Nextcloud Server is a self hosted personal cloud system. The OAuth2 client secrets were stored in a recoverable way, so that an attacker that got access to a backup of the database and the Nextcloud config file, would be able to decrypt them. It is recommended that the Nextcloud Server is upgraded to 28.0.10 or 29.0.7 and Nextcloud Enterprise Server is upgraded to 27.1.11.8, 28.0.10 or 29.0.7.
https://github.com/nextcloud/server/pull/47635
https://github.com/nextcloud/server/commit/09b8aea8f6783514bffe00df6abbf9fa542faac5
https://github.com/nextcloud/security-advisories/security/advisories/GHSA-fvpc-8hq6-jgq2