Occam’s Razor: A principle that generally recommends that, from among competing hypotheses, selecting the one that makes the fewest new assumptions usually provides the correct one, and that the simplest explanation will be the most plausible until evidence is presented to prove it false.  [Wikipedia]

Intrusion analysts face a unique problem: discovering and tracking adversaries not wanting to be discovered or tracked.  These adversaries will take significant action to prevent their operations from being discovered [see Insertion, Evasion, and Denial of Service: Eluding Network Intrusion Detection for examples].  Metasploit, a common exploitation framework, works hard to prevent their tool and modules from detection and signature including creating an Secure Sockets Layer (SSL) encrypted command-and-control channel.

In this environment, the cyber adversary is using Occam’s Razor against the analyst to elude detection.  Given a set of network traffic or events, an adversary wants any unintended observer to believe their traffic is normal and benign.  An adversary practices Denial and Deception in every packet.

Therefore, if an intrusion analyst relies too heavily on Occam’s Razor to describe a network event as benign, but a clever adversary is hiding their activities in that traffic – the adversary wins.

On the other hand, if an analyst does not employ Occam’s Razor effectively in their work, every packet will look suspicious wasting their precious time on unimportant events.

Richards Heuer’s preeminent work on the Psychology of Intelligence Analysis describes a very effective framework for employing the theory of Competing Hypothesis to decide which possible conclusion is the best given a set of facts and assumptions.  However, a common attack on Heuer’s framework is that an adversary utilizing Denial and Deception can easily defeat the analyst and have them select the conclusion the adversary wishes and not the one best describing the activity.

In the end, an intrusion analyst must use both a questioning and suspicious mind with a modicum of Occam’s Razor.