[Chapter 24] Discovering a Break-in
Author: Simson Garfinkel & Gene Spafford
Date: 1999-02-04T00:18:36Z

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| Chapter 24 |

# 24. Discovering a Break-in

Contents:
Prelude
Discovering an Intruder
The Log Files: Discovering an Intruder's Tracks
Cleaning Up After the Intruder
An Example
Resuming Operation
Damage Control

This chapter describes what to do if you discover that someone has broken into your computer system: how to catch the intruder; how to figure out what, if any, damage has been done; and how to repair the damage, if necessary. We hope that you'll never have to use the techniques mentioned here.

## 24.1 Prelude

There are three major rules for handling security breaches.

### 24.1.1 Rule #1: DON'T PANIC

After a security breach, you are faced with many different choices. No matter what has happened, you will only make things worse if you act without thinking.

Before acting, you need to answer certain questions and keep the answers firmly in mind:

- Did you really have a breach of security? Something that appears to be the action of an intruder might actually be the result of human error or software failure.

- Was any damage really done? With many security breaches, the perpetrator gains unauthorized access but doesn't actually access privileged information or maliciously change the contents of files.

- Is it important to obtain and protect evidence that might be used in an investigation?

- Is it important to get the system back into normal operation as soon as possible?

- Are you willing to take the chance that files have been altered or removed? If not, how can you tell for sure if changes have been made?

- Does it matter if anyone within the organization hears about this incident? If somebody outside hears about it?

- Can it happen again?

The answers to many of these questions may be contradictory; for example, protecting evidence and comparing files may not be possible if the goal is to get the system back into normal operation as soon as possible. You'll have to decide what's best for your own site.

### 24.1.2 Rule #2: DOCUMENT

Start a log, immediately. Take a notebook and write down everything you find, always noting the date and time. If you examine text files, print copies and then sign and date the hardcopy. If you have the necessary disk space, record your entire session with the script command, too. Having this information on hand to study later may save you considerable time and aggravation, especially if you need to restore or change files quickly to bring the system back to normal.

This chapter and the two chapters that follow present a set of guidelines for handling security breaches. In the following sections, we describe the mechanisms you can use to help you detect a break-in, and handle the question of what to do if you discover an intruder on your system. In Chapter 25, Denial of Service Attacks and Solutions, we'll describe denial of service attacks - ways in which attackers can make your system unusable without actually destroying any information. Finally, in Chapter 26, Computer Security and U.S. Law, we'll discuss legal approaches and considerations you may need to consider after a security incident.

### 24.1.3 Rule #3: PLAN AHEAD

A key to effective response in an emergency is advance planning. When a security problem occurs, there are some standard steps to be taken. You should have these steps planned out in advance so there is little confusion or hesitation when an incident occurs.

In larger installations, you may want to practice your plans. For example, along with standard fire drills, you may want to have "virus drills" to practice coping with the threat of a virus, or "break-in drills." The following basic steps should be at the heart of your plan:

Step 1: Identify and understand the problem.

  If you don't know what the problem is, you cannot take action against it. This rule does not mean that you need to have perfect understanding, but you should understand at least what form of problem you are dealing with. Cutting your computer's network connection won't help you if the problem is being caused by a revenge-bent employee with a terminal in his office.

Step 2: Contain or stop the damage.

  If you've identified the problem, take immediate steps to halt or limit it. For instance, if you've identified the employee who is deleting system files, you'll want to turn off his account, and probably take disciplinary action as well. Both are steps to limit the damage to your data and system.

Step 3: Confirm your diagnosis and determine the damage.

  After you've taken steps to contain the damage, confirm your diagnosis of the problem and determine the damage it caused. Are files still disappearing after the employee is discharged? You may never be 100% sure if two or more incidents are actually related. Furthermore, you may not be able to identify all of the damage immediately, if ever.

Step 4: Restore your system.

  After you know the extent of the damage, you need to restore the system and data to a consistent state. This may involve reloading portions of the system from backups, or it may mean a simple restart of the system. Before you proceed, be certain that all of the programs you are going to use are "safe." The attacker may have replaced your restore program with a Trojan horse that deletes both the files on your hard disk and on your backup tape!

Step 5: Deal with the cause.

  If the problem occurred because of some weakness in your security or operational measures, you'll want to make changes and repairs after your system has been restored to a normal state. If the cause was a person making a mistake, you will probably want to educate him or her to avoid a second occurrence of the situation. If someone purposefully interfered with your operations, you may wish to involve law enforcement authorities.

Step 6: Perform related recovery.

  If what occurred was covered by insurance, you may need to file claims. Rumor control, and perhaps even community relations, will be required at the end of the incident to explain what happened, what breaches occurred, and what measures were taken to resolve the situation. This step is especially important with a large user community, because unchecked rumors and fears can often damage your operations more than the problem itself.

VI. Handling Security Incidents |  | 24.2 Discovering an Intruder