Probable TCP NULL Firewall Alert

We have a third party company that scans and alerts us of our syslogging however once in a while something comes up that I can't really describe.

Probable TCP NULL scan detected
System Affected(DIP):
         UID                IP                   SIP                DIP              Dport
STLinternet
      10.2.2.25
       10.1.1.77
      10.2.2.25
      80

10.2.2.25 is our hardware firewall. 10.1.1.77 is our SIEM EIQ. Could someone help me make sense of this alert?
LVL 2
stlhostAsked:
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btanExec ConsultantCommented:
Typical of an attacker doing this TCP NULL scan is to determine if ports are closed on the target machine. These TCP segments does not have flags in the packet header and is actually illegal packets generated. In a production environment, there will never be a TCP packet that doesn’t contain a flag.

The expected result of a Null Scan on an open port is no response. Since there are no flags set, the target will not know how to handle the request. It will discard the packet and no reply will be sent. If the port is closed, the target will send an RST packet in response.

FW will picked up and alert upon detecting this attempt and likely the scan is spawning the list of IPs...there is no compromise (yet) as it is fingeprinting and identify which is the best target to penetrate subseq. good to find out the source sending these and if need to blacklist if the scan period is long and peristent and src ip is rather static. Check that PC is the src IP address belong to the certain user segment - hopefully not a malware or can be false pos if someone is performing some testing...
 
More info on null scan - https://capec.mitre.org/data/definitions/304.html

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stlhostAuthor Commented:
tyvm
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