Nextcloud Server is a self hosted personal cloud system. After an attacker got access to the session of a user or administrator, the attacker would be able to create, change or delete external storages without having to confirm the password. It is recommended that the Nextcloud Server is upgraded to 28.0.12, 29.0.9 or 30.0.2.
https://hackerone.com/reports/2602973
https://github.com/nextcloud/server/pull/48992
https://github.com/nextcloud/server/pull/48788
https://github.com/nextcloud/server/pull/48373
https://github.com/nextcloud/security-advisories/security/advisories/GHSA-vrhf-532w-99rg