Collabora Online is a collaborative online office suite based on LibreOffice technology. Users of Nextcloud with `Collabora Online - Built-in CODE Server` app can be vulnerable to attack via proxy.php. The bug was fixed in Collabora Online - Built-in CODE Server (richdocumentscode) release 23.5.601. Users are advised to upgrade. There are no known workarounds for this vulnerability.
https://github.com/CollaboraOnline/online/security/advisories/GHSA-8xm5-pgfr-8mjr