Nextcloud server is an open source self hosted cloud style services platform. In affected versions an attacker can cause a denial of service by uploading specially crafted files which will cause the server to allocate too much memory / CPU. It is recommended that the Nextcloud Server is upgraded to 21.0.8 22.2.4 or 23.0.1. Users unable to upgrade should disable preview generation with the `'enable_previews'` config flag.