Forensic Image / Disk Image

A forensic image is a bit-for-bit, sector-by-sector copy of a storage device that preserves all data including active files, deleted files, file slack space, unallocated areas, and filesystem metadata. Forensic images serve as the evidentiary foundation for digital forensic analysis during incident response and must be created using write-blocking tools to ensure the original media is not modified during the acquisition process.

Why Forensic Images Matter

Standard file copies only capture visible, active files. A forensic image captures everything on the storage media, including data that has been deleted but not yet overwritten, fragments of files in slack space, and metadata that reveals when files were created, accessed, modified, and deleted. This additional data is often critical for reconstructing attacker activity, identifying exfiltrated data, and building a complete incident timeline. Without a proper forensic image, this evidence may be permanently lost when the system is rebuilt or returned to service.

Creating a Forensic Image

Forensic imaging follows a strict process to maintain evidentiary integrity. A hardware or software write blocker is attached between the source media and the imaging workstation to prevent any writes to the original device. The imaging tool creates a complete sector-by-sector copy, typically in a forensic format such as E01 (EnCase) or AFF4 that includes built-in integrity verification. SHA-256 hash values are calculated for both the original media and the resulting image, and these values must match exactly. The process, tools used, hash values, and analyst identity are documented as part of the chain of custody record.

Forensic Images in Cloud Environments

Cloud environments present unique challenges for forensic imaging. Traditional disk imaging may not be possible when the organization does not have physical access to the underlying storage. Cloud-specific forensic techniques include snapshot-based acquisition, API-driven log collection, and memory capture through cloud provider tools. The forensic principles remain the same -- preserve evidence integrity, document the acquisition process, and maintain chain of custody -- but the technical methods differ from traditional on-premises imaging.

When to Create Forensic Images

Forensic images should be created as early as possible during an incident, before containment and recovery actions modify the state of affected systems. The decision to image specific systems should be guided by the incident commander and DFIR team based on which systems are most likely to contain critical evidence. In practice, imaging every affected system may not be feasible due to time and storage constraints, so prioritization is essential. At minimum, image systems where the initial compromise occurred, systems where the attacker achieved their objective, and any system that will be needed for regulatory or legal proceedings.