The Discourse Code Review Plugin allows users to review GitHub commits on Discourse. Prior to commit eed3a80, an attacker can execute arbitrary JavaScript on users' browsers by posting links to malicious GitHub commits. This problem is patched in commit eed3a80 of the discourse-code-review plugin. As a workaround, one may disable the plugin.
https://github.com/discourse/discourse-code-review/security/advisories/GHSA-358v-cwvc-gxh5
https://github.com/discourse/discourse-code-review/commit/eed3a801f8fee217fe782212d8950eb1bd236e43