Telegram Desktop before 6.9.4 allows XSS in the HTML exporter. (The first fixed stable version is 7.0.1.) This occurs in button.text.toUtf8 in export_output_html.cpp. Exploitation cannot occur unless HTML export was used by a victim. However, the exploit payload can be exported if a message were forwarded into a group by a member (it is not necessary for the message author to be a member of a group).
Analyzing vulnerability vectors…