A malicious user with permission to create documents in a database is able to attach a HTML attachment to a document. If a CouchDB admin opens that attachment, any JavaScript code embedded in that HTML attachment will be executed within the security context of that admin. A similar route is available with the already deprecated _show and _list functionality. This privilege escalation vulnerability allows an attacker to add or remove data in any database or make configuration changes.

External Reference:

https://docs.couchdb.org/en/stable/cve/2021-38295.html