The server-side scan is an extension to our remote malware/blacklist scan and it runs once per day.
Enabling it is very easy, just follow these steps.
Server Side Scan File
The Sucuri scan file must be located in your root web directory. Once uploaded, the scan file will be accessed by the monitor server using curl over ports 80/443.
If you need to reupload your server-side scan file, go to your site Monitoring Overview -> Settings -> Server Side Scanner to download a copy.
Temporary Scan File
You might notice that during scans, a temporary file is created in your root web directory. The file will be called sucuri-sss-resume-(LONGSTRING).php. This allows the server-side scan resume feature to help overcome hosting resource limitations.
If you are still not sure what do, please feel free to open a support ticket with the Sucuri team.