FEATURE: Allow hotlinked media to be blocked (#16940)

This commit introduces a new site setting: `block_hotlinked_media`. When enabled, all attempts to hotlink media (images, videos, and audio) will fail, and be replaced with a linked placeholder. Exceptions to the rule can be added via `block_hotlinked_media_exceptions`.

`download_remote_image_to_local` can be used alongside this feature. In that case, hotlinked images will be blocked immediately when the post is created, but will then be replaced with the downloaded version a few seconds later.

This implementation is purely server-side, and does not impact the composer preview.

Technically, there are two stages to this feature:

1. `PrettyText.sanitize_hotlinked_media` is called during `PrettyText.cook`, and whenever new images are introduced by Onebox. It will iterate over all src/srcset attributes in the post HTML and check if they're allowed. If not, the attributes will be removed and replaced with a `data-blocked-hotlinked-src(set)` attribute

2. In the `CookedPostProcessor`, we iterate over all `data-blocked-hotlinked-src(set)` attributes and check whether we have a downloaded version of the media. If yes, we update the src to use the downloaded version. If not, the entire media element is replaced with a placeholder. The placeholder is labelled 'external media', and is a link to the offsite media.

app/assets/stylesheets/common/base/topic-post.scss

@@ -1309,14 +1309,25 @@ a.mention-group {
   }
 }
 
-.broken-image {
-  display: inline-flex;
+.broken-image,
+.blocked-hotlinked-placeholder {
+  &:not(a) {
     color: var(--primary-low-mid-or-secondary-high);
+  }
+
+  display: inline-flex;
+  flex-direction: column;
   border: 1px solid var(--primary-low);
-  font-size: $font-up-5;
+  align-items: center;
+  justify-content: center;
 
   .d-icon {
     margin: 16px;
+    font-size: var(--font-up-5);
+  }
+
+  .notice {
+    margin: 0 0.5em 0.5em 0.5em;
   }
 }
 

app/jobs/regular/pull_hotlinked_images.rb

@@ -23,7 +23,7 @@ module Jobs
       changed_hotlink_records = false
 
       extract_images_from(post.cooked).each do |node|
-        download_src = original_src = node['src'] || node['href']
+        download_src = original_src = node['src'] || node[PrettyText::BLOCKED_HOTLINKED_SRC_ATTR] || node['href']
         download_src = "#{SiteSetting.force_https ? "https" : "http"}:#{original_src}" if original_src.start_with?("//")
         normalized_src = normalize_src(download_src)
 
@@ -130,7 +130,7 @@ module Jobs
     def extract_images_from(html)
       doc = Nokogiri::HTML5::fragment(html)
 
-      doc.css("img[src], a.lightbox[href]") -
+      doc.css("img[src], [#{PrettyText::BLOCKED_HOTLINKED_SRC_ATTR}], a.lightbox[href]") -
         doc.css("img.avatar") -
         doc.css(".lightbox img[src]")
     end

app/models/post_analyzer.rb

@@ -46,7 +46,11 @@ class PostAnalyzer
       onebox
     end
 
-    cooked = result.to_html if result.changed?
+    if result.changed?
+      PrettyText.sanitize_hotlinked_media(result.doc)
+      cooked = result.to_html
+    end
+
     cooked
   end
 

config/locales/server.en.yml

@@ -727,6 +727,11 @@ en:
   post:
     image_placeholder:
       broken: "This image is broken"
+      blocked_hotlinked_title: "Image hosted on another site. Click to open in a new tab."
+      blocked_hotlinked: "External Image"
+    media_placeholder:
+      blocked_hotlinked_title: "Media hosted on another site. Click to open in a new tab."
+      blocked_hotlinked: "External Media"
     hidden_bidi_character: "Bidirectional characters can change the order that text is rendered. This could be used to obscure malicious code."
     has_likes:
       one: "%{count} Like"
@@ -1514,9 +1519,11 @@ en:
     contact_email: "Email address of key contact responsible for this site. Used for critical notifications, and also displayed on <a href='%{base_path}/about' target='_blank'>/about</a> for urgent matters."
     contact_url: "Contact URL for this site. Displayed on the <a href='%{base_path}/about' target='_blank'>/about</a> page for urgent matters."
     crawl_images: "Retrieve images from remote URLs to insert the correct width and height dimensions."
-    download_remote_images_to_local: "Convert remote images to local images by downloading them; this prevents broken images."
+    download_remote_images_to_local: "Convert remote (hotlinked) images to local images by downloading them; This preserves content even if the images are removed from the remote site in future."
     download_remote_images_threshold: "Minimum disk space necessary to download remote images locally (in percent)"
     disabled_image_download_domains: "Remote images will never be downloaded from these domains. Pipe-delimited list."
+    block_hotlinked_media: "Prevent users from introducing remote (hotlinked) media in their posts. Remote media which is not downloaded via 'download_remote_images_to_local' will be replaced with a placeholder link."
+    block_hotlinked_media_exceptions: "A list of base URLs which are exempt from the block_hotlinked_media setting. Include the protocol (e.g. https://example.com)."
     editing_grace_period: "For (n) seconds after posting, editing will not create a new version in the post history."
     editing_grace_period_max_diff: "Maximum number of character changes allowed in editing grace period, if more changed store another post revision (trust level 0 and 1)"
     editing_grace_period_max_diff_high_trust: "Maximum number of character changes allowed in editing grace period, if more changed store another post revision (trust level 2 and up)"

config/site_settings.yml

@@ -1326,6 +1326,12 @@ files:
     type: list
     list_type: simple
     default: ""
+  block_hotlinked_media:
+    default: false
+  block_hotlinked_media_exceptions:
+    default: ""
+    type: list
+    regex: '\A((https?:\/\/.+)(\|https?:\/\/.+[|$])*)?\z'
   create_thumbnails: true
   clean_up_uploads: true
   clean_orphan_uploads_grace_period_hours: 48

lib/cooked_post_processor.rb

@@ -42,6 +42,7 @@ class CookedPostProcessor
       remove_full_quote_on_direct_reply if new_post
       post_process_oneboxes
       post_process_images
+      add_blocked_hotlinked_media_placeholders
       post_process_quotes
       optimize_urls
       remove_user_ids
@@ -120,7 +121,7 @@ class CookedPostProcessor
 
   def extract_images
     # all images with a src attribute
-    @doc.css("img[src]") -
+    @doc.css("img[src], img[#{PrettyText::BLOCKED_HOTLINKED_SRC_ATTR}]") -
     # minus data images
     @doc.css("img[src^='data']") -
     # minus emojis
@@ -371,7 +372,7 @@ class CookedPostProcessor
 
   def process_hotlinked_image(img)
     @hotlinked_map ||= @post.post_hotlinked_media.preload(:upload).map { |r| [r.url, r] }.to_h
-    normalized_src = PostHotlinkedMedia.normalize_src(img["src"])
+    normalized_src = PostHotlinkedMedia.normalize_src(img["src"] || img[PrettyText::BLOCKED_HOTLINKED_SRC_ATTR])
     info = @hotlinked_map[normalized_src]
 
     still_an_image = true
@@ -384,11 +385,37 @@ class CookedPostProcessor
       still_an_image = false
     elsif info&.downloaded? && upload = info&.upload
       img["src"] = UrlHelper.cook_url(upload.url, secure: @with_secure_media)
+      img.delete(PrettyText::BLOCKED_HOTLINKED_SRC_ATTR)
     end
 
     still_an_image
   end
 
+  def add_blocked_hotlinked_media_placeholders
+    @doc.css([
+      "[#{PrettyText::BLOCKED_HOTLINKED_SRC_ATTR}]",
+      "[#{PrettyText::BLOCKED_HOTLINKED_SRCSET_ATTR}]",
+    ].join(',')).each do |el|
+      src = el[PrettyText::BLOCKED_HOTLINKED_SRC_ATTR] ||
+        el[PrettyText::BLOCKED_HOTLINKED_SRCSET_ATTR]&.split(',')&.first&.split(' ')&.first
+
+      if el.name == "img"
+        add_blocked_hotlinked_image_placeholder!(el)
+        next
+      end
+
+      if ["video", "audio"].include?(el.parent.name)
+        el = el.parent
+      end
+
+      if el.parent.classes.include?("video-container")
+        el = el.parent
+      end
+
+      add_blocked_hotlinked_media_placeholder!(el, src)
+    end
+  end
+
   def is_svg?(img)
     path =
       begin

lib/cooked_processor_mixin.rb

@@ -40,6 +40,8 @@ module CookedProcessorMixin
       end
     end
 
+    PrettyText.sanitize_hotlinked_media(@doc)
+
     oneboxed_images.each do |img|
       next if img["src"].blank?
 
@@ -251,6 +253,31 @@ module CookedProcessorMixin
     true
   end
 
+  def add_blocked_hotlinked_image_placeholder!(el)
+    el.name = "a"
+    el.set_attribute("href", el[PrettyText::BLOCKED_HOTLINKED_SRC_ATTR])
+    el.set_attribute("class", "blocked-hotlinked-placeholder")
+    el.set_attribute("title", I18n.t("post.image_placeholder.blocked_hotlinked_title"))
+    el << "<svg class=\"fa d-icon d-icon-link svg-icon\" aria-hidden=\"true\"><use href=\"#link\"></use></svg>"
+    el << "<span class=\"notice\">#{CGI.escapeHTML(I18n.t("post.image_placeholder.blocked_hotlinked"))}</span>"
+
+    true
+  end
+
+  def add_blocked_hotlinked_media_placeholder!(el, src)
+    placeholder = Nokogiri::XML::Node.new("a", el.document)
+    placeholder.name = "a"
+    placeholder.set_attribute("href", src)
+    placeholder.set_attribute("class", "blocked-hotlinked-placeholder")
+    placeholder.set_attribute("title", I18n.t("post.media_placeholder.blocked_hotlinked_title"))
+    placeholder << "<svg class=\"fa d-icon d-icon-link svg-icon\" aria-hidden=\"true\"><use href=\"#link\"></use></svg>"
+    placeholder << "<span class=\"notice\">#{CGI.escapeHTML(I18n.t("post.media_placeholder.blocked_hotlinked"))}</span>"
+
+    el.replace(placeholder)
+
+    true
+  end
+
   def oneboxed_images
     @doc.css(".onebox-body img, .onebox img, img.onebox")
   end

lib/pretty_text.rb

@@ -18,6 +18,9 @@ module PrettyText
   ].freeze
   DANGEROUS_BIDI_REGEXP = Regexp.new(DANGEROUS_BIDI_CHARACTERS.join("|")).freeze
 
+  BLOCKED_HOTLINKED_SRC_ATTR = "data-blocked-hotlinked-src"
+  BLOCKED_HOTLINKED_SRCSET_ATTR = "data-blocked-hotlinked-srcset"
+
   @mutex = Mutex.new
   @ctx_init = Mutex.new
 
@@ -318,6 +321,7 @@ module PrettyText
     add_nofollow = !options[:omit_nofollow] && SiteSetting.add_rel_nofollow_to_user_content
     add_rel_attributes_to_user_content(doc, add_nofollow)
     strip_hidden_unicode_bidirectional_characters(doc)
+    sanitize_hotlinked_media(doc)
 
     if SiteSetting.enable_mentions
       add_mentions(doc, user_id: opts[:user_id])
@@ -348,6 +352,25 @@ module PrettyText
     end
   end
 
+  def self.sanitize_hotlinked_media(doc)
+    return if !SiteSetting.block_hotlinked_media
+
+    allowed_pattern = allowed_src_pattern
+
+    doc.css("img[src], source[src], source[srcset], track[src]").each do |el|
+      if el["src"] && !el["src"].match?(allowed_pattern)
+        el[PrettyText::BLOCKED_HOTLINKED_SRC_ATTR] = el.delete("src")
+      end
+
+      if el["srcset"]
+        srcs = el["srcset"].split(',').map { |e| e.split(' ', 2)[0].presence }
+        if srcs.any? { |src| !src.match?(allowed_pattern) }
+          el[PrettyText::BLOCKED_HOTLINKED_SRCSET_ATTR] = el.delete("srcset")
+        end
+      end
+    end
+  end
+
   def self.add_rel_attributes_to_user_content(doc, add_nofollow)
     allowlist = []
 
@@ -673,4 +696,25 @@ module PrettyText
     mentions
   end
 
+  def self.allowed_src_pattern
+    allowed_src_prefixes = [
+      Discourse.base_path,
+      Discourse.base_url,
+      GlobalSetting.s3_cdn_url,
+      GlobalSetting.cdn_url,
+      SiteSetting.external_emoji_url.presence,
+      *SiteSetting.block_hotlinked_media_exceptions.split("|")
+    ]
+
+    patterns = allowed_src_prefixes.compact.map do |url|
+      pattern = Regexp.escape(url)
+
+      # If 'https://example.com' is allowed, ensure 'https://example.com.blah.com' is not
+      pattern += '(?:/|\z)' if !pattern.ends_with?("\/")
+
+      pattern
+    end
+
+    /\A(data:|#{patterns.join("|")})/
+  end
 end

spec/integration/blocked_hotlinked_media_spec.rb

@@ -0,0 +1,156 @@
+# frozen_string_literal: true
+
+describe "hotlinked media blocking" do
+  let(:hotlinked_url) { "http://example.com/images/2/2e/Longcat1.png" }
+  let(:onebox_url) { "http://example.com/onebox" }
+  let(:png) { Base64.decode64("R0lGODlhAQABALMAAAAAAIAAAACAAICAAAAAgIAAgACAgMDAwICAgP8AAAD/AP//AAAA//8A/wD//wBiZCH5BAEAAA8ALAAAAAABAAEAAAQC8EUAOw==") }
+
+  before do
+    SiteSetting.download_remote_images_to_local = false
+    stub_request(:get, hotlinked_url).to_return(body: png, headers: { "Content-Type" => "image/png" })
+    stub_image_size
+  end
+
+  it "normally allows hotlinked images" do
+    post = Fabricate(:post, raw: "<img src='#{hotlinked_url}'>")
+    expect(post.cooked).to have_tag("img", with: { "src" => hotlinked_url })
+  end
+
+  context "with hotlinked media blocked, before post-processing" do
+    before do
+      SiteSetting.block_hotlinked_media = true
+      Oneboxer.stubs(:cached_onebox).returns("<aside class='onebox'><img src='#{hotlinked_url}'></aside>")
+    end
+
+    it "blocks hotlinked images" do
+      post = Fabricate(:post, raw: "<img src='#{hotlinked_url}'>")
+      expect(post.cooked).not_to have_tag("img[src]")
+      expect(post.cooked).to have_tag("img", with: { PrettyText::BLOCKED_HOTLINKED_SRC_ATTR => hotlinked_url })
+    end
+
+    it "blocks hotlinked videos with src" do
+      post = Fabricate(:post, raw: "![alt text|video](#{hotlinked_url})")
+      expect(post.cooked).not_to have_tag("video source[src]")
+      expect(post.cooked).to have_tag("video source", with: { PrettyText::BLOCKED_HOTLINKED_SRC_ATTR => hotlinked_url })
+    end
+
+    it "blocks hotlinked videos with srcset" do
+      srcset = "#{hotlinked_url} 1x,https://example.com 2x"
+      post = Fabricate(:post, raw: "<video><source srcset='#{srcset}'></video>")
+      expect(post.cooked).not_to have_tag("video source[srcset]")
+      expect(post.cooked).to have_tag("video source", with: { PrettyText::BLOCKED_HOTLINKED_SRCSET_ATTR => srcset })
+    end
+
+    it "blocks hotlinked audio" do
+      post = Fabricate(:post, raw: "![alt text|audio](#{hotlinked_url})")
+      expect(post.cooked).not_to have_tag("audio source[src]")
+      expect(post.cooked).to have_tag("audio source", with: { PrettyText::BLOCKED_HOTLINKED_SRC_ATTR => hotlinked_url })
+    end
+
+    it "blocks hotlinked onebox content when cached (post_analyzer)" do
+      post = Fabricate(:post, raw: "#{onebox_url}")
+      expect(post.cooked).not_to have_tag("img[src]")
+      expect(post.cooked).to have_tag("img", with: { PrettyText::BLOCKED_HOTLINKED_SRC_ATTR => hotlinked_url })
+    end
+
+    it "allows relative URLs" do
+      src = "/assets/images/blah.png"
+      post = Fabricate(:post, raw: "![](#{src})")
+      expect(post.cooked).to have_tag("img", with: { src: src })
+    end
+
+    it "allows data URIs" do
+      src = "data:image/png;base64,abcde"
+      post = Fabricate(:post, raw: "![](#{src})")
+      expect(post.cooked).to have_tag("img", with: { src: src })
+    end
+
+    it "allows an exception" do
+      post = Fabricate :post, raw: <<~RAW
+        ![](https://example.com)
+        ![](https://example.com/myimage.png)
+        ![](https://example.com.malicious.com/myimage.png)
+        ![](https://malicious.invalid/https://example.com)
+      RAW
+
+      expect(post.cooked).not_to have_tag("img[src]")
+
+      SiteSetting.block_hotlinked_media_exceptions = "https://example.com"
+
+      post.rebake!
+      post.reload
+      expect(post.cooked).to have_tag("img", with: { "src" => "https://example.com" })
+      expect(post.cooked).to have_tag("img", with: { "src" => "https://example.com/myimage.png" })
+      expect(post.cooked).to have_tag("img", with: { PrettyText::BLOCKED_HOTLINKED_SRC_ATTR => "https://example.com.malicious.com/myimage.png" })
+      expect(post.cooked).to have_tag("img", with: { PrettyText::BLOCKED_HOTLINKED_SRC_ATTR => "https://malicious.invalid/https://example.com" })
+    end
+
+    it "allows multiple exceptions" do
+      post = Fabricate :post, raw: <<~RAW
+        ![](https://example.com)
+        ![](https://exampleb.com/myimage.png)
+      RAW
+
+      expect(post.cooked).not_to have_tag("img[src]")
+
+      SiteSetting.block_hotlinked_media_exceptions = "https://example.com|https://exampleb.com"
+
+      post.rebake!
+      post.reload
+      expect(post.cooked).to have_tag("img", with: { "src" => "https://example.com" })
+      expect(post.cooked).to have_tag("img", with: { "src" => "https://exampleb.com/myimage.png" })
+    end
+  end
+
+  context "with hotlinked media blocked, with post-processing" do
+    before do
+      SiteSetting.block_hotlinked_media = true
+      Jobs.run_immediately!
+      Oneboxer.stubs(:onebox).returns("<aside class='onebox'><img src='#{hotlinked_url}'></aside>")
+    end
+
+    it "renders placeholders for all media types (CookedPostProcessor)" do
+      post = Fabricate :post, raw: <<~RAW
+        <img src='#{hotlinked_url}'>
+
+        ![alt text|video](#{hotlinked_url})
+
+        ![alt text|audio](#{hotlinked_url})
+
+        #{onebox_url}
+      RAW
+      post.rebake!
+      post.reload
+      expect(post.cooked).not_to have_tag("img")
+      expect(post.cooked).not_to have_tag("video")
+      expect(post.cooked).not_to have_tag("audio")
+      expect(post.cooked).to have_tag(
+        "a.blocked-hotlinked-placeholder[href^='http://example.com'][rel='noopener nofollow ugc']",
+        count: 4
+      )
+    end
+  end
+
+  context "with hotlinked media blocked, and download_remote_images_to_local enabled" do
+    before do
+      SiteSetting.block_hotlinked_media = true
+      SiteSetting.download_remote_images_to_local = true
+      Oneboxer.stubs(:onebox).returns("<aside class='onebox'><img src='#{hotlinked_url}'></aside>")
+      Jobs.run_immediately!
+    end
+
+    it "can still download remote images after they're blocked" do
+      post = Fabricate :post, raw: <<~RAW
+        <img src='#{hotlinked_url}'>
+
+        #{onebox_url}
+      RAW
+      post.rebake!
+      post.reload
+      expect(post.uploads.count).to eq(1)
+      upload = post.uploads.first
+      expect(post.cooked).to have_tag("img", count: 2)
+      expect(post.cooked).to have_tag("img[src$=\"#{upload.url}\"]", count: 2)
+    end
+  end
+end