Authors: Andreas Dewald, Sabine Seufert
DFRWS EU 2017
In forensic computing, especially in the field of postmortem file system forensics, the reconstruction of lost or deleted files plays a major role. The techniques that can be applied to this end strongly depend on the specifics of the file system in question. Various file systems are already well-investigated, such as FAT16/32, NTFS for Microsoft Windows systems and Ext2/3 as the most relevant file system for Linux systems. There also exist tools, such as the famous Sleuthkit (Carrier), that provide file recovery features for those file systems by interpreting the file system internal data structures. In case of an Ext file system, the interpretation of the so-called superblock is essential to interpret the data. The Ext4 file system can mainly be analyzed with the tools and techniques that have been developed for its predecessor Ext3, because most principles and internal structures remained unchanged. However, a few innovations have been implemented that have to be considered for file recovery. In this paper, we investigate those changes with respect to forensic file recovery and develop a novel approach to identify files in an Ext4 file system even in cases where the superblock is corrupted or overwritten, e.g. because of a re-formatting of the volume. Our approach applies heuristic search patterns for utilizing methods of file carving and combines them with metadata analysis. We implemented our approach as a proof of concept and integrated it into the Sleuthkit framework.