Database resource exhaustion for logged-in users via sharee recommendations with circles

How it was found

Summary:

Registered users can generate massive database load

Steps To Reproduce:

1. create 9 circles and 6 folders (circles * folder > 50)
2. share all created folders with all created circles
3. open an other folder and open the share tab, so the URI /ocs/v2.php/apps/files_sharing/api/v1/sharees_recommended is requested
4. this requests results in a loop that runs as long as the php value max_execution_time is set; the recommended value for this is 3600 seconds (1h)
5. a small number of these requests will stress even large servers

Tested with Nextcloud 23.0.8

Impact

Attacker slow down the system by generating a lot of database/cpu load.