diff --git a/app/lib/admin/system_check.rb b/app/lib/admin/system_check.rb
index f512635abb..89dfcef9f1 100644
--- a/app/lib/admin/system_check.rb
+++ b/app/lib/admin/system_check.rb
@@ -2,6 +2,7 @@
 
 class Admin::SystemCheck
   ACTIVE_CHECKS = [
+    Admin::SystemCheck::MediaPrivacyCheck,
     Admin::SystemCheck::DatabaseSchemaCheck,
     Admin::SystemCheck::SidekiqProcessCheck,
     Admin::SystemCheck::RulesCheck,
diff --git a/app/lib/admin/system_check/media_privacy_check.rb b/app/lib/admin/system_check/media_privacy_check.rb
new file mode 100644
index 0000000000..1df05b120e
--- /dev/null
+++ b/app/lib/admin/system_check/media_privacy_check.rb
@@ -0,0 +1,105 @@
+# frozen_string_literal: true
+
+class Admin::SystemCheck::MediaPrivacyCheck < Admin::SystemCheck::BaseCheck
+  include RoutingHelper
+
+  def skip?
+    !current_user.can?(:view_devops)
+  end
+
+  def pass?
+    check_media_uploads!
+    @failure_message.nil?
+  end
+
+  def message
+    Admin::SystemCheck::Message.new(@failure_message, @failure_value, @failure_action, true)
+  end
+
+  private
+
+  def check_media_uploads!
+    if Rails.configuration.x.use_s3
+      check_media_listing_inaccessible_s3!
+    else
+      check_media_listing_inaccessible!
+    end
+  end
+
+  def check_media_listing_inaccessible!
+    full_url = full_asset_url(media_attachment.file.url(:original, false))
+
+    # Check if we can list the uploaded file. If true, that's an error
+    directory_url = Addressable::URI.parse(full_url)
+    directory_url.query = nil
+    filename = directory_url.path.gsub(%r{.*/}, '')
+    directory_url.path = directory_url.path.gsub(%r{/[^/]+\Z}, '/')
+    Request.new(:get, directory_url, allow_local: true).perform do |res|
+      if res.truncated_body&.include?(filename)
+        @failure_message = use_storage? ? :upload_check_privacy_error_object_storage : :upload_check_privacy_error
+        @failure_action = 'https://docs.joinmastodon.org/admin/optional/object-storage/#FS'
+      end
+    end
+  rescue
+    nil
+  end
+
+  def check_media_listing_inaccessible_s3!
+    urls_to_check = []
+    paperclip_options = Paperclip::Attachment.default_options
+    s3_protocol = paperclip_options[:s3_protocol]
+    s3_host_alias = paperclip_options[:s3_host_alias]
+    s3_host_name  = paperclip_options[:s3_host_name]
+    bucket_name = paperclip_options.dig(:s3_credentials, :bucket)
+
+    urls_to_check << "#{s3_protocol}://#{s3_host_alias}/" if s3_host_alias.present?
+    urls_to_check << "#{s3_protocol}://#{s3_host_name}/#{bucket_name}/"
+    urls_to_check.uniq.each do |full_url|
+      check_s3_listing!(full_url)
+      break if @failure_message.present?
+    end
+  rescue
+    nil
+  end
+
+  def check_s3_listing!(full_url)
+    bucket_url = Addressable::URI.parse(full_url)
+    bucket_url.path = bucket_url.path.delete_suffix(media_attachment.file.path(:original))
+    bucket_url.query = "max-keys=1&x-random=#{SecureRandom.hex(10)}"
+    Request.new(:get, bucket_url, allow_local: true).perform do |res|
+      if res.truncated_body&.include?('ListBucketResult')
+        @failure_message = :upload_check_privacy_error_object_storage
+        @failure_action  = 'https://docs.joinmastodon.org/admin/optional/object-storage/#S3'
+      end
+    end
+  end
+
+  def media_attachment
+    @media_attachment ||= begin
+      attachment = Account.representative.media_attachments.first
+      if attachment.present?
+        attachment.touch # rubocop:disable Rails/SkipsModelValidations
+        attachment
+      else
+        create_test_attachment!
+      end
+    end
+  end
+
+  def create_test_attachment!
+    Tempfile.create(%w(test-upload .jpg), binmode: true) do |tmp_file|
+      tmp_file.write(
+        Base64.decode64(
+          '/9j/4QAiRXhpZgAATU0AKgAAAAgAAQESAAMAAAABAAYAAAA' \
+          'AAAD/2wCEAAEBAQEBAQEBAQEBAQEBAQEBAQEBAQEBAQEBAQEBAQEBAQEBAQEBAQEBAQEBA' \
+          'QEBAQEBAQEBAQEBAQEBAQEBAQEBAQEBAQEBAQEBAQEBAQEBAQEBAQEBAQEBAQEBAQEBAQE' \
+          'BAQEBAQEBAQEBAQEBAQEBAQEBAQEBAQEBAQEBAQEBAf/AABEIAAEAAgMBEQACEQEDEQH/x' \
+          'ABKAAEAAAAAAAAAAAAAAAAAAAALEAEAAAAAAAAAAAAAAAAAAAAAAQEAAAAAAAAAAAAAAAA' \
+          'AAAAAEQEAAAAAAAAAAAAAAAAAAAAA/9oADAMBAAIRAxEAPwA/8H//2Q=='
+        )
+      )
+      tmp_file.flush
+      Account.representative.media_attachments.create!(file: tmp_file)
+    end
+  end
+end
diff --git a/app/lib/admin/system_check/message.rb b/app/lib/admin/system_check/message.rb
index bfcad3bf3d..ad8d4b6073 100644
--- a/app/lib/admin/system_check/message.rb
+++ b/app/lib/admin/system_check/message.rb
@@ -1,11 +1,12 @@
 # frozen_string_literal: true
 
 class Admin::SystemCheck::Message
-  attr_reader :key, :value, :action
+  attr_reader :key, :value, :action, :critical
 
-  def initialize(key, value = nil, action = nil)
-    @key    = key
-    @value  = value
-    @action = action
+  def initialize(key, value = nil, action = nil, critical = false)
+    @key      = key
+    @value    = value
+    @action   = action
+    @critical = critical
   end
 end
diff --git a/app/views/admin/dashboard/index.html.haml b/app/views/admin/dashboard/index.html.haml
index 8354f0b9f5..425472abde 100644
--- a/app/views/admin/dashboard/index.html.haml
+++ b/app/views/admin/dashboard/index.html.haml
@@ -12,7 +12,7 @@
 - unless @system_checks.empty?
   .flash-message-stack
     - @system_checks.each do |message|
-      .flash-message.warning
+      .flash-message{ class: message.critical ? 'alert' : 'warning' }
         = t("admin.system_checks.#{message.key}.message_html", value: message.value ? content_tag(:strong, message.value) : nil)
         - if message.action
           = link_to t("admin.system_checks.#{message.key}.action"), message.action
diff --git a/config/locales/en.yml b/config/locales/en.yml
index 6332f0f01d..6f246f5fd5 100644
--- a/config/locales/en.yml
+++ b/config/locales/en.yml
@@ -756,6 +756,12 @@ en:
         message_html: You haven't defined any server rules.
       sidekiq_process_check:
         message_html: No Sidekiq process running for the %{value} queue(s). Please review your Sidekiq configuration
+      upload_check_privacy_error:
+        action: Check here for more information
+        message_html: "<strong>Your web server is misconfigured. The privacy of your users is at risk.</strong>"
+      upload_check_privacy_error_object_storage:
+        action: Check here for more information
+        message_html: "<strong>Your object storage is misconfigured. The privacy of your users is at risk.</strong>"
     tags:
       review: Review status
       updated_msg: Hashtag settings updated successfully