In the nggallery-manage-gallery page in the Photocrati NextGEN Gallery plugin 2.1.15 for WordPress, XSS is possible for remote authenticated administrators via the images[1][alttext] parameter.
https://github.com/cybersecurityworks/Disclosed/issues/5
https://euvd.enisa.europa.eu/vulnerability/EUVD-2015-9082
https://cybersecurityworks.com/zerodays/cve-2015-9229-nextgen-gallery.html