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Ch a p t e r 2 3 - D a t a R e c o v e r y

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Figure 214, Recover Folders results

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23.3

NTFS DATA RECOVERY

When a file is deleted in a NTFS file system, the data content of the file remains

available for recovery from the newly unallocated clusters. The original data will

remain in each cluster up until such time as it is used to store new data and the

previous content overwritten.

If only a percentage of clusters are reused, then partial recovery, or the recovery of a

͞data fragment͟, may still be possible. If all clusters are re-used, all original content is

overwritten and destroyed.

23.3.1

NTFS - DELETED FILES

Each file and folder on an NTFS drive has an ͞allocation status͟ set by a flag in the

Master File Table (MFT) record header. The flag identifies whether it is an ͞allocated͟

(active) file, or ͞unallocated͟ (deleted). To display deleted files, Forensic Explorer

reads the MFT to find ͞unallocated entries͟.

Allocation status flag values are shown in the tables Table 1 and Table 2 below:

Table 1, Allocation status for a file

Flag Value for a file

Hex

Binary

Status

00000000

Unallocated

00000001

Allocated

Table 2, Allocation status for a folder

Flag value for a folder

Hex

Binary

Status

00000010

Unallocated

00000011

Allocated

In Forensic Explorer, the allocation status of a file is shown in Filesystem Record view

when the file is highlighted:

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Figure 215, Forensic Explorer Record view showing decoded MFT allocation status (an allocated file)

When the MFT record is marked as unallocated, both the MFT record and clusters

used to store the data (for non-resident files) become available to store new data.

However, importantly:



the file attributes within the unallocated MFT record remain intact;



the data for the file remains untouched.

When new data is written to the MFT record or the clusters holding the data, the

possibility for successful recovery of the deleted file is diminished.

23.3.2

NTFS - ORPHANS

In Folders view a folder is created by Forensic Explorer called ͞Orphans͟. Orphans are

deleted folders and files for which the original parent folder is unknown.

From the investigators perspective, an orphaned file can be treated in an investigation

the same way as any other deleted file. The only difference is that it is longer possible

to determine the location of the file or folder within the directory structure prior to

deletion.

An example of how NTF folders and file can become orphaned is as follows:

A folder on an NTFS drive, ͞PARENT-1͟ is deleted by the user. At this point

PARENT-1 and its content, ͞ HILD-FOLDER-1͟, are deleted files.

The user then saves a new file. The MFT record for PARENT-1 is re-used to

store information for the new file. The MFT information for PARENT-1 is now

overwritten and destroyed.

The computer is then forensically imaged and examined.

Forensic Explorer reads the file system and CHILD-FOLDER-1 is located.

Forensic Explorer then tries to trace the parent folder, but determines that

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the MFT record for the parent folder has been re-used by another file and

the original information for the parent is no longer available.

CHILD-FOLDER-1 and its content are available, but Forensic Explorer cannot

determine where in the tree structure it belongs. The Orphans folder is

created by Forensic Explorer to hold CHILD-FOLDER-1 and its content.

23.3.3

NTFS - RECOVER FOLDERS

͞Recover Folders͟ is a method of searching unallocated clusters to find deleted or

missing folders and their content. Recover Folders will often locate multilevel folder

and sub folder structures and make them visible to the investigator within the Forensic

Explorer module. For this reason it is recommended that a Recover Folders search be

one of the first tasks undertaken by an investigator in a new case.

To run a Recover Folders search, click the Recover Folders toolbar icon in the File

System module:

Figure 216, Recover Folders File System module toolbar icon

This opens the Folder Carve options window:

Figure 217, Recover Folders options

When the ͞Recover Folders͟ command is executed on a NTFS partition in Forensic

Explorer, the program searches unallocated clusters for MFT records.

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The process is identical to that described in ͞NTFS Orphans͟ above. The only

difference is that instead of working with files in existing MFT records, the MFT

records themselves are recovered from unallocated space.

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23.4

FILE CARVING

File carving is a well-known computer forensics term used to describe the

identification and extraction of file types from unallocated clusters using file

signatures. A file signature, also commonly referred to as a magic number, is ͞a

constant numerical or text value used to identify a file format or protocol͟ (16).

An example of a file signature is shown in Figure 218, which is the beginning of a .jpg

file in Hex view:

Figure 218, View of .jpg file header

The object of the carving exercise is to identify and extract (carve) the file based on

this signature information alone. Carrier (2005) describes File carving as:

͞…a process where a chunk of data is searched for signatures that correspond

to the start and end of known file types. The result of this analysis process is a

collection of files that contain one of the signatures. This is commonly

performed on the unallocate space of a file system and allows the investigator

to recover files that hav no metadata structures pointing to them͟. (2)

23.4.1

CARVING ADVANTAGES AND LIMITATIONS

File carving has both advantages and limitations. These include:

File system independent

File carving is essentially file system independent. A file type will exhibit the

same file signature and structure on under FAT, NTFS, HFT, EXT2 or other file

systems and can be data carved accordingly. File carving is also effective

method of recovery when the file system is corrupt or destroyed.

Time Required:

A drawback of file carving is that it can take a considerable amount of time to

process a large drive. The lower the level of search (i.e. cluster v͛s sector v͛s

byte), and the greater the number of file signatures searched for

simultaneously, the longer the search.

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False Positives:

File carving always brings with it the risk of false positives, where identified

file signatures are not true identifiers for the start of a file. Searching at the

lower levels of sector and byte may increase the number of false positives

because it removes the validation requirement that the signatures must start

near cluster boundaries.

Data Fragmentation:

Without file system records, it is difficult to track a fragmented files. File

carving relies on the information contained in the file structure and to a lesser

extent it͛s on disk layout.

No Original File Names

As file names are stored only as part of the file system, data carved files

cannot be recovered with their original name.

23.4.2

FORENSIC EXPLORER FILE CARVING ENGINE

Forensic Explorer has an inbuilt file carving engine capable of carving more than 300

file types.

To run a file carve using the Forensic Explorer file carving engine:

Switch to the File System module;

Click the File Carve button on the ribbon;

Figure 219, File System module, File Carve button

The ͞File arving͟ selection window, shown in Figure 220 will open:

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Figure 220, File carving file signature selection window

CARVE NAME

The carve name is the name of the folder which holds the carve results. This folder is

displayed in Folders view of the File System module. The default name, ͞ arve 1͟ can

be edited during setup of the search.

Ch a p t e r 2 3 - D a t a R e c o v e r y

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Figure 221, File Carve results

SOURCE

A File Carve is usually run on unallocated space. However it is possible to carve on a

specific file, such as the Windows page file, or a backup file, by first checking the file in

the File System module and then selecting to carve the checked items.

CARVE SEARCH MODE:

Cluster based file carving

In a cluster based file system like FAT or NTFS a new file must start in a new

cluster. It follows then that the file signature appears near a cluster boundary.

Carving speed is therefore achieved by searching for file signatures only near

cluster boundaries.

Sector based file carving (recommended)

It is recommended to perform a lower level search for sector-aligned file

signatures. This search may recover additional files, for example files from a

previous volume which had a different cluster layout and is no longer aligned

to current cluster boundaries.

NOTE: Carving in sector mode will increase the length of the search.

Byte based file carving

In certain situations it is necessary to data carve at a byte by byte level. This

will locate additional files where the file signature is neither aligned with a

cluster or sector boundary.

Sector carving is used to recover files from mobile/cell phone image files.

NOTE: Carving in byte mode will greatly increase the length of the search.

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SELECTING FILE TYPES TO CARVE

Select the required file signatures by placing a tick in the selection box and click OK to

begin the search.

NOTE: It is recommended that in order to maintain search speed, no more than 10 file

signatures be selected at one time.

CARVE PROGRESS

The progress of the data carve is shown in the processes window. To stop a data carve

click the stop button in this window.

DEFAULT SIZE ALLOCATION

When a file signature of a selected file is located, Forensic Explorer will analyze the file

structure in an attempt to locate the end of the file. If the file end is not found, but

sufficient information is found within the file to suggest it will at minimum be partially

recovered, it is assigned a pre-determined default file size according to that file type.

LOGGING AND PRIORITY

See 7.5 - Process Logging and Priority.

23.4.3

CARVING USING SCRIPTS

The second file carving method available in Forensic Explorer is to use a custom file

carving script. An investigator may use, modify or write a script to suit their data

recovery needs.

For more information on scripts, please refer to Chapter 18 - Scripts Module.

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