Fix possible acct: uri usurpation in ActivityPub account discovery (#5208)

Signed-off-by: Eugen Rochko <eugen@zeonfederated.com>

app/services/activitypub/fetch_remote_account_service.rb

@@ -30,14 +30,12 @@ class ActivityPub::FetchRemoteAccountService < BaseService
     return true if @username.casecmp(confirmed_username).zero? && @domain.casecmp(confirmed_domain).zero?
 
     webfinger                            = Goldfinger.finger("acct:#{confirmed_username}@#{confirmed_domain}")
-    confirmed_username, confirmed_domain = split_acct(webfinger.subject)
+    @username, @domain                   = split_acct(webfinger.subject)
     self_reference                       = webfinger.link('self')
 
+    return false unless @username.casecmp(confirmed_username).zero? && @domain.casecmp(confirmed_domain).zero?
     return false if self_reference&.href != @uri
 
-    @username = confirmed_username
-    @domain   = confirmed_domain
-
     true
   rescue Goldfinger::Error
     false