People often try to draw analogies between computer security and the military or warfare. Lets put aside for a moment the fact that I don't know anything about the military and continue on with this analogy.
Ask yourself for a moment: "What does the average person in the military spend their time doing?" And the answer I believe is training, drilling and exercising. They don't spend the vast majority of their time in heated battle. In fact only small spurts of time, I'd imagine, are spent that way.
Does your defence team spend all its time engaged in cyber battle? If not do they spend most of their time training, exercising and practising for future incidents? If not why not?
In my experience most defensive teams are in meetings, playing with tools, creating presentations, maintaining systems or perhaps doing some ad hoc analysis. Occasionally they might be engaged in research.
It is my belief that much like soldiers, these teams should spend a large majority of their time in training. And the best way to do this training is to have an outside entity play the adversary much like the Airforce Aggressor Squadrons.
From wikipedia: "Aggressor squadrons use enemy tactics, techniques, and procedures to give a realistic simulation of air combat (as opposed to training against one's own forces)." (http://en.wikipedia.org/wiki/Aggressor_squadron)
Traditional penetration testing does NOT use enemy tactics, techniques and procedures. Penetration testing in general these days is simply patch management verification. Penetration testing often focuses on known exploits and real attackers do not. Attackers either use 0days, complex configuration/design issues or malware.
What's nice about the computer security realm is that it is much easier to replicate adversary "equipment" than with aircraft. The best methods to acquire this equipment is to conduct incident response engagements and/or to have global sources that provide samples and intrusion information.
These samples can then be reverse engineered, their functionality recreated and used in ongoing drills to keep defensive teams sharp.
I have come to believe that defence teams should be constantly drilling against adversary teams. This is the best way they can get better, find institutional deficiencies, improve and validate procedures, etc. This sort of ongoing training is more expensive than penetration testing for sure, but far outstrips traditional penetration testing in benefits.
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Adversary team sneaks a person into the client facility and embeds a device that provides a command and control foothold out to the internet.
The C2 is designed to appear like a specific attacker's behaviour such as a beacon which non-SSL encryption cipher over port 443 with a specific user-agent.
The adversary team begins lateral attack using a custom tool similar to psexec along with a special LSASS injection tool.
The team then sets up persistence using a non-public (but used by real attackers) registry related method along with an RDP related backdoor.
Next the team indexes all documents and stores them in a semi-hidden location on the hard drive in a cd sized chunk using a non-english language version of winrar and a password captured from an incident response event. The team searches out, identifies and compromises, systems, users and data of interest. Each drill may have a different target such as PCI, engineering related intellectual property, executive communications.
Finally the team ex-filtrates this data and prepares the notification document.
The team notifies the client that the week's drill is complete, likely has a conference call or VTC and answers questions related to the exercise. The notification stage includes data that can be used in signatures and alerts such as PCAPS, indicators of compromise, etc. The team and client then discuss what if anything was detected and what could have been done to improve performance, procedures, etc. Plans to tune and improve defensive system configurations can be developed at this stage as well.
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If your defensive staff is not doing something along these lines at LEAST once a quarter if not once a month then your soldiers are untrained and likely to get slaughtered when its time for the real battle.