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Lecture 4Lecture 4

Forensic Analysis of Windows SystemsForensic Analysis of Windows Systems

Prof. Shamik Sengupta

Office 4210N

ssengupta@jjay.cuny.edu

http://jjcweb.jjay.cuny.edu/ssengupta/

Fall 2010

What we will cover today

Understanding Hard Disk File concept Structure of a file Concept and detecting file types File system traces (Understanding MAC traces)

Hands-on Practice.

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Understanding Hard Disk

Richest source of digital evidence on computers among various storage media

Disk drives are made up of one or more platters coated with magnetic material

Hard disk drives are organized as a concentric stack of disks (platters)

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Understanding Disk Drives

Disk drive components

– Platter or disk– Each disk has two surfaces: top and bottom

– Accessed by two heads

– Tracks

– Cylinders

– Sectors

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Hard Disk (Continued)

Data are recorded on a platter in concentric circles called tracks– Tracks are numbered, starting from zero, starting at the outside of

the platter and increasing as you go in

– A modern hard disk has tens of thousands of tracks on each platter

– Each track broken into smaller units called sectors (typically 512 bytes)

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Hard Disk (Continued): Cylinders

A hard disk is usually made up of multiple platters– Because of this arrangement, often the track location of the

heads is not referred to as a track number but rather as a cylinder number

A cylinder is basically the set of all tracks that all the heads are currently located at

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Hard Disk (Continued)

Disk capacity?– Multiply the number of cylinders, heads, sectors and

512(block size)– Ex) What’s the size of disk which has 12,495 cylinders, 16 heads,

63 sectors?

12,495 cylinders * 16 heads * 63 sectors * 512 bytes = about 6GB

CHS– Each storage unit on a disk can be identified by a 3-

coordinate system identifying – Cylinder

– Head

– Sector

Zone Bit Recording (ZBR) in Hard Disks

The earlier concept of uniform sectoring was not efficient– every track had the same number of sectors

– But the ones on the outside of the platter are much larger than the ones on the inside

– Outer tracks are underutilized

New technique Zoned bit recording (ZBR)

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Understanding Files

File system– Gives OS a road map to data on a disk

Determines how data is stored on the disk

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File system Basics

File– Named collection of data objects

– Typically stored on secondary storage devices, though they can exist exclusively in volatile main memory

– Operations– Open, close, create, destroy, copy, rename, list, etc

File systems– Organizes files and manages access to data

– Directory– File containing the name and location of other files in the file

system

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File system Basics: FAT

FAT file systems– Simplest windows file systems:

– A FAT basically is an array that keeps track of what clusters belong to which file

– To locate data on a volume, directories and a FAT are used

– FAT: Older, for management of smaller disks– Compatible with 9X/2000/NT, MS-DOS, OS2, etc

– The original FAT had many limitations– No subdirectories

– Limited to very small disks

– Hard to recover the disk if allocation tables were damaged

– FAT32: enhanced version– Available with Win95

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Examining FAT Disks (continued)

When the OS stores data in a FAT file system, it assigns a starting cluster position to a file– Data for the file is written to the first sector of the first assigned

cluster

When this first assigned cluster is filled and runs out of room– FAT assigns the next available cluster to the file

If the next available cluster isn’t contiguous to the current cluster– File becomes fragmented

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FAT-based File system: Schematic

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Deleting FAT Files

In Microsoft OSs, when a file is deleted– Directory entry is marked as a deleted file

– With the HEX E5 (σ) character replacing the first letter of the filename

Data in the file remains on the disk drive

Area of the disk where the deleted file resides becomes unallocated disk space– Available to receive new data from newly created files or other

files needing more space

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FAT: Characteristics

FAT12– For floppy disks– 12-bit fields for each entry in FAT

FAT16 (MS-DOS, Win95/98/NT/2000/XP)– 16-bit fields to identify a particular cluster in the FAT– For hard disks

FAT32 (Win95/98/2000/XP)– 28-bit fields to identify a particular cluster in the FAT

– 4 bits of the 32-bit fields are “reserved”

– For larger hard disks– No limit on root directory size– Limited time/date information

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Examining NTFS Disks

New Technology File System (NTFS)– Introduced with Windows NT

– Primary file system for Windows Vista

Improvements over FAT file systems– NTFS provides more information about a file

– NTFS gives more control over files and folders

Different from FAT file system– Storing information in MFT (Master File Table)

– In NTFS, everything written to the disk is considered a file

NTFS was Microsoft’s move toward a journaling file system

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File system Basics: NTFS (Continued)

NTFS uses several metadata files to keep track of both files and folders on a given volume– Virtually everything is a file in NTFS including its

component parts

NTFS represents all characters using 16-bit Unicode including file and directory name– FAT file systems use the 8-bit ASCII/ANSI character set

for the most part– Long file names are represented with 16-bit Unicode characters

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Supported File Systems for MS OS

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File Allocation and Deletion in NTFS

Master File Table (MFT)– System file created during the formatting of an NTFS volume– Table that describes files and directories– Each file and directory has an entry in MFT

– Including an entry for itself and other metadata files– These metadata files are located in the root folder of a volume– Have names beginning with ‘$’ and not generally visible– E.g. Windows 2000 metadata file (In earlier versions of NTFS, these files are visible with,

e.g., “dir /ah” but invisible in XP)MFT File Record No.

File Name Description

0 $MFT Master File Table (MFT)

1 $MFTMIRR Copy of the first 16 records of the MFT

2 $LOGFILE List of file system transactions

3 $VOLUME Information about the volume, including NTFS version, volume name and creation time

4 $ATTRDEF Table of attribute definitions

5 . Root folder

6 $BITMAP Bitmap representation of used and unused clusters on volume

7 $BOOT Boot record with bootstrap loader code if the volume is bootable

8 $BADCLUS List of the bad clusters in the volume

9 $SECURE Stores security descriptions (Windows 2000 only)

10 $UPCASE Conversion table for converting lowercase characters to matching uppercase Unicode characters

11 $EXTEND Enables file system extensions such as volume quotas (Windows 2000 only)

22File Allocation and Deletion in NTFS (Continued)

MFT records store attributes of files and folder including– the name in the $FILENAME

– MAC times and other file characteristics in the $STANDARD_INFORMATION

– some or all of the data in a file using the $DATA attribute– A small file may exist entirely within its MFT record with no associated data

elsewhere on disk

E.g. Simplified MFT record for a file

Header $FILENAME $STANDARD-INFORMATION

$DATA Attribute list

23File Allocation and Deletion in NTFS (Continued)

$BITMAP file– System file created during formatting of NTFS volume to keep track

of cluster usage

– Uses one bit to record the status of each cluster on the volume– E.g. If a cluster in an NTFS volume is used, the corresponding bit in the

$BITMAP file is changed to 1

24File Allocation and Deletion in NTFS (Continued)

Steps to allocate a file on an NTFS volume– 1) $BITMAP file must be modified to reflect that the used

clusters are allocated

– 2) Allocated MFT record must be created for the file

– 3) An index entry must be created for the file name in the parent’s folder’s MFT record

– 4) cluster extent entries must be created in the file’s MFT record if the file is non-resident

When a file is deleted– 1) Its cluster reference in the $BITMAP file are changed to 0

– 2) MFT record for that file is marked for deletion

– 3) Its index entry is deleted

Analyzing the structure of a file: Hands-on investigation

Understanding the structure of a file for digital investigation

Data are stored on computers as files

But files can be corrupted meaningfully to hide data

To understand the corruption, we must understand the structure of a file

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Analyzing the structure of a file

The simplest structure of a file consists of:– Filename

– filename is a unique identifier which allows the computer to correctly identify each file

– File header– information which describes the type of the file

– Important for file header signature

– Important for OS to understand which application to be associated

– File content– Actual data

files can be corrupted meaningfully to hide data– Change or delete the file type (today’s lab)

– Delete the data!

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Case Study: Hiding a pdf

There are simple ways to hide a pdf from OS

– Change the file type

– Delete the file type

– Let’s check the file type by looking at Hexadecimal signature values

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Fun lab

Download the test.zip from the class website and see if there is any corruption done with the files. If yes, can you detect the file types?

http://www.garykessler.net/library/file_sigs.html

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File System Traces

An individual’s actions on a computer leave traces– Temporal traces is very important to understand the file’s

life– When it was created/downloaded

– When accessed

– When modified etc.

– An understanding of the cause-effect relationship in a reverse manner is particularly important for digital investigator

– What is cause-effect relationship in forward manner?– You changed something, you see the changed result

– What is cause-effect relationship in forward manner?– You see the changed result, you have to guess what caused it

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MAC Times in NTFS

Windows records the date and time of a file’s– Creation : created

– The date and time that a file was created on the current volume

– Last modification: modified– The date and time that a file was last modified

– Last accessed: accessed– The date that a file was last accessed

An examination of the MAC times of a file can provide insight into – the file’s history on a computer

– the extent of the user’s knowledge of the file’s existence and contents

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MAC Times in NTFS (Continued)

Case Example – MAC times

In a prosecution for the possession and distribution of “CP”, a Supervising Special Agent for the California Department of Justice charted the MAC times of several files.

The defendant asserted at trial that he merely downloaded files of unknown content and then forwarded them to others without having viewed them himself.

During a trial recess, the prosecution and defense attorneys held a conference to disclose and discuss the Supervising Special Agent’s scheduled testimony regarding the results of his forensic examination.

The agent informed and described how many files had been accessed at times far remote from their dates of creation.

As a result of this discussion, the defendant entered a plea of guilty prior to the agent’s testimony

In a prosecution for the possession and distribution of “CP”, a Supervising Special Agent for the California Department of Justice charted the MAC times of several files.

The defendant asserted at trial that he merely downloaded files of unknown content and then forwarded them to others without having viewed them himself.

During a trial recess, the prosecution and defense attorneys held a conference to disclose and discuss the Supervising Special Agent’s scheduled testimony regarding the results of his forensic examination.

The agent informed and described how many files had been accessed at times far remote from their dates of creation.

As a result of this discussion, the defendant entered a plea of guilty prior to the agent’s testimony

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MAC Times in NTFS (Continued)

Example – MAC times

Can be viewed by– dir /TC

– dir /TA

– dir /TW

MAC times can also be– viewed and sorted using

Forensic Tools

Understanding the temporal traces

File moved within a volume– No change in last-modified, last-accessed, last-created

File moved/copied across volume– Last-accessed and last-created time change

– NO change in Last-modified!

– However, for parent directory listing– Last-accessed and last-modified updated. WHY?

Homework Assignment #1 will be up by tomorrow

Next up, more forensic examination of Windows and EnCase

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