Inline footnotes wrapped in <a> tags can cause errors in discourse-footnotes

Vulnerability Description

discourse-footnote is a library providing footnotes for posts in Discourse. ### Impact When posting an inline footnote wrapped in `<a>` tags (e.g. `<a>^[footnote]</a>`, the resulting rendered HTML would include a nested `<a>`, which is stripped by Nokogiri because it is not valid. This then caused a javascript error on topic pages because we were looking for an `<a>` element inside the footnote reference span and getting its ID, and because it did not exist we got a null reference error in javascript. Users are advised to update to version 0.2. As a workaround editing offending posts from the rails console or the database console for self-hosters, or disabling the plugin in the admin panel can mitigate this issue.

References

• https://github.com/discourse/discourse-footnote/security/advisories/GHSA-58vr-c56v-qr57
• https://github.com/discourse/discourse-footnote/commit/796617e0131277011207541313522cd1946661ab