diff --git a/app/helpers/jsonld_helper.rb b/app/helpers/jsonld_helper.rb
index 6c7c38070a..9530ad9f30 100644
--- a/app/helpers/jsonld_helper.rb
+++ b/app/helpers/jsonld_helper.rb
@@ -39,6 +39,10 @@ module JsonLdHelper
     !json.nil? && equals_or_includes?(json['@context'], ActivityPub::TagManager::CONTEXT)
   end
 
+  def unsupported_uri_scheme?(uri)
+    !uri.start_with?('http://', 'https://')
+  end
+
   def canonicalize(json)
     graph = RDF::Graph.new << JSON::LD::API.toRdf(json)
     graph.dump(:normalize)
diff --git a/app/javascript/mastodon/features/compose/index.js b/app/javascript/mastodon/features/compose/index.js
index c3e936ab9b..3f75a218d0 100644
--- a/app/javascript/mastodon/features/compose/index.js
+++ b/app/javascript/mastodon/features/compose/index.js
@@ -94,7 +94,7 @@ export default class Compose extends React.PureComponent {
           <div className='drawer__inner' onFocus={this.onFocus}>
             <NavigationContainer onClose={this.onBlur} />
             <ComposeFormContainer />
-            <div className='mastodon' />
+            {multiColumn && <div className='mastodon' />}
           </div>
 
           <Motion defaultStyle={{ x: -100 }} style={{ x: spring(showSearch ? 0 : -100, { stiffness: 210, damping: 20 }) }}>
diff --git a/app/lib/activitypub/activity/create.rb b/app/lib/activitypub/activity/create.rb
index 3a985c19b6..64c4294202 100644
--- a/app/lib/activitypub/activity/create.rb
+++ b/app/lib/activitypub/activity/create.rb
@@ -1,11 +1,11 @@
 # frozen_string_literal: true
 
 class ActivityPub::Activity::Create < ActivityPub::Activity
-  SUPPORTED_TYPES = %w(Article Note).freeze
-  CONVERTED_TYPES = %w(Image Video).freeze
+  SUPPORTED_TYPES = %w(Note).freeze
+  CONVERTED_TYPES = %w(Image Video Article).freeze
 
   def perform
-    return if delete_arrived_first?(object_uri) || unsupported_object_type?
+    return if delete_arrived_first?(object_uri) || unsupported_object_type? || invalid_origin?(@object['id'])
 
     RedisLock.acquire(lock_options) do |lock|
       if lock.acquired?
@@ -213,7 +213,14 @@ class ActivityPub::Activity::Create < ActivityPub::Activity
 
   def object_url
     return if @object['url'].blank?
-    url_to_href(@object['url'], 'text/html')
+
+    url_candidate = url_to_href(@object['url'], 'text/html')
+
+    if invalid_origin?(url_candidate)
+      nil
+    else
+      url_candidate
+    end
   end
 
   def content_language_map?
@@ -245,6 +252,15 @@ class ActivityPub::Activity::Create < ActivityPub::Activity
     @skip_download ||= DomainBlock.find_by(domain: @account.domain)&.reject_media?
   end
 
+  def invalid_origin?(url)
+    return true if unsupported_uri_scheme?(url)
+
+    needle   = Addressable::URI.parse(url).host
+    haystack = Addressable::URI.parse(@account.uri).host
+
+    !haystack.casecmp(needle).zero?
+  end
+
   def reply_to_local?
     !replied_to_status.nil? && replied_to_status.account.local?
   end
diff --git a/app/services/activitypub/process_account_service.rb b/app/services/activitypub/process_account_service.rb
index 0fbf18c001..f43edafe7b 100644
--- a/app/services/activitypub/process_account_service.rb
+++ b/app/services/activitypub/process_account_service.rb
@@ -6,7 +6,7 @@ class ActivityPub::ProcessAccountService < BaseService
   # Should be called with confirmed valid JSON
   # and WebFinger-resolved username and domain
   def call(username, domain, json)
-    return if json['inbox'].blank?
+    return if json['inbox'].blank? || unsupported_uri_scheme?(json['id'])
 
     @json        = json
     @uri         = @json['id']
@@ -107,7 +107,21 @@ class ActivityPub::ProcessAccountService < BaseService
 
   def url
     return if @json['url'].blank?
-    url_to_href(@json['url'], 'text/html')
+
+    url_candidate = url_to_href(@json['url'], 'text/html')
+
+    if unsupported_uri_scheme?(url_candidate) || mismatching_origin?(url_candidate)
+      nil
+    else
+      url_candidate
+    end
+  end
+
+  def mismatching_origin?(url)
+    needle   = Addressable::URI.parse(url).host
+    haystack = Addressable::URI.parse(@uri).host
+
+    !haystack.casecmp(needle).zero?
   end
 
   def outbox_total_items
diff --git a/spec/lib/activitypub/activity/create_spec.rb b/spec/lib/activitypub/activity/create_spec.rb
index 3c3991c13f..51f54a398e 100644
--- a/spec/lib/activitypub/activity/create_spec.rb
+++ b/spec/lib/activitypub/activity/create_spec.rb
@@ -6,7 +6,7 @@ RSpec.describe ActivityPub::Activity::Create do
   let(:json) do
     {
       '@context': 'https://www.w3.org/ns/activitystreams',
-      id: 'foo',
+      id: [ActivityPub::TagManager.instance.uri_for(sender), '#foo'].join,
       type: 'Create',
       actor: ActivityPub::TagManager.instance.uri_for(sender),
       object: object_json,
@@ -16,6 +16,8 @@ RSpec.describe ActivityPub::Activity::Create do
   subject { described_class.new(json, sender) }
 
   before do
+    sender.update(uri: ActivityPub::TagManager.instance.uri_for(sender))
+
     stub_request(:get, 'http://example.com/attachment.png').to_return(request_fixture('avatar.txt'))
     stub_request(:get, 'http://example.com/emoji.png').to_return(body: attachment_fixture('emojo.png'))
   end
@@ -28,7 +30,7 @@ RSpec.describe ActivityPub::Activity::Create do
     context 'standalone' do
       let(:object_json) do
         {
-          id: 'bar',
+          id: [ActivityPub::TagManager.instance.uri_for(sender), '#bar'].join,
           type: 'Note',
           content: 'Lorem ipsum',
         }
@@ -52,7 +54,7 @@ RSpec.describe ActivityPub::Activity::Create do
     context 'public' do
       let(:object_json) do
         {
-          id: 'bar',
+          id: [ActivityPub::TagManager.instance.uri_for(sender), '#bar'].join,
           type: 'Note',
           content: 'Lorem ipsum',
           to: 'https://www.w3.org/ns/activitystreams#Public',
@@ -70,7 +72,7 @@ RSpec.describe ActivityPub::Activity::Create do
     context 'unlisted' do
       let(:object_json) do
         {
-          id: 'bar',
+          id: [ActivityPub::TagManager.instance.uri_for(sender), '#bar'].join,
           type: 'Note',
           content: 'Lorem ipsum',
           cc: 'https://www.w3.org/ns/activitystreams#Public',
@@ -88,7 +90,7 @@ RSpec.describe ActivityPub::Activity::Create do
     context 'private' do
       let(:object_json) do
         {
-          id: 'bar',
+          id: [ActivityPub::TagManager.instance.uri_for(sender), '#bar'].join,
           type: 'Note',
           content: 'Lorem ipsum',
           to: 'http://example.com/followers',
@@ -108,7 +110,7 @@ RSpec.describe ActivityPub::Activity::Create do
 
       let(:object_json) do
         {
-          id: 'bar',
+          id: [ActivityPub::TagManager.instance.uri_for(sender), '#bar'].join,
           type: 'Note',
           content: 'Lorem ipsum',
           to: ActivityPub::TagManager.instance.uri_for(recipient),
@@ -128,7 +130,7 @@ RSpec.describe ActivityPub::Activity::Create do
 
       let(:object_json) do
         {
-          id: 'bar',
+          id: [ActivityPub::TagManager.instance.uri_for(sender), '#bar'].join,
           type: 'Note',
           content: 'Lorem ipsum',
           inReplyTo: ActivityPub::TagManager.instance.uri_for(original_status),
@@ -151,7 +153,7 @@ RSpec.describe ActivityPub::Activity::Create do
 
       let(:object_json) do
         {
-          id: 'bar',
+          id: [ActivityPub::TagManager.instance.uri_for(sender), '#bar'].join,
           type: 'Note',
           content: 'Lorem ipsum',
           tag: [
@@ -174,7 +176,7 @@ RSpec.describe ActivityPub::Activity::Create do
     context 'with mentions missing href' do
       let(:object_json) do
         {
-          id: 'bar',
+          id: [ActivityPub::TagManager.instance.uri_for(sender), '#bar'].join,
           type: 'Note',
           content: 'Lorem ipsum',
           tag: [
@@ -194,7 +196,7 @@ RSpec.describe ActivityPub::Activity::Create do
     context 'with media attachments' do
       let(:object_json) do
         {
-          id: 'bar',
+          id: [ActivityPub::TagManager.instance.uri_for(sender), '#bar'].join,
           type: 'Note',
           content: 'Lorem ipsum',
           attachment: [
@@ -218,7 +220,7 @@ RSpec.describe ActivityPub::Activity::Create do
     context 'with media attachments missing url' do
       let(:object_json) do
         {
-          id: 'bar',
+          id: [ActivityPub::TagManager.instance.uri_for(sender), '#bar'].join,
           type: 'Note',
           content: 'Lorem ipsum',
           attachment: [
@@ -239,7 +241,7 @@ RSpec.describe ActivityPub::Activity::Create do
     context 'with hashtags' do
       let(:object_json) do
         {
-          id: 'bar',
+          id: [ActivityPub::TagManager.instance.uri_for(sender), '#bar'].join,
           type: 'Note',
           content: 'Lorem ipsum',
           tag: [
@@ -263,7 +265,7 @@ RSpec.describe ActivityPub::Activity::Create do
     context 'with hashtags missing name' do
       let(:object_json) do
         {
-          id: 'bar',
+          id: [ActivityPub::TagManager.instance.uri_for(sender), '#bar'].join,
           type: 'Note',
           content: 'Lorem ipsum',
           tag: [
@@ -284,7 +286,7 @@ RSpec.describe ActivityPub::Activity::Create do
     context 'with emojis' do
       let(:object_json) do
         {
-          id: 'bar',
+          id: [ActivityPub::TagManager.instance.uri_for(sender), '#bar'].join,
           type: 'Note',
           content: 'Lorem ipsum :tinking:',
           tag: [
@@ -310,7 +312,7 @@ RSpec.describe ActivityPub::Activity::Create do
     context 'with emojis missing name' do
       let(:object_json) do
         {
-          id: 'bar',
+          id: [ActivityPub::TagManager.instance.uri_for(sender), '#bar'].join,
           type: 'Note',
           content: 'Lorem ipsum :tinking:',
           tag: [
@@ -333,7 +335,7 @@ RSpec.describe ActivityPub::Activity::Create do
     context 'with emojis missing icon' do
       let(:object_json) do
         {
-          id: 'bar',
+          id: [ActivityPub::TagManager.instance.uri_for(sender), '#bar'].join,
           type: 'Note',
           content: 'Lorem ipsum :tinking:',
           tag: [
diff --git a/spec/services/activitypub/fetch_remote_status_service_spec.rb b/spec/services/activitypub/fetch_remote_status_service_spec.rb
index ad26abc5b2..a533e8413a 100644
--- a/spec/services/activitypub/fetch_remote_status_service_spec.rb
+++ b/spec/services/activitypub/fetch_remote_status_service_spec.rb
@@ -21,6 +21,8 @@ RSpec.describe ActivityPub::FetchRemoteStatusService do
 
   describe '#call' do
     before do
+      sender.update(uri: ActivityPub::TagManager.instance.uri_for(sender))
+
       stub_request(:head, 'https://example.com/watch?v=12345').to_return(status: 404, body: '')
       subject.call(object[:id], prefetched_body: Oj.dump(object))
     end
@@ -48,13 +50,13 @@ RSpec.describe ActivityPub::FetchRemoteStatusService do
             {
               type: 'Link',
               mimeType: 'application/x-bittorrent',
-              href: 'https://example.com/12345.torrent',
+              href: "https://#{valid_domain}/12345.torrent",
             },
 
             {
               type: 'Link',
               mimeType: 'text/html',
-              href: 'https://example.com/watch?v=12345',
+              href: "https://#{valid_domain}/watch?v=12345",
             },
           ],
         }
@@ -64,8 +66,8 @@ RSpec.describe ActivityPub::FetchRemoteStatusService do
         status = sender.statuses.first
 
         expect(status).to_not be_nil
-        expect(status.url).to eq 'https://example.com/watch?v=12345'
-        expect(strip_tags(status.text)).to eq 'Nyan Cat 10 hours remix https://example.com/watch?v=12345'
+        expect(status.url).to eq "https://#{valid_domain}/watch?v=12345"
+        expect(strip_tags(status.text)).to eq "Nyan Cat 10 hours remix https://#{valid_domain}/watch?v=12345"
       end
     end
   end