07 July, 2009

The "Does it make sense?" test

I was composing the next installment of my series on building an incident response team and started to include this, but then decided it deserves a separate entry.

Some time ago, my boss came up with what he calls the "Does it make sense?" test as a cheat-sheet for help training new analysts and to use as a quick reference. When we refer to traffic making sense, we are asking whether the traffic is normal for the network.

This is very simple and covers some of the quickest ways an analyst can investigate a possible incident. Consider it a way to triage possible NSM activity or incidents. Using something like this can easily eliminate a lot of unnecessary and time-consuming analysis, or point out when the extra analysis is needed.

The "does it make sense" test:

  1. Determine the direction of the network traffic.
  2. Determine the IP addresses involved.
  3. Determine the locations of the systems (e.g. internal, external, VPN, whois, GeoIP).
  4. Determine the functions of the systems involved (e.g. web server, mail server, workstation).
  5. Determine protocols involved and whether they are "normal" protocols and ports that should be seen between the systems.
  6. When applicable, look at the packet capture and compare it to the signature/rule.
  7. Use historical queries on NSM systems and searches of documentation to determine past events that may be related to the current one.
Based on the above knowledge, does the traffic that caused the alert make sense or is it abnormal? Simple examples:
  1. A file server sending huge amounts of SMTP traffic over port 25 probably does not make sense, whether because of malicious activity or a misconfiguration.
  2. Someone connecting to a workstation on port 21 with FTP probably does not make sense.
  3. A DNS server sending and receiving traffic to another DNS server over port 53 does make sense. However, an analysis of the alert and the DNS traffic may still be needed to verify whether the traffic is malicious or not.
Remember, traffic that makes sense and is normal on one network may not be normal on another network. Having a good baseline of your network traffic is extremely important before you can accurately determine what traffic makes sense and what traffic does not makes sense. Even traffic that does not make sense is not automatically malicious.

Also remember, traffic that makes sense is not always friendly. A good attacker will make his network traffic look like it fits in with the the baseline traffic, making the traffic less likely to stick out.

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