Improve shared status verification (#2525)

* Instead of parsing shared status contents verbatim, make roundtrip
to purported original URL. Confirm that the "original" URL is from the
same domain as the author it claims to be from.

* Fix obvious typo, add comment

* Use URI look-up first

* Add test, update Goldfinger dependency to make less useless HTTP requests per Webfinger lookup

1 files changed, 11 insertions, 1 deletions

diff --git a/app/services/fetch_remote_status_service.rb b/app/services/fetch_remote_status_service.rb
index c666961ad..5a454808e 100644
--- a/app/services/fetch_remote_status_service.rb
+++ b/app/services/fetch_remote_status_service.rb
@@ -39,9 +39,19 @@ class FetchRemoteStatusService < BaseService
 
     Rails.logger.debug "Going to webfinger #{username}@#{domain}"
 
-    return FollowRemoteAccountService.new.call("#{username}@#{domain}")
+    account = FollowRemoteAccountService.new.call("#{username}@#{domain}")
+
+    # If the author's confirmed URLs do not match the domain of the URL
+    # we are reading this from, abort
+    return nil unless confirmed_domain?(domain, account)
+
+    account
   rescue Nokogiri::XML::XPath::SyntaxError
     Rails.logger.debug 'Invalid XML or missing namespace'
     nil
   end
+
+  def confirmed_domain?(domain, account)
+    domain.casecmp(account.domain).zero? || domain.casecmp(Addressable::URI.parse(account.remote_url).normalize.host).zero?
+  end
 end