I'm having a bit of a problem getting my head round what this vulnerability means, can someone help me understand this?
I'm particularly puzzled by the RESULTS section. Why would source port 25 be any different from a random source port, they're both originating from the outside world?
TCP Source Port Pass Firewall
Your firewall policy seems to let TCP packets with a specific source port pass through.
Some types of requests can pass through the firewall. The port number listed in the results section of this vulnerability report is the source port that unauthorized users can use to bypass your firewall.
Make sure that all your filtering rules are correct and strict enough. If the firewall intends to deny TCP connections to a specific port, it should be configured to block all TCP SYN packets going to this port, regardless of the source port.
The host responded 4 times to 4 TCP SYN probes sent to destination port 22 using source port 25. However, it did not respond at all to 4 TCP SYN probes sent to the same destination port using a random source port.