required by Mastodon -->
+  <policy domain="coder" rights="none" pattern="*" />
+  <policy domain="coder" rights="read | write" pattern="{PNG,JPEG,GIF,HEIC,WEBP}" />
+  <policy domain="coder" rights="write" pattern="{HISTOGRAM,RGB,INFO}" />
+</policymap>

config/initializers/paperclip.rb

@@ -153,3 +153,10 @@ unless defined?(Seahorse)
     end
   end
 end
+
+# Set our ImageMagick security policy, but allow admins to override it
+ENV['MAGICK_CONFIGURE_PATH'] = begin
+  imagemagick_config_paths = ENV.fetch('MAGICK_CONFIGURE_PATH', '').split(File::PATH_SEPARATOR)
+  imagemagick_config_paths << Rails.root.join('config', 'imagemagick').expand_path.to_s
+  imagemagick_config_paths.join(File::PATH_SEPARATOR)
+end

lib/paperclip/media_type_spoof_detector_extensions.rb

@@ -0,0 +1,22 @@
+# frozen_string_literal: true
+
+module Paperclip
+  module MediaTypeSpoofDetectorExtensions
+    def calculated_content_type
+      return @calculated_content_type if defined?(@calculated_content_type)
+
+      @calculated_content_type = type_from_file_command.chomp
+
+      # The `file` command fails to recognize some MP3 files as such
+      @calculated_content_type = type_from_marcel if @calculated_content_type == 'application/octet-stream' && type_from_marcel == 'audio/mpeg'
+      @calculated_content_type
+    end
+
+    def type_from_marcel
+      @type_from_marcel ||= Marcel::MimeType.for Pathname.new(@file.path),
+                                                 name: @file.path
+    end
+  end
+end
+
+Paperclip::MediaTypeSpoofDetector.prepend(Paperclip::MediaTypeSpoofDetectorExtensions)

lib/paperclip/transcoder.rb

@@ -19,10 +19,7 @@ module Paperclip
     def make
       metadata = VideoMetadataExtractor.new(@file.path)
 
-      unless metadata.valid?
-        Paperclip.log("Unsupported file #{@file.path}")
-        return File.open(@file.path)
-      end
+      raise Paperclip::Error, "Error while transcoding #{@file.path}: unsupported file" unless metadata.valid?
 
       update_attachment_type(metadata)
       update_options_from_metadata(metadata)

spec/models/media_attachment_spec.rb

@@ -152,6 +152,26 @@ RSpec.describe MediaAttachment, paperclip_processing: true do
     end
   end
 
+  describe 'mp3 with large cover art' do
+    let(:media) { described_class.create(account: Fabricate(:account), file: attachment_fixture('boop.mp3')) }
+
+    it 'detects it as an audio file' do
+      expect(media.type).to eq 'audio'
+    end
+
+    it 'sets meta for the duration' do
+      expect(media.file.meta['original']['duration']).to be_within(0.05).of(0.235102)
+    end
+
+    it 'extracts thumbnail' do
+      expect(media.thumbnail.present?).to be true
+    end
+
+    it 'gives the file a random name' do
+      expect(media.file_file_name).to_not eq 'boop.mp3'
+    end
+  end
+
   describe 'jpeg' do
     let(:media) { described_class.create(account: Fabricate(:account), file: attachment_fixture('attachment.jpg')) }