In Drupal 7 versions prior to 7.65; Drupal 8.6 versions prior to 8.6.13;Drupal 8.5 versions prior to 8.5.14. Under certain circumstances the File module/subsystem allows a malicious user to upload a file that can trigger a cross-site scripting (XSS) vulnerability.
https://www.synology.com/security/advisory/Synology_SA_19_13
https://www.drupal.org/sa-core-2019-004
https://lists.debian.org/debian-lts-announce/2019/04/msg00003.html