Published: January 11, 2007
This chapter discusses how to acquire the data that is necessary for the investigation. Some computer investigation data is fragile, highly volatile, and can be easily modified or damaged. Therefore, you need to ensure that the data is collected and preserved correctly prior to analysis. Use the three-step process shown in the following figure.
Figure 2.1. Acquisition phase of the computer investigation model
Build a Computer Investigation Toolkit
Your organization will need a collection of hardware and software tools to acquire data during an investigation. Such a toolkit might contain a laptop computer with appropriate software tools, operating systems and patches, application media, write-protected backup devices, blank media, basic networking equipment, and cables. Ideally, such a toolkit will be created in advance, and team members will be familiar with the tools before they have to conduct an investigation.
See the "Preparing Your Organization for a Computer Investigation" and "Tools" sections of Appendix: Resources in this guide for a list of suggested software tools that can be included in the computer investigation toolkit and for guidelines to follow when building a toolkit.
Collect the Data
Data collection of digital evidence can be performed either locally or over a network. Acquiring the data locally has the advantage of greater control over the computer(s) and data involved. However, it is not always feasible (for example, when computers are in locked rooms or other locations, or when high availability servers are involved). Other factors, such as the secrecy of the investigation, the nature of the evidence that must be gathered, and the timeframe for the investigation will ultimately determine whether the evidence is collected locally or over the network.
When using tools to collect data, it is important to first determine whether or not a rootkit has been installed. Rootkits are software components that take complete control of a computer and conceal their existence from standard diagnostic tools. Because rootkits operate at a very low hardware level, they can intercept and modify system calls. You cannot find a rootkit by searching for its executable, because the rootkit removes itself from the list of returned search results. Port scans do not reveal that the ports the rootkit uses are open, because the rootkit prevents the scanner from detecting the open port. Therefore, it is difficult to ensure that no rootkits exist. One available tool you can use is the Microsoft® Windows® Sysinternals RootkitRevealer.
When acquiring data over a network, you need to consider the type of data to be collected and the amount of effort to use. Consider what data you need to obtain that would support the prosecution of the offending parties. For example, it might be necessary to acquire data from several computers through different network connections, or it might be sufficient to copy a logical volume from just one computer.
The recommended data acquisition process is as follows:
- Create accurate documentation that will later allow you to identify and authenticate the evidence you collect. Ensure that you note any items of potential interest and log any activities that might be of importance later in the investigation. Key to a successful investigation is proper documentation, including information such as the following:
- Who performed the action and why they did it. What were they attempting to accomplish?
- How they performed the action, including the tools they used and the procedures they followed.
- When they performed the action (date and time) and the results.
- Determine which investigation methods to use. Typically, a combination of offline and online investigations is used.
- In offline investigations, additional analysis is performed on a bit-wise copy of the original evidence. (A bit-wise copy is a complete copy of all the data from the targeted source, including information such as the boot sector, partition, and unallocated disk space.) You should use the offline investigation method whenever possible because it mitigates the risk of damaging the original evidence. However, this method is only suitable for situations in which an image can be created, so it cannot be used to gather some volatile data.
- In an online investigation, analysis is performed on the original live evidence. You should be especially careful when performing online analysis of data because of the risk of altering evidence that might be required to prove a case.
- Identify and document potential sources of data, including the following:
- Servers. Server information includes server role, logs (such as event logs), files, and applications.
- Logs from internal and external facing network devices, such as firewalls, routers, proxy servers, network access servers (NAS), and intrusion detection systems (IDS) that may be used in the possible attack path.
- Internal hardware components, such as network adapters (which include media access control (MAC) address information) and PCMCIA cards. Also note external port types, such as Firewire, USB, and PCMCIA.
- Storage devices that need to be acquired (internal and external), including hard disks, network storage devices, and removable media. Don’t forget portable mobile devices such as PocketPC, Smartphone devices, and MP3 players such as Zune™.
When you must capture volatile data, carefully consider the order in which you collect the data. Volatile evidence can be easily destroyed. Information such as running processes, data loaded into memory, routing tables, and temporary files can be lost forever when the computer is shut down. Information about tools and commands that can help gather this information is available in the "Tools" section of Appendix: Resources in this guide.
- Use the following methods to collect data from storage media and record storage media configuration information:
Verify the data you collect. Create checksums and digital signatures when possible to help establish that the copied data is identical to the original. In certain circumstances (for example, when a bad sector exists on the storage media) it may be impossible to create a perfect copy. Ensure that you have obtained the best copy possible with the available tools and resources. You can use the Microsoft File Checksum Integrity Verifier (FCIV) tool to compute an MD5 or SHA1 cryptographic hash of the content of a file.
Store and Archive
When evidence is collected and ready for analysis, it is important to store and archive the evidence in a way that ensures its safety and integrity. You should follow any storage and archival procedures that exist within your organization.
Best practices for data storage and archival include the following:
- Physically secure and store the evidence in a tamperproof location.
- Ensure that no unauthorized personnel has access to the evidence, over the network or otherwise. Document who has physical and network access to the information.
- Protect storage equipment from magnetic fields. Use static control storage solutions to protect storage equipment from static electricity.
- Make at least two copies of the evidence you collected, and store one copy in a secure offsite location.
- Ensure that the evidence is physically secured (for example, by placing the evidence in a safe) as well as digitally secured (for example, by assigning a password to the storage media).
Clearly document the chain of custody of the evidence. Create a check-in / check-out list that includes information such as the name of the person examining the evidence, the exact date and time they check out the evidence, and the exact date and time they return it. A sample Worksheet - Chain of Custody Log Documentation document is included with the worksheets referenced in Appendix: Resources in this guide.