Sometimes, security best practices protect you against attacks that are very improbable. In these scenarios, how do you defend the implementation of such security measures?

For example, password-protecting access to the BIOS of a thinclient. A BIOS without a password is a risk because an attacker with physical access can change BIOS configuration in order to boot from USB and access the data of the system without authentication. In this case, the attacker needs physical access, the thinclient does not store too much important information, etc. The risk is very low.

Other examples may be related to some measures when you harden systems.

In this case, is it better to not enforce this security measure to be reasonable with the rest of the company or you are opening little holes that will punch you in the face in the future?

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    – schroeder
    Commented Dec 14, 2015 at 16:18

10 Answers 10


The potential impact of this is not low, regardless of how much information the thin client stores.

Specifically the risk is that an attacker installs something like a rootkit or software keylogger in the operating system of the thin client, which you are unlikely to be able to discover by inspection. (This is a variant of the Evil Maid attack).

Should any administrator ever use that client in the future, it will be game over for the network. The thin client can also be used as a beachhead to launch further attacks against the network, or to conduct reconnaisance.

Protecting the BIOS prevents this from happening, by protecting the integrity of the thin client OS.

The wider lesson here: Best practice saves you the expense and difficulty of assessing every risk, which as this question demonstrates, is hard to do.

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    If you believe best practices do not apply to your situation, you may be correct, or you may simply not understand the full reasons. Of course you can investigate the reasons, and you may still think they do not apply. But you must consider: "How likely is it that I am mistaken?" One cannot know everything, it is not cost-effective. Investigating the reasons with the hope to avoid the practice, may in the end be more effort than implementing the practice.
    – Ben
    Commented Dec 11, 2015 at 12:37
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    Best practice represents the expertise of others, and the accumulated knowledge and experience of many years. It is better to have understanding, but in the absence of undestanding, received wisdom generally suffices.
    – Ben
    Commented Dec 11, 2015 at 13:00
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    Or you might say that best practice is things that are so frequently the correct thing to do, that it is typically easier to do them than to decide whether or not to do them and then to ensure that decision remains correct in the face of future changes. Commented Dec 11, 2015 at 21:51
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    @SteveJessop - Exactly. I think "future changes" is one of the most important considerations. Suppose you did all the research, and (today) you could be 100% confident and even actually correct, that some "best practice" need not be followed. But some small change tomorrow, by someone else (even a vendor), might make that omitted best practice important. Commented Dec 12, 2015 at 3:18
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    I had the same idea (keylogger rootkit) as you. However my conclusion differs a bit: risk cascades. Any small risk can often be exploited in a way that creates more vulnerabilities and the end result can be unpredictable. Assume any known risk can cascade ten-fold.
    – Reaces
    Commented Dec 13, 2015 at 16:39

Costs are the basis to evaluate risks and their mitigations. If the costs (monetary costs, operational costs, ease-of-use costs etc.) to implement a defence (best practice, etc.) exceed the damage caused by the risk being realized, then you are justified in choosing the accept the risk.

This is pretty basic concept in risk management.

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    They are not difficult to justify. Why do you think they are difficult to justify?
    – schroeder
    Commented Dec 11, 2015 at 15:58
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    you assess the risks (taking probability into account and other mitigating measures) and make a decision based on cost/benefit.
    – schroeder
    Commented Dec 11, 2015 at 15:59
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    Your response does not answer my question
    – schroeder
    Commented Dec 11, 2015 at 16:54
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    Risk is a combination of probability and impact. Impact is easily scientifically measured. Probability is almost impossible to measure scientifically in infosec risk. It's based on experience and judgement. I'm still confused about what your question really is, though! Even if risk is difficult to measure, you can still assign a risk level and weigh the costs of a "best practice" against it.
    – schroeder
    Commented Dec 11, 2015 at 18:25
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    @Eloy: once you have "attacker has prepared a pendrive", the rest is redundant since someone who is prepared has covered all those things (including the time issue: if they're properly prepared they just stick the pendrive in, power-cycle the machine, select to boot from the drive and then leave it to do its thing). It sounds like you're trying to justify not doing this by treating an attacker as if they're relying on a whole lot of coincidences coming together ("oh look, I just so happen to have a pendrive with me!"). But of course they're prepared: they're an attacker, it's their job. Commented Dec 11, 2015 at 21:56

Best practices are not laws but recommendations. They usually come with an explanation why they are recommended. If you feel that the explanation does not apply in your case your are free to ignore the recommendation. You might even be right with this feeling and thus you can save costs.

But be aware that depending on your working environment you cannot simply ignore the recommendation, but you need to document why you feel that it can be ignored. It might also be that you are responsible in person if something bad happens because the best practices got ignored. Thus it is often easier and less risky to follow the recommendations than to ignore them.

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    I take your advice on not applying a best practice if it has not sense to me but I disagree in two things: 1) In my experience best practices usually come without the explanation about why it is important...if is just accepted. 2) I don't think that it is easier to follow recommendations than to ignore them...it is easier to ignore them...specially if you have to fight against other departments to implement some security measure not correctly justified. Commented Dec 11, 2015 at 12:16
  • @EloyRoldánParedes: it highly depends on your working environment (for example industry vs. small company) and who is responsible if something goes bad. If you can shift the responsibility to somebody else it might be easier for you to ignore the recommendation. Commented Dec 11, 2015 at 12:21
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    @EloyRoldánParedes - Some industries require certain things (HIPPA for healthcare, PCI for credit cards, etc). Often, there's some wiggle room in the requirements, where you can choose not to do something, but only if you document why you aren't and how it doesn't pose a risk.
    – Bobson
    Commented Dec 11, 2015 at 13:28

There are a number of factors to be considered here:

  1. What's the cost of implementing the measure? Protecting all BIOSes can be a pain, but isn't a significant outlay of money or effort.

  2. what's the risk of a bad thing happening? The chances are fairly low, but the impact is moderate (keyloggers are mentioned, there are a number of other issues with someone executing their own code on your system, namely that it is no longer your system). The actual risk is low-to-medium.

  3. What's the impact of implementation? Are there any legitimate reasons for the average user to be accessing the BIOS of their thin client? Not really.

So we've got something where mitigation doesn't cost much, doesn't have a major impact, and protects against a low-to-medium risk. I'd say that's a good argument to implement it, personally.

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    There is a fourth relevant factor: What is the effectiveness of implementation? If implementing the measure completely and perfectly eliminates the risk evaluated in point 2, then that's a very different situation to evaluate than if implementing the measure just shaves a small amount off either the severity or the probability of that risk, and the former can justify a much higher cost (point 1) and impact (point 3) than can the latter. I would put the given example of BIOS passwords somewhere in the middle of that scale, perhaps a bit closer to the latter than the former extreme. Commented Dec 12, 2015 at 19:52

In many cases it boils down to a question about what very low risk means. If you have an exact knowledge of how low the risk is, you can do calculations on the expected cost of not mitigating the risk.

In reality you rarely know the exact risk. Often potential security problems are dismissed as very low risk without any attempt at an actual assessment of the risk.

In many cases it requires less effort to solve the security problem than to evaluate the risk with sufficient accuracy to make an informed decision on whether to address the security vulnerability. For me that is the primary argument for addressing even security vulnerabilities of very low risk, because it means you save most of the cost associated with evaluating the risk.

Who can always tell the difference between a security problem known to be low risk and one assumed to be low risk?

Sometimes it is more productive to evaluate security problems against each other rather than evaluating each individual security problem against a low risk threshold.

In your specific example you could compare the risk of malicious change of BIOS settings to the risk of network connection on the device being bugged with a MITM device.

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    "In reality you rarely know the exact risk. Often potential security problems are dismissed as very low risk without any attempt at an actual assessment of the risk." Would give +1000 if I could. In fact, the whole concept of creating a safety margin when dealing with systems--whether the security of a network against a local attacker, the resistance of a building to earthquake, etc--is based on building in some degree of prudent protection against the risk that, well, your assessment/s of risk themselves be off somewhat. Commented Dec 12, 2015 at 4:16

Sometimes, security best practices protect you against risks that are very unprobable. In that scenarios, how do you defend the implementation of these security measure?

You shouldn't have to defend anything; let the numbers speak for themselves:

  1. Probability of attack X occurring = P.
  2. Total Cost of X if it occurs = TC. (This needs to include diagnosing and fixing the breach, lost revenue, reputation, lawsuits, crisis management, etc.)
  3. Cost to take security measures to prevent X in the first place = C.

The equation is simply:

C < P * TC

When the equation is true, the security measure should be implemented. Of course, the difficult part is calculating P and TC to begin with, but you may find you can be overly conservative on TC and overly optimistic on P and it will still be a worthwhile investment. Plus you also get the added benefit of publicizing the security measures to investors and/or customers.

This is obviously an oversimplification of all the factors to consider, but from a ROI point of view, the logic should hold up.

  • I'm sorry but being able to calculate a scientific probability of an attack seems to me a falacy. For example, looking in past events very usually brings to a probability of 0 and this is not real. Other possibility is "inventing" the probability or performing an "approximate calculation"...also not very scientific IMHO. I know that this is the standard in risk methodologies but I disagree in this approach. If we agree that we need a metric to measure I like more RAV calculation from OSSTMM. Commented Dec 11, 2015 at 17:09
  • @EloyRoldánParedes - Just because we don't know a good way to calculate the probability doesn't mean it isn't possible to do so. And if the probability really approaches 0 then maybe you shouldn't worry about it unless it's comparatively easy to mitigate it. That's basically the point of the equation. For example, the owner of Ashley Madison once claimed the value of his company to be $1B. Arguably the cost of the breach could be the value of the entire company. If that breach had a P of 1 in 1,000 then the cost to add better security needed to be less than $1M - should have done it...
    – TTT
    Commented Dec 11, 2015 at 17:28
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    Another way of looking at it is this: If C / TC < P, you should implement it. C should be very well known, TC should be pretty well known (at least within Fermi estimation limits), and P is a risk level you consider appropriate for having to pay TC, be it measured in money, trust, lawsuits or apologetic emails.
    – l0b0
    Commented Dec 11, 2015 at 23:49

The posture you adopt is based on the environment you are operating in. At the very least, the security policies you create are proportional to the budget you have and the threat models you have in mind. Without an assessment of the dollar value you are protecting, you will be spending an exorbitant amount of time figuring out where the holes are. On the other hand, criminal elements use low/no protected elements to search for higher value targets.

Also bear in mind that vendors purporting to help mitigate risk do not come with the silver bullet to solve every single problem. Ultimately you and the business are the only ones responsible for the security. From a business angle, you could pass on the costs of security errors/flaws, but it might not help in the long run. It's a pretty hoary world out there at the moment.


So this is kind of an extension of schroeder's answer.

I think there's a slight mix-up here in what we mean by 'risk'. You're looking at risk from the perspective of the computer system (password the BIOS, install AV, use a firewall, etc). The perspective that Schroeder is talking about is from the business' perspective: once the machine has been compromised, what is the associated cost?

When a business has a 'risk', there are a few things it can do:

  • Accept the risk - assume nothing bad will happen, and if it does it won't be a huge problem to deal with.

  • Risk avoidance - Avoid exposure to the risk at all.

  • Risk limitation(mitigation) - Reduce the likelihood of the risk happening.

  • Risk transference (CYA) - Make sure someone else is responsible for the risk.

To be clear, the two types of risk that are being discussed in the thread:

  1. The risk a business has should a particular system be compromised. This can include customer financial information (credit card numbers, etc.), as well as proprietary information, trade secrets, classified information, etc.

  2. The risk OP is proposing: the risk of a computer being compromised in a specific way.

Here's the nuance I'm getting at: #1 is the business' risk. The strategy to dealing with this risk is typically through risk limitation (mitigation). That's where #2 comes in. #2 mitigates the likelihood that #1 will occur. We consider many different methods in which a computer can be compromised in #2, and implement preventive measures in order to mitigate #1 as much as possible. Since a lot of 'best practices' are cheap or no-brainers (i.e. their cost is easily justified), it makes sense to follow them even if on an individual basis their attack vector is highly improbable.

  • Sometimes the business risk is not very clear for devices that do not store business data or perform business critical functions but are a first door to an infrastructure that is protected with other security measures. In this scenario I suppose that it is better not to implement the best practice but I'm worried of not implementing a low of non-important best practices and then have an accumulated very high risk. Commented Dec 11, 2015 at 17:20
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    @EloyRoldánParedes The problem there is that you essentially need to treat all of your systems equally. You can try to justify lower security practices on Jim's computer since he works on the factory floor and only uses it for email, but a hacker will see a computer with a lower security profile as a prime target. They will hack Jim's computer and use it as a launching point towards your servers, financial information, etc.
    – Shaz
    Commented Dec 11, 2015 at 17:24

In regards to your example:

All access to a system/network/forest/thing that contains secure data, must be kept secure from end to end, no matter how small the end is.

In regards to security in general:

All access to a system/network/forest/thing that contains secure data, must be kept secure.


There's a reason for this. A single access point to a network that contains secure data, is itself in need of security. Information of what has happened on the system as a WHOLE must be secure to keep the secure data all the way in the smallest part secure. If a single link in the chain breaks, the chain is broken.

But how do I break that chain if there is no security information on the client? Simple, by observing the network. If I can see what goes on in the network, I can learn what keys will get me through what locks, how, and in what ways. Then I have an easy way to get into your network, and get your data. That is why, from end to end, ALL activity must be kept secure.

Let's go to the example of punching you in the face:

  1. You think your home is secure.
  2. You have bio metric finger print scanners(LIKE ON TV) that mean only your fingers get you inside
  3. You touch a door knob on a store front 70 million miles away, six years ago
  4. I lift that print
  5. I identify your print(through some long drawn out process)
  6. I use that print to open your home while you sleep
  7. I punch you in the face

Even though it was miles away, and that print had many other prints, and so forth and on wards, I could still eventually find your print in there, and use it to open your home. This is very improbable, but if I REALLY want to, I can still punch you in the face with time, effort, and some patience.

Still though this doesn't do more than present an example. Let's instead state the truth of security:

The most secure system in the world in buried underground, in an unknown location, burnt to a crisp, and destroyed.

Sure that's a little tongue in cheek, but it gets the idea across. So yes this security best practice should still be used. After all your system isn't the most secure system in the world, so you should try and get close.


As it has already been said, security is mainly a risk vs cost approach.

But IMHO, there is another point to considere: as in building security, a reinforced door is little use if the window is left open. To correctly evaluate a risk you must know what to want to protect against. So protecting BIOS with a password because it could be a vector for an attacker that could have a physical access. IMHO, if an attacker can gain physical access to a computer, this computer is compromised because anything could happer: a physical key logger in the keyboard itself, an network analyser between the internal connector and the external plug, a malicious USB device inside the box, etc.

But BIOS password is still good practice because it can prevent further infection if an attack reaches the post, and mainly because it prevents a careless or clumsy user to break or compromise himself its own terminal.

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