There are 4 open security issues in buster.
4 issues left for the package maintainer to handle:
A cross-site scripting error in Nextcloud Desktop client 2.6.4 allowed to present any html (including local links) when responding with invalid data on the login attempt.
A cleartext storage of sensitive information in Nextcloud Desktop Client 2.6.4 gave away information about used proxies and their authentication credentials.
Missing sanitization of a server response in Nextcloud Desktop Client 2.6.4 for Linux allowed a malicious Nextcloud Server to store files outside of the dedicated sync directory.
Nextcloud Desktop Client prior to 3.1.3 is vulnerable to resource injection by way of missing validation of URLs, allowing a malicious server to execute remote commands. User interaction is needed for exploitation.
You can find information about how to handle these issues in the security team's documentation.