Basics of File System Forensics
Use write blockers during imaging to preserve the original evidence, and document every step for chain of custody. File system forensics involves the meticulous examination of storage media to uncover how data was created, modified, and deleted. Whether investigating insider threats, external intrusions, or data leakage, analysts rely on a thorough understanding of file system structures to locate key evidence. While techniques vary by file system type—such as NTFS, FAT32, or ext4—the overarching concepts remain similar: identify artifacts, preserve data integrity, and build a clear timeline of events.
The first step in any forensic investigation is to create a bit-for-bit image of the target drive. Tools like dd or FTK Imager can be used to capture every sector of the disk, ensuring that you preserve hidden areas and slack space. Always use a write blocker to prevent accidental modification of the original media. Maintaining a pristine copy allows you to verify your results and demonstrate to courts or stakeholders that the evidence has not been altered.
Once the image is acquired, analysts parse the file system metadata to understand how files are organized. In NTFS, the Master File Table (MFT) stores records that describe each file and directory, including timestamps, permissions, and data locations. For ext4, inodes serve a similar purpose. Examining these structures reveals when files were created, modified, or deleted. Deleted entries might still reside in the MFT or inode table until the space is overwritten, offering a chance to recover lost data.
Time stamps are critical in building timelines. NTFS provides multiple timestamp attributes such as Created, Modified, Accessed, and Entry Modified (also known as $MFT Changed). These values can be cross-referenced with application logs, web history, or system events to determine user activity. Tools like log2timeline convert various logs and metadata into a single timeline that can be reviewed with a tool like Plaso, aiding investigators in identifying patterns or anomalies.
Forensic analysis also extends to unallocated space. Even after files are deleted, remnants may linger until the blocks are overwritten. By scanning for file signatures or patterns in unallocated space, you can reconstruct fragments of documents, images, or executable code. Hash sets and known file databases help differentiate legitimate system files from potential malware or contraband.
Chain of custody is paramount throughout the process. Every transfer of evidence—from initial acquisition to subsequent analysis—must be documented. Detailed notes should include the date and time of acquisition, the tools used, and any hash values calculated for integrity verification. These records prove that the data remained unchanged and that the investigation followed accepted best practices. Failure to maintain a clear chain of custody can render evidence inadmissible.
Finally, reporting is as important as analysis. Investigators compile their findings into a formal report that describes the methodology, evidence discovered, and conclusions drawn. Screenshots of directory listings, excerpts from log files, and recovered artifacts bolster the narrative. A well-structured report enables decision-makers to take appropriate action, whether that means pursuing legal proceedings or tightening internal security controls. Proficiency in file system forensics not only helps uncover wrongdoing but also guides organizations in preventing future incidents. \nMastery of these techniques ensures digital investigations remain reliable and thorough.