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Building Internet Firewalls

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Previous: 13.3 Pursuing and Capturing the Intruder Chapter 13
Responding to Security Incidents
Next: 13.5 Being Prepared
 

13.4 Planning Your Response

All of the actions we've outlined in the previous sections sound fine in theory, but you can't actually do any of them reliably without an incident response plan. You may personally be able to mount a sensible response to an attack, but you aren't necessarily going to be the person who discovers one. You may not even be available at the time. How will your organization react if someone attacks your system? Unless you have an incident response plan in place, the people involved will waste valuable time trying to figure out what to do first.

If you already have a plan in place for disaster or emergency response of any kind (e.g., fire, earthquake, electrical problems), you're probably not going to have to change it significantly to meet your security needs. If you don't have such a plan already, you can probably use your security incident response plan with only minor modifications for most emergencies.

Your incident response plan need not be an elaborate document, but you need to have something , even if it's only an email message that records and confirms the details you've all worked out over lunch at the local sushi bar. You'll be better off than many sites even if you do nothing more than think about the issues and discuss them with the relevant people.

What's in your plan?

The response plan is primarily concerned with two issues: authority and communication. For each part of the incident response, the plan should say who's in charge, and who they're supposed to talk to. Although you'll specify a few steps people will take, incidents vary so much that the response plan mostly details who's going to make decisions, and who they're going to contact after they've decided - not what they're going to decide. This section summarizes the different parts of a response plan.

13.4.1 Planning for Detection

An incident starts when somebody detects an intruder or attacker. That person might be a system administrator, but more often it's someone with no official responsibility. If you've properly educated the people who use your computers, they know they're supposed to report weird events. Somebody then needs to sort run-of-the-mill peculiarities from a security incident in progress. Who are the users going to report to? Who are those people going to report to if they're still not sure? What are they authorized to do if they are sure?

The two cases you really want to plan for are these:

  • Somebody notices a real security incident in progress at 3 A.M.

  • Somebody notices one of your perfectly legitimate users who happens to be doing vital work from halfway across the globe at 3 A.M. local time. (In Australia, where he's consulting at the moment, it's a reasonable 5 P.M. )

In the first case, you need a procedure which is going to reliably start up a full incident response immediately. Don't waste any time. It's going to be embarrassing and expensive if you don't actually get around to doing anything until your senior security person arrives in the next morning, takes in enough caffeine to become able to think, and gets around to looking at some report. (And that's if there is a report in the first place; without a response plan, it may be weeks before anyone actually tells someone who can begin to do something about the situation.)

In the second case, it's going to be embarrassing and expensive if you disconnect the network and get five people out of bed, all to prevent somebody from doing the work they're paid to do.

Either way, it's not a decision you probably want made by a night operator, or by a user acting alone because he can't figure out how to call somebody who knows how to tell a real incident from a false alarm.

At a small site, you might want to simply post a number that users can call to get help outside of office hours (for instance, a pager number). Users might be encouraged to shut down personal machines if they suspect an attack and know how to shut the machine down gracefully. You want to be very cautious about this, however, because an ungraceful shutdown, particularly of a multiuser machine, may be more damaging than an intruder.

At a larger site, one that has on-site support after hours, you should instruct the on-site support people to call a senior person if they see a possible security incident. They should be told explicitly not to do anything more than that unless circumstances are extreme, but to keep trying to contact senior personnel until they get somebody who can take a look at what's going on.

13.4.2 Planning for Evaluation of the Incident

Who's going to decide that you don't just have a suspicious situation - you actually have a security problem? You need to designate one specific person who will have responsibility for making the important decisions. It's tempting to pick one specific person in advance and put his or her name in your plan. But, what if that person isn't available in the event of an actual incident? Who, then, will have the responsibility?

Teamwork is great, but emergencies call for leadership. You don't want to have everybody doing their own thing and nobody in charge, and you certainly can't afford to stand around arguing about it. If your senior technical person is absent, do you want someone less senior but more technical to do the evaluation, or do you want someone more senior but less technical? How much time are you going to spend searching for the senior technical person when you have an emergency to deal with, before proceeding to your next candidate for the hot seat?

At a small site, you may not have a lot of options; if there's only one person who has the skills necessary to do something about an attack, your policy will simply list that person as the one in charge in case of a security incident. If that person is unavailable, authority should go to somebody levelheaded and calm who can take stopgap actions and arrange for assistance (for example, from a relevant response team). In this situation, technical skills would be nice, but resourcefulness and calm are more important.

At a larger site, there is probably more than one person who could be in charge. Your plan may want to say that the most senior of them will be in charge by default, or that whoever is specified as being on call will be in charge. Either way, the plan should state that if the default person in charge is unavailable, the first of the other possible people to respond is in charge. Specifying what order they're going to be contacted in is probably overkill; let whoever is trying to reach these people use his or her knowledge of the situation. If none of those people are available, you'll usually want to work up the organizational hierarchy rather than down. (A manager, particularly a technical one, is probably better equipped to cope than an operator.)

In a small organization, you will pick your fallback candidates by name. In a large one, you will usually specify fallbacks by job title. If job title is your criterion, it's important to base your decision on the characteristics of the job, not of the person currently in it. Don't write into your plan that the janitor should decide, on the theory that the current janitor also is the most sensible and technical of those who aren't system administrators. The next janitor might be an airhead with a mop.

13.4.3 Planning for Disconnecting or Shutting Down Machines

Your response plan needs to specify what kind of situation warrants disconnecting or shutting down, and who can make the decision to do that. Most importantly, as we've discussed in "Pursuing and Capturing the Intruder," are you ever willing to allow a known intruder to remain connected to your systems? If you're not, are you going to take down the system, or are you going to disconnect from the network altogether?

If you are at a site with multiple computer facilities, do you want to take the entire site off the Internet if one facility has been compromised, or is it better (or even possible) to take just that facility off the Internet?

At most sites, the reasonable plan is to disconnect the site as a whole from the network as soon as you know for sure that you have an intruder connected to your systems. You may have a myriad of internal connections, with a triply redundant, diversely cabled, UPS -protected routing mesh, which can make "disconnecting" a daunting prospect (the system keeps "fixing" itself). On the other hand, you probably only have one (or a small handful) of connections to the outside world, which can be more easily severed.

Your plan needs to say how to disconnect the network, and how the machines should be shut down. Be very careful about this. You do not want to tell people to respond to a mildly suspicious act by hitting the circuit breakers and powering off every machine in the machine room. On the other hand, if an intruder is currently removing all the files on the machine, you don't want them to give that intruder a 15-minute warning for a graceful shutdown.

This is one case in which you need clear, security-specific instructions in your plan. Here's what we recommend you do:

  • In most security emergencies, the correct way to shut down the machine is to do an immediate but graceful shutdown, with no explanations or warnings sent. Your plan should state that and specify the appropriate commands to issue.

  • If the intruder is actively destroying things, you want people to shut the machine down by the fastest method possible. If they are physically near the machine, cutting off the power to the machine or the disk drive is completely appropriate, despite the damage it may cause. This implies that the relevant power switches must be easy to locate; a master switch for each machine is a good idea.

Whoever is going to disconnect the network needs to know how to do that. Often, the safest and easiest way is to unplug cables and clean up the side effects afterwards. With networks, this tends to result in voluminous error messages but to cause no actual damage. You do have to unplug the relevant cables, however, and the voluminous error messages may make it difficult to determine whether or not the cables that were unplugged were actually the correct ones. Your plan needs to tell people what to unplug, and how to make things functional afterwards.

13.4.4 Planning for Notification of People Who Need to Know

Your incident response plan needs to specify who you're going to notify, who's going to do the notification, when they're going to do it, and what method they're going to use. As we described earlier in this chapter, you may need to notify:

  • People within your own organization

  • CERT-CC or other incident response teams

  • Vendors and service providers

  • People at other sites

13.4.4.1 Your own organization

To start with, you need to notify the people who are going to be involved in the response. You'll have an urgent need to get hold of them, so you need telephone and pager numbers. Be sure you have all the relevant phone numbers; in addition to home and work numbers, check to see if people have car phones or portable phones at which they might be reached. This list includes anybody who manages computers within your site and anybody who manages those people, plus anybody else who might be needed to provide resources (to sign off on emergency purchases or to unlock doors, for example). Ideally, the list - or at least the key portions of it - should be reduced down so it's small enough to carry easily (for example, it might be laser-printed onto business card-sized stock). Obviously, the list isn't much use unless it's kept up to date.

If there are many people to be notified, you may wish to use a phone tree or an alert tree. In such a tree, shown in Figure 13.2 , each person notifies two or three other people; this is a geometric progression, so a large number of people can be rapidly notified with relatively little work to any one person. Everybody should have a copy of the entire tree, so that if people are unavailable their calls can be taken over by someone else (usually the person above them on the tree). It's best to set it up so as many calls as possible are toll-free, and so that people are notifying other people they know relatively well (which increases their chances of knowing how to get through). There's no need for an alert tree to reflect an organizational chart or a chain of command.

Figure 13.2: An alert tree

Figure 13.2

Next, you're going to notify other people within your organization who need to know, starting with the users of your computer facility. For that, you'll use whatever your organization normally uses for relatively urgent notifications to everybody, whether that's memos or electronic mail. Your plan should specify how to do that (system administrators rarely send memos to all personnel and may not know how).

Your plan should also show a sample notification message for the users of your systems. This can sometimes be tricky. Your message needs to contain enough information so that legitimate users understand what's happening. They need to know:

  • What has been taken out of service

  • Why you're making their lives miserable

  • Exactly which things that they normally do aren't going to work

  • When service will be restored.

  • What they're supposed to do (including leave you alone so that you can concentrate on the response)

  • That you realize you're making life unpleasant for them

  • That you're doing everything possible to improve matters

  • That you're going to tell them the details later

Things that are obvious to you may not be obvious to your users (e.g., they might not even understand why it's so bad to have an intruder). Writing an appropriate message (see Figure 13.3 ) is not easy, particularly if you're busy and tired.

Figure 13.3: A notification message

Figure 13.3

For the remaining people within your organization - people from other computer facilities, legal, audit, public relations, or security - the plan needs to specify who gets notified. Do you need to call the legal department? If so, who should you talk to? Who are the administrators for other sites within your organization? During the Morris worm incident in 1988, at least one large government lab was reduced to having the guards hand out flyers at the gate to everyone as they came to work, asking "Are you a system administrator?" because they had no idea who all the system administrators were, much less how to contact them.

Think about how you are going to send your message. If you send it via electronic mail, remember that the intruder may see it. Even if you know that your own systems are clean, don't assume that other people's are. Don't say anything in your message that you don't want the attacker to know. Better yet, use a telephone.

Some sites use a simple code phrase to announce a system attack that they can include in electronic mail. This can rapidly degenerate into bad spy fiction, but if you have an agreed-upon phrase that isn't going to alert an intruder (and isn't going to cause people who don't know it or don't remember it to give the game away by asking what on earth you're talking about), it can be effective. Something like "We're having a pizza party; call 3-4357 to RSVP " should serve the purpose.

Should you contact your organization's security department? At some organizations, the security department is responsible only for physical security. You'll want to have a contact number for them in case you need doors unlocked, for example, but they are unlikely to be trained in helping with an emergency of this kind, so you probably won't need to notify them routinely of every computer security incident. However, if there is a group responsible for computer security within your organization, you are probably required to notify them. Find out ahead of time when they want to be notified and how, and put that information in the plan. Even if that group cannot help you respond to your particular type of incident (e.g., they may be personal-computer specialists, or government-security specialists), it's advisable to at least brief them on the incident after you have finished responding to it.

13.4.4.2 CERT-CC and other incident response teams

Your plan should also specify what emergency response team, if any, you're served by and how to contact them. CERT-CC and many teams in the FIRST have 24-hour numbers, and they prefer to be called immediately if a security incident occurs.

13.4.4.3 Vendors and service providers

Your plan should also contain the contact numbers for your vendors and Internet service providers. These people probably do not need to be called immediately, unless you need their help. However, if you have any reason to suspect that your Internet provider itself has been compromised, you should contact them immediately.

Many vendors and service providers have special contact procedures for security incidents. Using these procedures will yield much faster results than going through normal support channels. Be sure to research these procedures ahead of time, and include the necessary information in your response plan.

13.4.4.4 Other sites

You will not ordinarily need to talk to other sites as part of the immediate incident response. Instead, you'll call them after the immediate emergency is over, when you have time to work without needing everything written down in the plan. In addition, no plan could cover all the information needed to find out what other sites were involved and to contact them. Therefore, your plan doesn't need to say much about informing other sites.

If you are providing Internet service for other sites, however, or have special network connections to other sites, you should have contact information in the plan and should contact them promptly. They need to know what happened to their service and to check that the attacker didn't reach them through your site.

13.4.5 Planning for Snapshots

Your incident response plan should specify how you're going to do snapshots of the compromised system. Make sure that your plan contains the answers to these questions:

  • Where are the necessary supplies and what program are you going to use?

  • How should the snapshot be labeled and where should it be stored?

  • How should they be preserved against tampering, for possible later use in legal proceedings?

13.4.6 Planning for Restoration and Recovery

Different incidents are going to require different amounts of recovery. Your response plan should provide some general guidelines.

Reinstalling an operating system from scratch is time-consuming, unpleasant, and often exposes underlying problems. For example, you may discover that you no longer know where some of your programs came from. For this reason, people are extremely reluctant to do it. Unless your incident response plan says explicitly that they need to reinstall the operating system, they probably won't. The problem is, this leads to situations where you have to get rid of the same intruder over and over again, because the system hasn't been properly cleaned up. Your response plan should specify what's acceptable proof that the operating system hasn't been tampered with (for instance, a comparison against cryptographic checksums of an operating system known to be uncompromised). If you don't have those tools, which are discussed in Chapter 5, Bastion Hosts , or if you can't pass the inspection, then you must install a clean operating system, and the plan should say so.

The plan should also provide the information needed to reinstall the operating system; for example:

  • Where are the distribution media kept?

  • How do you find out how to install the operating system?

  • Where are the backups, and how do you restore from them?

  • Where are the records that will let you reconstruct third-party or locally written programs?

13.4.7 Planning for Documentation

Your plan should include the basic instructions on what documentation methods you intend to use and where to find the supplies. If you might pursue legal action, your plan should also include the instructions on dating, labeling, signing, and protecting the documentation. Remember that you aren't likely to know when you start out whether or not there will be legal action, so you will always need to document if you ever want to be able to take legal steps; this is not something you can go back and "fix" later on.

13.4.8 Periodic Review of Plans

However solid your security incident response plans may seem to be, make sure to go back and review them periodically. Changes - in requirements, priorities, personnel, systems, data, and other resources - are inevitable, and you need to be sure that your response plans keep up with these changes. The right question to ask about each item isn't "Has it changed?," but " How has it changed?"

A good time to review your incident response plan is after a live drill, which may have exposed weaknesses or problems in the plan. (See the section called "Do Drills" at the end of this chapter.) For example, a live drill may uncover any of the following:

  • That you've changed all your storage since the plan was written.

  • That you can't actually restore your operating system from scratch.

  • That your plan relies on the ability to use the network to reach external sites, but at the same time instructs you to disconnect the network.


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