In Wikibase Wikidata Query Service GUI before 0.3.6-SNAPSHOT 2019-11-07, when mathematical expressions in results are displayed directly, arbitrary JavaScript execution can occur, aka XSS.
ui/ResultView.js in Wikibase Wikidata Query Service GUI before 0.3.6-SNAPSHOT 2019-11-07 allows HTML injection when reporting the number of results and number of milliseconds.