Chapter 3: Analyze the Data

Published: January 11, 2007

This chapter discusses different approaches and well-accepted industry best practices for analyzing the evidence that is gathered during the Acquire the Data phase of an internal investigation. Use the three-step process shown in the following figure.

 

Analysis phase of the computer investigation

Figure 3.1. Analysis phase of the computer investigation model

 

 Important   Online analysis of data, which examines a computer directly while it is running, is often necessary. Online analysis is typically performed because of time constraints on an investigation or to capture volatile data. You should be especially careful when performing online analysis to ensure that you minimize the risk to other evidence.

Analyze Network Data

In many investigations it is not necessary to analyze network data. Instead, the investigations focus on and examine images of the data. When network analysis is required, use the following procedure:

  1. Examine network service logs for any events of interest. Typically, there will be large amounts of data, so you should focus on specific criteria for events of interest such as username, date and time, or the resource being accessed.
  2. Examine firewall, proxy server, intrusion detection system (IDS), and remote access service logs. Many of these logs contain information from monitored incoming and outgoing connections and include identifying information, such as IP address, time of the event, and authentication information. You might want to examine the log data in a tool that is suited for data analysis, such as Microsoft® SQL Server™ 2005.
  3. View any packet sniffer or network monitor logs for data that might help you determine the activities that took place over the network. In addition, determine whether connections you examine are encrypted—because you will not be able to read the contents of an encrypted session. However, you can still derive the time of the connection and whether a suspected party established a session with a specific server.

Analyze Host Data

Host data includes information about such components as the operating system and applications. Use the following procedure to analyze the copy of the host data you obtained in the Acquire the Data phase.

  1. Identify what you are looking for. There will likely be a large amount of host data, and only a portion of that data might be relevant to the incident. Therefore, you should try to create search criteria for events of interest. For example, you might use the Microsoft Windows® Sysinternals Strings tool to search the files located in the \Windows\Prefetch folder. This folder contains information such as when and where applications were launched. For information about how to use the Strings tool and send or pipe the results to a text file, see Chapter 5: Applied Scenario Example in this guide.
  2. Examine the operating system data, including clock drift information, and any data loaded into the host computer's memory to see if you can determine whether any malicious applications or processes are running or scheduled to run. For example, you can use the Windows Sysinternals AutoRuns tool to show you what programs are configured to run during the boot process or login.
  3. Examine the running applications, processes, and network connections. For example, you can look for running processes that might have an appropriate name but are running from non-standard locations. Use tools such as Windows Sysinternals ProcessExplorer, LogonSession, and PSFile to perform these tasks. See the "Tools" section in Appendix: Resources of this guide for information about these tools.

Analyze Storage Media

The storage media you collected during the Acquire the Data phase will contain many files. You need to analyze these files to determine their relevance to the incident, which can be a daunting task because storage media such as hard disks and backup tapes often contain hundreds of thousands of files.

Identify files that are likely to be relevant, which you can then analyze more closely. Use the following procedure to extract and analyze data from the storage media you collected:

  1. Whenever possible, perform offline analysis on a bit-wise copy of the original evidence.
  2. Determine whether data encryption was used, such as the Encrypting File System (EFS) in Microsoft Windows. Several registry keys can be examined to determine whether EFS was ever used on the computer. For a list of the specific registry keys, see the "Determining If EFS is Being Used on a Machine" section in the article "Encrypting File System in Windows XP and Windows Server 2003" on Microsoft TechNet. If you suspect data encryption was used, then you need to determine whether or not you can actually recover and read the encrypted data. Your ability to do so will depend upon different circumstances, such as the version of Windows, whether or not it is a domain-joined computer, and how EFS was deployed. For more information about EFS see "The Encrypting File System" on Microsoft TechNet. External EFS recovery tools are also available, such as Advanced EFS Data Recovery by Elcomsoft.
  3. If necessary, uncompress any compressed files and archives. Although most forensic software can read compressed files from a disk image, you might need to uncompress archive files to examine all files on the media you are analyzing.
  4. Create a diagram of the directory structure. It might be useful to graphically represent the structure of the directories and files on the storage media to effectively analyze the files.
  5. Identify files of interest. If you know which files were affected by the security incident, you can focus the investigation on these files first. The hash sets created by the National Software Reference Library can be used to compare well-known files (such as operating system and application files) to the originals. Those files that match can normally be eliminated from the investigation. You can also use informational sites such as filespecs.com, Wotsit's Format, ProcessLibrary.com, and Microsoft DLL Help to help you categorize and collect information about existing file formats as well as to identify files.
  6. Examine the registry, the database that contains Windows configuration information, for information about the computer boot process, installed applications (including those loaded during startup), and login information such as username and logon domain. For registry background information and detailed descriptions of registry content, see the Windows Server 2003 Resource Kit Registry Reference. Various tools are available for analyzing the registry, including RegEdit, which ships with the Windows operating system, Windows Sysinternals RegMon for Windows, and Registry Viewer by AccessData.
  7. Search the contents of all gathered files to help identify files that may be of interest. Various intelligent searches can be performed using tools described in the "Tools" section in Appendix: Resources of this guide. For example, you can use the Windows Sysinternals Streams tool to reveal whether there are any NTFS alternate data streams used on files or folders. NTFS alternate data streams can hide information within a file by causing it to appear to contain zero bytes of data when viewed through Windows Explorer although the file actually contains hidden data.
  8. Study the metadata of files of interest, using tools such as Encase by Guidance Software, The Forensic Toolkit (FTK) by AccessData, or ProDiscover by Technology Pathways. File attributes such as timestamps can show the creation, last access, and last written times, which can often be helpful when investigating an incident.
  9. Use file viewers to view the content of the identified files, which allow you to scan and preview certain files without the original application that created them. This approach protects files from accidental damage, and is often more cost effective than using the native application. Note that file viewers are specific to each type of file; if a viewer is not available, use the native application to examine the file.

After you analyze all of the available information, you may be able to reach a conclusion. However, it is important to be very cautious at this stage and ensure that you do not blame the wrong party for any damages. However, if you are certain of your findings, you will be ready to begin the Report the Investigation phase.

This accelerator is part of a larger series of tools and guidance from Solution Accelerators.

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