Fix invalid blurhash handling in Create activity (#16583)

app/lib/activitypub/activity/create.rb

@@ -446,10 +446,14 @@ class ActivityPub::Activity::Create < ActivityPub::Activity
   end
 
   def supported_blurhash?(blurhash)
-    components = blurhash.blank? ? nil : Blurhash.components(blurhash)
+    components = blurhash.blank? || !blurhash_valid_chars?(blurhash) ? nil : Blurhash.components(blurhash)
     components.present? && components.none? { |comp| comp > 5 }
   end
 
+  def blurhash_valid_chars?(blurhash)
+    /^[\w#$%*+-.:;=?@\[\]^{|}~]+$/.match?(blurhash)
+  end
+
   def skip_download?
     return @skip_download if defined?(@skip_download)