DiscoTOC is a Discourse theme component that generates a table of contents for topics. Users that can create topics in TOC-enabled categories (and have sufficient trust level - configured in component's settings) are able to inject arbitrary HTML on that topic's page. The issue has been fixed on the `main` branch. Admins can update the theme component through the admin UI (Customize -> Themes -> Components -> DiscoTOC -> Check for Updates). Alternatively, admins can temporarily disable the DiscoTOC theme component.