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I have been using the same computer with the same isp and firewall for quite some time now. However since the past 2 days I kepp getting atleast one alert log telling me that a fin scan is underway. It gives me the remote ip too and the network isnt even on my isp. Why is this happening so often. I am using http://www.f-secure.com/en/web/home_global

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Some more info would be good: do you have a virusscanner installed ? Malware scanner ? Which one ? –  pleinolijf Jan 28 '13 at 12:27
    
I am using f-secure.com/en/web/home_global –  Sab Jan 28 '13 at 12:28
    
Please edit your original question to incorporate this information. Thanks. –  pleinolijf Jan 28 '13 at 12:30
    
Possibly related: superuser.com/questions/293642/… –  pleinolijf Jan 28 '13 at 13:41
    
Checked that already. The remote ip is the local ip. –  Sab Jan 28 '13 at 15:22

1 Answer 1

TCP FIN Scan

Hackers use the TCP FIN scan to identify listening TCP port numbers based on how the target device reacts to a transaction close request for a TCP port (even though no connection may exist before these close requests are made). This type of scan can get through basic firewalls and boundary routers that filter on incoming TCP packets with the Finish (FIN) and ACK flag combination. The TCP packets used in this scan include only the TCP FIN flag setting.

If the target device's TCP port is closed, the target device sends a TCP RST packet in reply. If the target device's TCP port is open, the target device discards the FIN and sends no reply.

Scan your computer for backdoors, as there is a good chance some piece of malware is calling home. Malwarebytes' Anti-Malware is pretty good. Do a full scan.

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I checked a little more and the port is the port on which skype is running and I also observed that this behaviour is shown only when I have skype running. Any ideas why. I did a full scan but it could not find anything –  Sab Jan 29 '13 at 5:54

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