Let's review some of the answers you might hear to this question. I'll give an opinion regarding the utility of the answer as well.
For the purpose of this exercise let's assume it is possible to answer "yes" to this question. In other words, we just don't answer "no." We could all make arguments as to why it's impossible to be secure, but does that really mean there is no acceptable level of perceived risk in which you could operate? I doubt it.
So, are you secure? Prove it.
- Yes. Then, crickets (i.e., silence for you non-imaginative folks.) This is completely unacceptable. The failure to provide any kind of proof is security by belief. We want security by fact.
- Yes, we have product X, Y, Z, etc. deployed. This is better, but it's another expression of belief and not fact. The only fact here is that technologies can be abused, subverted, and broken. Technologies can be simultaneously effective against one attack model and completely worthless against another.
- Yes, we are compliant with regulation X. Regulatory compliance is usually a check-box paperwork exercise whose controls lag attack models of the day by one to five years, if not more. A compliant enterprise is like feeling an ocean liner is secure because it left dry dock with life boats and jackets. If regulatory compliance is more than a paperwork self-survey, we approach the realm of real of evidence. However, I have not seen any compliance assessments which measure anything of operational relevance.
- Yes, we have logs indicating we prevented attacks X, Y, and Z. This is getting close to the right answer, but it's still inadequate. For the first time we have some real evidence (logs) but these will probably not provide the whole picture. Sure, logs indicate what was stopped, but what about activities that were allowed? Were they all normal, or were some malicious but unrecognized by the preventative mechanism?
- Yes, we do not have any indications that our systems are acting outside their expected usage patterns. Some would call this rationale the definition of security. Whether or not this answer is acceptable depends on the nature of the indications. If you have no indications because you are not monitoring anything, then this excuse is hollow. If you have no indications and you comprehensively track the state of an asset, then we are making real progress. That leads to the penultimate answer, which is very close to ideal.
- Yes, we do not have any indications that our systems are acting outside their expected usage patterns, and we thoroughly collect, analyze, and escalate a variety of network-, host-, and memory-based evidence for signs of violations. This is really close to the correct answer. The absence of indications of intrusion is only significant if you have some assurance that you've properly instrumented and understood the asset. You must have trustworthy monitoring systems in order to trust that an asset is "secure." If this is really close, why isn't it correct?
- Yes, we do not have any indications that our systems are acting outside their expected usage patterns, and we thoroughly collect, analyze, and escalate a variety of network-, host-, and memory-based evidence for signs of violations. We regularly test our detection and response people, processes, and tools against external adversary simulations that match or exceed the capabilities and intentions of the parties attacking our enterprise (i.e., the threat). Here you see the reason why number 6 was insufficient. If you assumed that number 6 was ok, you forgot to ensure that your operations were up to the task of detecting and responding to intrusions. Periodically you must benchmark your perceived effectiveness against a neutral third party in an operational exercise (a "red team" event). A final assumption inherent in all seven answers is that you know the assets you are trying to secure, which is no mean feat.
Incidentally, this post explains why deploying a so-called IPS does nothing for ensuring "security." Of course, you can demonstrate that it blocked attacks X, Y, and Z. But, how can you be sure it didn't miss something?
If you want to spend the least amount of money to take the biggest step towards Magnificent Number 7, you should implement Network Security Monitoring.