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what it means it shows all the time ,what can i do to prevent this.
Security Services Alert Probable TCP FIN scan detected
the above log indicates that there might be a possible computer which may be scanning for FIN Packets (a type of TCP Flag).
A FIN scan is a type of Port Scan to detect whether specific ports are open/closed on a target machine and it can be used to attack the target.
I would recommend you to check on the source of the FIN Scan (you can see this in the firewall logs) and investigate further.
Note: sometimes this can be a false positive but it's always better to investigate and double check.