The remote proxy server can be used to connect to arbitrary ports.
The proxy allows everyone to perform HTTP requests to arbitrary ports, such as:
GET http://www.acunetix.com:25This may allow attackers to bypass your firewall and connect to sensitive ports like 23 (telnet), 25 (sendmail) using the proxy.
Restrict proxy access to valid users and/or hosts. Deny access to non-authorized ports.