Discourse is an open source platform for community discussion. This vulnerability is not exploitable on the default install of Discourse. A custom feature must be enabled for it to work at all, and the attacker’s payload must pass the CSP to be executed.

However, if an attacker succeeds in embedding Javascript that does pass the CSP, it could result in session hijacking for any users that view the attacker’s post. The vulnerability is patched in the latest tests-passed, beta and stable branches. Users are advised to upgrade.

Users unable to upgrade should enable and/or restore your site's CSP to the default one provided with Discourse. Remove any embed-able hosts configured.