Friday, September 27, 2019

Security: Excessive Inbound Connections Denied by Firewalls

This rule is intended for organizations that are concerned with monitoring heavy inbound network traffic.

Description
This alert rule is triggered whenever an excessive denied inbound connections across a firewall is noticed. This rule finds host machines of potential intruders and also detects if a particular user is trying, and subsequently failing, to access a resource inside a firewall.

Log Source

  • All Firewalls
  • IDS/IPS


Alert Logic
Monitor Firewall/IDS/IPS event that happens to deny a connection.
Create a list of FirewallDevices so that update in list will ensure that this rule will work with any new firewall device that may be added in future. Also, create a baseline of average denied connection to your infrastructure. If connection attempt seems to increase by let's say 20% trigger this alert.

What should you monitor in case this alert triggers?
  • Check the source IP address. Validate if this traffic is expected one or needs further investigation.
  • Analyze the source IP addresses and destination ports. Multiple source IP addresses
    with similar destination ports could indicate malicious activity.

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