Htcia an introduction to the microsoft ex fat file system 1.01 final

HTCIA 2014 International Conf
Hyatt Lost Pines Resort, Austin Texas
Tuesday August 26, 2014 8:00am
Introduction to the Microsoft exFAT
HTCIA 2014 Conf - Aug 26, 2014
File System
Robert Shullich
CPP, CISSP, CRISC, GSEC, GCFA, CEH, CHFI, CCFP-US

Agenda
• About me, the paper and the presentation
• The need for a new generation of FAT
• Digital Forensics Relevance
• Exponents and Standards
• exFAT Overview
• Linux Development
• Memory Cards & Flash Memory
• exFAT File System Internals
• Closing

About me, the paper and the presentation
About Me
About the Presentation
About the SANS Paper
A Gold Standard
Another Paper Reference
Disclaimer
EXFAT

About Me
• I have been in the IT field for 40+ Years, and in
InfoSec for over 20 Years
• I carry many IT and InfoSec certifications
• This research was originally for a class term
project towards my D4CS MS degree
• I then expanded that term paper into a practical
paper for my SANS “Gold” GCFA certification
• Links to the SANS paper and my blog are
provided at the end of this presentation

About the Presentation
What I call the exFAT Road Show
• The New York Forensics Computer Show 4/20/2010
• Techno Security and Digital Investigations 6/7/2010
• SANS What Works in Forensics and IR Summit 7/8/2010
• HTCIA International Training Conference & Expo 9/20/2010
• The New York Forensics Computer Show 4/19/2011
http://techchannel.att.com/play-video.cfm/2011/8/16/Conference-TV-Computer-
Forensics-Show:-Introduction-to-exFAT
• NYC4SEC 6/11/2014
• HTCIA International Training Conference & Expo 8/26/2014

About the SANS Paper
• Consider it “exFAT – the missing manual”
• Very little published about exFAT today
• Two current forensics books mention exFAT:
• Wiley - Mastering Windows Network Forensics and Investigation
• Sybex - EnCase Computer Forensics - The Official EnCE: EnCase
Certified Examiner
“For those seeking an in-depth understanding of the exFAT file
system, you should read the SANS paper entitled “Reverse
Engineering the Microsoft Extended FAT File System (exFAT) by
Robert Shullich”

A Gold Standard
• 2005 Book considered the
authority on different file
systems
• The book’s Author
developed the open-source
TSK forensics tools (The
Sleuth Kit) & Autopsy
• This year adding exFAT to
TSK

Another Paper Reference

Disclaimer
• The released specification and implementation is Release
1.00 of exFAT
• The specification mentions additional features that were
not implemented yet, but may at a future time/ Some of
these are Windows CE holdovers
• Both may be presented today
• Some directory entries will be skipped
• Focus is Microsoft Desktop/Server implementation
• Will talk about Flash/Solid State, but high level
• For exFAT, tried to stay with the patent terminology

The need for a new generation
Legacy FAT
Why do we need a new file system?
Why do we need Faster I/O and Higher Capacity?
Hi-definition movie recording MPEG-4. H.264
EXFAT

Legacy FAT
• FAT 8
• 1977 Bill Gates and Marc McDonald
• Floppy based
• FAT 12
• 1980
• FAT 16
• 1984 with release of PC/AT & MS DOS 3
• FAT 16B
• 1987 Compaq DOS 3.31
• FAT 16X
• 1995 PC DOS 7.0/Win 95 – LBA Addressing
• FAT 32
• 1996 Windows 95 OSR2, 98, ME, MS DOS 7.1 – CHS Addressing
• FAT 32X
• LBA Addressing

Why do we need a new file system?
• Current Limits Exhausted (Ran Out of Bits!)
• Larger volumes (>2TB) (Scale to Larger Capacity)
• Larger files sizes (>4GB)
• Faster I/O
• (UHS-I: 104 MB/s - UHS-II: 312MB/s)
• Removable Media
• Flash/Solid State Media
• Flexibility
• Extensibility (Difficult to add new features)
• NTFS Features without the overhead
• Easier to implement FS in firmware

Why do we need Faster I/O and
Higher Capacity?
http://www.cnet.com/news/what-is-4k-uhd-next-generation-resolution-explained/

Hi-def movie recording MPEG-4. H.264
2 GB 4 GB 8 GB 16 GB 32 GB
Fine mode
(13Mbps/CBR)
20 min 40 min 80 min 160 min 320 min
Normal Mode
(9Mbps/VBR)
Economy mode
(6Mbps/VBR)

Digital Forensics Relevance
Relevance to Forensics Study
What happens when you have exFAT formatted media and no
exFAT support?
Forensics Challenges in 2009
Forensics Challenges Today
EXFAT

Relevance to Forensics Study
• Digital Evidence Extraction
• Finding the evidence
• Including the hiding places
• Validation
• Completeness
• Daubert Expert Testimony
• Need to know and understand file org
• Establish Credibility
• New Media (SD Cards) will drive exFAT adoption, and
the potential for CP investigations.
• Larger Media Capacity also driving exFAT adoption

Trust but Verify

What happens when you have exFAT
formatted media and no exFAT support?

Forensics Challenges 2009
• In 2009, in regards to exFAT:
• No tools (RAW)
• No documentation or Training
• No expertise
• Evidence backlog

Forensics Challenges Today
Today
• exFAT Misunderstood
• Linux OS Support
• Tuxera drivers may help (Embedded)
• FUSE and No-FUSE hacks
• Most Distributions – No native support
• Mac OS Support (Nov 2010) OS/X 10.6.5+
• Implementation Deviations, No Standards
• Open Source Tools
• Commercial Tools
• Encase (6.14.3 Dec 2009)
• Encase (6.18.0.59) NIST Test March 2014
• FTK (3.2 Oct 2010)
• FTK (3.3) NIST Test April 2014
• Cross Vendor Compatibility

NIST Computer Forensics Tool Testing
• Cyber Fetch
• AAFS-2013 Conference 02/21/2013
• Deleted File Recovery Tool
Testing Results
• One Summary Item:
Support for ExFAT, ext3 &
ext4 is sometimes lacking.

Test Results for Deleted File Recovery and
Active File Listing
• 17 Basic Tests
• March 12, 2014 – Encase 6.18.0.59
• MAC differed by 9 hours
• April 3, 2014 – FTK 3.3.0.33124
• MAC differed by 4 hours
• The exFAT partition and HFS+ created on OS/X 10.6
• exFAT: ctime meta-data replaced with the time of
file deletion [I was unable to recreate]
• Vendor Tool or Apple Implementation?
• Who Validates the Test?

Who Validates the Validator?
Superman: Easy, Miss, I've got you
Lois Lane: You...you've got me, who's got you?

Exponents and Standards
Base 2 or 10?
Exponents
International System of Units (SI) Table
IEC 60027-2
Reference Standards
Endian
Microsoft Math
More Math – exFAT
WinCE
EXFAT

Base 2 or 10?

Exponents
• 102 = 10 times 10 = 100
• 103 = 10 times 10 times 10 = 1000 (1K)
• 22 = 2 times 2 = 4
• 29 = 2*2*2*2*2*2*2*2*2 = 512
• 210 = 2*2*2*2*2*2*2*2*2*2 = 1024 (1K)
• 212 = 2*2*2*2*2*2*2*2*2*2*2*2 = 4096

International System of Units (SI)
Table
• File System in
powers of 2
• Device
characteristics in
power of 10
Shorthand Longhand Nth Bytes
KiB Kibibyte 210 1024
MiB Mebibyte 220 1024 KiB
GiB Gibibyte 230 1024 MiB
TiB Tebibyte 240 1024 GiB
PiB Pebibyte 250 1024 TiB
EiB Exbibyte 260 1024 PiB
ZiB Zebibyte 270 1024 EiB
YiB Yobibyte 280 1024 ZiB

IEC 60027-2
Prefixes for binary multiples
Factor Name Symbol Origin Derivation
210 kibi Ki kilobinary: (210)1 kilo: (103)1
220 mebi Mi megabinary: (210)2 mega: (103)2
230 gibi Gi gigabinary: (210)3 giga: (103)3
240 tebi Ti terabinary: (210)4 tera: (103)4
250 pebi Pi petabinary: (210)5 peta: (103)5
260 exbi Ei exabinary: (210)6 exa: (103)6
Examples and comparisons with SI prefixes
one kibibit 1 Kibit = 210 bit = 1024 bit
one kilobit 1 kbit = 103 bit = 1000 bit
one mebibyte 1 MiB = 220 B = 1 048 576 B
one megabyte 1 MB = 106 B = 1 000 000 B
one gibibyte 1 GiB = 230 B = 1 073 741 824 B
one gigabyte 1 GB = 109 B = 1 000 000 000 B
http://physics.nist.gov/cuu/Units/binary.html

How far off are we?
When we say but mean we're this far off
1 kilobyte 210 bytes 2.4%
1 megabyte 220 bytes 4.9%
1 gigabyte 230 bytes 7.4%
1 terabyte 240 bytes 10.0%
1 petabyte 250 bytes 12.6%
1 exabyte 260 bytes 15.3%
http://cnx.org/content/m13081/1.1/

Reference Standards
• Bits are numbered right to left
76543210
• Decimal Offsets (zero based)
• Little-Endian numbers
• Unsigned numbers
• Sectors vs. Clusters
• Strings are 16 bit Unicode
• Strings not Terminated

Endian
• Numbering order may vary based on processor
type, is determined by the order the data bytes
are read from the register.
• A 32 bit number is read as 4 8-bit bytes
• If I have the number 0x11 22 33 44
• Big-Endian will store it as:
0x 11 22 33 44
• Little-Endian will store it as:
0x 44 33 22 11

Microsoft Math
KB184006 Limitations of FAT32 File System
The maximum possible number of clusters on a volume using the
FAT32 file system is 268,435,445. With a maximum of 32 KB per
cluster with space for the file allocation table (FAT), this equates to a
maximum disk size of approximately 8 terabytes (TB).
512B Sectors in a 32 KB cluster = 64
228 (268,435,445) * 26 (64) * 29 (512) = 243 = 8,796,093,022,208
Size of FAT32 FS specified in BPB as sectors (32 bit number)

More Math, exFAT
KB955704
Description of the exFAT file system driver update package
• Support for volumes that are larger than 32 GB, the theoretical
maximum volume size for FAT32 in Windows XP
• The theoretical maximum volume size is 64 ZB.
• The recommended maximum volume size is 512 TB.
• Support for files that are larger than 4 GB, the theoretical maximum
file size for FAT32 in Windows XP
• The theoretical maximum file size is 64 ZB.
• The recommended maximum file size is 512 TB.

WinCE
Version Released End of Support
1.0 November 18, 1996 December 31, 2001
2.0 September 29, 1997
2.11 September 30, 2002
2.12 September 30, 2005
3.0 June 15, 2000
4.X October 9, 2007
4.0 January 7, 2002 July 10, 2012
4.1 January 8, 2013
4.2 July 9, 2013
5.X August 2004 October 14, 2014
6.0 September 2006 April 10, 2018
7.0 March 2011 April 13, 2021
2013 June 2013 October 10, 2023

Overview
Features of exFAT 1.00
4K (4096) Sector Size
Supported Cluster Sizes
Features of exFAT 1.00 (cont’d)
Future Features of exFAT
MBR Partition Limitations
Advantages of exFAT
Disadvantages of exFAT
OS Support for exFAT
Key Dates for exFAT
EXFAT

Features of exFAT 1.00
• Maximum Volume Size (Increased Capacity)
• Architectural ≈ 128 PiB (232-11 * 225)
• Implementation = 512 TiB
• Sector sizes from 512 [SF] to 4096 bytes [AF]
• Clusters sizes to 32MiB (225)
• Subdirectories to 256MiB (Root not restricted)
• Maximum files on volume ≈ 232
• Maximum File Size 16 EiB-1
• Built for speed, less overhead than NTFS
• Catches up with some NTFS features
• Template-based metadata structures
• On-disk storage of file Valid Data Length (VDL)
• Speeds up storage allocation processes

4K (4096) Sector Size

Supported Cluster Sizes

Features of exFAT 1.00 (cont’d)
• OEM Parameters Sector for device dependent parameters
• 12 sector VBR, support of larger boot program
• Up to 2,796,202 files per sub-subdirectory
• File Names max to 255 Characters
• 16-Bit Unicode File Names and Volume Labels
• Optimized for Flash Memory
• Device Boundary Alignment
• No FAT32 minimum cluster (65,525) restriction
• No 8.3 file name support (only LFN)
• UTC Timestamp Support
• Vista/Server 2008 SP2+, XP/Server 2003 with KB
• Native in Windows 7, 8, 8.1, Server 2008 R2, 2012

Future Features of exFAT
• TexFAT (To be released later)
Exists in Windows CE
Transaction Safe exFAT
• ACL (To be released later)
Exists in Windows CE
• Compression & Encryption Support?
Not announced, but would be easy to add

MBR Partition Limitations
• Microsoft File Systems are limited when stored in
a MBR partition
• A partition is defined by a Master Boot Record
• A MBR uses a 4 byte value for number of sectors
• LBA as 32 bit # times 512 Sector limits to 2TiB
• To get the maximum volume size, exFAT cannot
be created within a MBR partition, Need GPT
GUID Partition, or Super floppy Mode
• ExFAT on GPT works on Mac

Advantages of exFAT
• Large volume, file and directory sizes
• Handle growing capacities in media, increasing capacity
to >32 GB.
• > 1000 files in a single directory.
• Speeds up storage allocation processes.
• Breaks file size 4 GB barrier.
• Supports interoperability with future desktop OSs.
• Provides an extensible format.
• Large cluster sizes
• Metadata integrity with checksums

Disadvantages of exFAT
• Not all Windows CE features implemented
• No direct conversion to or from other FS
• Cannot use CONVERT command to NTFS
• No Floppy Support
• Mostly a Microsoft Desktop and Server World
• No Support for Older MS systems (Pre-XP)
• Support for other devices, surfacing
• No Information Sector “Hint”
• Like all FAT – Finding Stuff is via brute force

OS Support for exFAT
• Windows XP & Server 2003
• KB955704 (requires SP2 or SP3)
• Vista & Server 2008 SP1
• Vista & Server 2008 SP2
• (Adds UTC timestamp support)
• Windows 7/Server 2008 R2 and later:
• RTM
• Mac OS/X 10.6.5 and later

Key Dates for exFAT
• September 2006 – Windows CE 6.0
• March 2008 – Windows Vista Service Pack 1
• January 2009 – Announcement at CES of SDXC specification
• January 2009 – Windows XP Drivers Available
• May 2009 – Windows Vista Service Pack 2
• August 2009 – Tuxera Signs File System IP Agreement with
Microsoft
• March 2009 – Pretec Releases first SDXC Cards
• December 2009 – Microsoft (re)announces exFAT license program
for third-parties
• December 2009 – SDXC laptops due soon
• December 2009 – Diskinternals releases exFAT recovery utility
• December 2009 – Encase support

More Key Dates for exFAT
• December 2009 Sony, Canon & Sanyo License
• January 2010 Funai License (LCD TV)
• February 2010 Panasonic License
• February 2010 Panasonic 64/48GB SDXC
• February 2010 Sony Memory Stick XC
• February 2010 SanDisk Ultra SDXC 64GB Card 3.0 Spec $350
• April 26, 2010 DCF Version 2.0 (Edition 2010)
• June 1st 2010 Tuxera Releases Linux & Android exFAT drivers
• June 3rd 2010 Kingston Releases Class 10 SDXC 64GB Card 60 MB/s
read, 35 MB/s write.
• October 11th, 2010 FTK 3.2 with exFAT support announced

More Key Dates
• Mar 16th 2011 Lexar Releases SDXC 128GB
• May 3rd, 2011 e.solutions (Volkswagen)
• Aug 8, 2012 Sharp for Android Smart Phones
• Sep 18, 2012 RIM (Blackberry) Smartphones
• Nov 7, 2012 Sharp, Sigma, NextoDi, Black Magic and Atomos Global
• Jan 16, 2013 BMW
• April 30, 2014 PS4 V1.7 update – hidden new feature: exFAT

Linux Development
FUSE Project
Samsung (No-FUSE)
EXFAT

Linux Development
• Open Source community developing FUSE
• FUSE – File System in User Space
• Samsung accidently leaks native exFAT
implementation, dubbed NO-FUSE
• Samsung source code on GitHUB with GPL
License
• Still legal issues because of patent protection

FUSE Project

Samsung (No-FUSE)

Memory Cards (Including SSD)
Applications (IOT)
exFAT Gone Wild
SD Card Association
Compact Flash
SDXC Storage Capabilities
Standard vs. Non-Standard
General Flash Notes
SD Card Notes
EXFAT

Applications (IOT)
• Camera (Still, Video)
• Entertainment Systems (Home, Plane, Train, & Automobiles)
• GPS, Navigation Systems
• Smart Phones, Audio/MP3 players
• Laptop, Monitor, Printers
• Handheld Computers (Tablets, Netbooks, Mobile)
• Smart TVs, Home Theaters
• Automatic inflight infotainment systems
• Game Consoles
• Medical Devices
• Measuring Equipment
• Other Consumer Electronics

exFAT Gone Wild
• Adoption Rate ↑
• Prevalence ↑
• Media Prices ↓
Storage Media larger than 32GB is being shipped out of the
factory door pre-formatted with the exFAT file system
NTFS, FAT32, and HFS+ are still used in some cases but to
a lesser degree

SD Card Association
• New Memory Card SDXC
• Consumer Appliances
• Follows SDHC
• Specification for 2TB
Maximum Capacity

http://anythingbutipod.com/2009/01/next-generation-sdxc-details/

Market for SD Cards to Reach
$21.3 Billion by 2018
The SD technology is employed by over 400
brands across numerous product categories
and over 8,000 models, making it the de-facto
industry standard. SD memory cards have been
able to meet the requirements of high-end
consumer devices.
http://www.storagenewsletter.com/rubriques/market-reportsresearch/global-industry-analysts-sd-cards/

Compact Flash
• Small Market
• Specification 5.0 (Feb 2010)
• Specification 6.0 (Nov 2010)
• 48-Bit Addressing
• Max Size 144PB (Up from 137GB)
• UltraDMA 7 (167MBytes/s)
• FAT32 won’t do (2TB Limit)
• SanDisk factory preformats 256GB CF using exFAT
• Not Sure Where the file system support will go, but
expect that exFAT will also become a FS of choice for
other media

SDXC Storage Capabilities
• From 32GB to 2TB on a card
• Exclusively exFAT File System
• 312 MB/s I/O Transfer (UHS-II)
• Storage (examples)
4,000 RAW images (14mb file size/64GB)
136,000 fine-grade photos
100 HD movies
480 hours of HD recording
On a single 2TB SDXC card

Standard vs. Non-Standard
• SDXC is supposed to be exFAT
• In computer, you can format as anything
• Many devices, will enforce standard
• Formatting SD card with OS Format has
issues and differences
• Don’t assume FS based on card type

General Flash Notes
• Write Endurance (Program Erase Cycles)
• Write Cliff
• Wear Leveling
• Pages (Unit of a write)
• Blocks (Unit of an erase)

SD Card Notes
• SDXC Maximum set at 2TB
• Two FAT Partitions within MBR
• “Protected Area” and “User Area”
• WinHex – Partition Offset ≠ 0
• VBR differences on format/factory
• AU (Allocation Unit) same as Cluster Size
• Max AU = 64MiB
• RU (Recording Unit) 16KB+
• FAT Write Cycle {FAT1/FAT2/DIR}
• exFAT Write Cycle {FAT/ABM/DIR}

File System Internals
Regions
FAT
VBR
Directories
Volume Label
Allocation Bit Map
UP Case Table
File Directory Entry Sets
EXFAT

File System Integrity
• Version Verified
• 4 Checksums
• VBR
• UP-Case Table
• Directory File Set entry
• Directory GUID entry
• Critical Directory Entries
• Other Checks and Balances
• File System should NOT mount if failures
• File System may mount R/O when dirty
• Dirty flags in VBR, not in the FAT

Data Hide Alert!
• FAT32 max cluster 64KiB
• exFAT max cluster 32MiB
This is an increase of 512 fold
• Potential for massive slack space

Volume Space Layout
• The Main Boot Region
• Contains main VBR
• The Backup Boot Region
• Contains backup VBR
• The FAT Region
• Contains FAT Table(s)
• The Data Region (Cluster Heap)
• This is where data resides

VBR – Volume Boot Record
• Contains 12 sectors
1 sector main boot sector
• Jump Code (3 bytes)
• Must be Zero (53 bytes)
• BPB (BIOS Parameter Block)
• Boot Strap Code
8 sectors main extended boot sectors (MEBS)
1 sector OEM parms
1 sector reserved
1 sector VBR Checksum

Boot Parameter Block (BPB)
• OEM Label “EXFAT ”
• Volume Length (64-bit) [sector]
• FAT Location & Size [sector]
• Heap Location & Size [sector, cluster]
• Volume Serial Number
• Location of Root Directory [cluster]
• Volume Flags
• Sector and Cluster Sizes [2-shift]
• Percent in use
• File System Revision (0x0010=1.00)

Sectors & Clusters
• A 2-Shift is a power of 2
• Another name for exponent
• Sector size and sectors per cluster
• Each stored in 1 byte
• Theoretical maximum is 2255
• Sector Size Maximum 212
• Sectors per cluster is derived
• Cluster Size Maximum is 225

Executable Boot Code
• First 3 bytes of Main Boot Sector
• Jump Code
• 0xEB7690
• Offset 120 size 390
• Remainder of boot code
• Offset 510
• End signature marker
• 0xAA55 = “55AA”
• Offset 512
• Unused if defined

More Bootable Code
• Up to 8 Main Extended Boot Sectors
• FAT32 had 3 sector VBR with 1 MEBS
• Entire sector can be used for boot code
• Last 8 bytes of sector is marker
• 0xAA550000 = “000055AA”
• Larger capacity for boot virus!

VBR Checksum Sector
• The 12th sector of the VBR
• Repeating 4 byte checksum
• Checksum of previous 11 sectors
• Flags and Percent excluded
• These are volatile and change often
• Boot Sector Virus & Checksum

VBR Checksum Sector
Offset 0 1 2 3 4 5 6 7 8 9 A B C D E F
00000000 C9 D0 18 8B C9 D0 18 8B C9 D0 18 8B C9 D0 18 8B ÉÐ.‹ÉÐ.‹ÉÐ.‹ÉÐ.‹
Lines 00000050 through 01BF repeated
000001C0 C9 D0 18 8B C9 D0 18 8B C9 D0 18 8B C9 D0 18 8B ÉÐ.‹ÉÐ.‹ÉÐ.‹ÉÐ.‹
000001D0 C9 D0 18 8B C9 D0 18 8B C9 D0 18 8B C9 D0 18 8B ÉÐ.‹ÉÐ.‹ÉÐ.‹ÉÐ.‹
000001E0 C9 D0 18 8B C9 D0 18 8B C9 D0 18 8B C9 D0 18 8B ÉÐ.‹ÉÐ.‹ÉÐ.‹ÉÐ.‹
000001F0 C9 D0 18 8B C9 D0 18 8B C9 D0 18 8B C9 D0 18 8B ÉÐ.‹ÉÐ.‹ÉÐ.‹ÉÐ.‹

FAT – File Allocation Table
• When it is used, same as legacy FAT
• Not used when file contiguous
• Never used for cluster allocation
• FAT 32 has 32 bit cells, uses 28 bits (LBA-28)
• exFAT has 32 bit cells, uses 32 bits (LBA-48)
• There is no 64 bit FAT
• Maximum clusters is 232-11
• With TexFAT – 2 FAT Tables (2 Bitmaps)
• 1st Addressed by pointer in VBR, 2nd Immed Follows
• Size stored in VBR

Reserved Cluster Index Values
• 0x00000000 – No significant meaning
• 0x00000001 – Not a valid cell value
• 0xFFFFFFF6 – Largest Value
• 0xFFFFFFF7 – Bad Block
• 0xFFFFFFF8 – Media Descriptor
• Fixed Disk
• 0xFFFFFFF9-0xFFFFFFFE – Not Defined
• 0xFFFFFFFF – End of Cluster Chain (EOC)

FAT Table Example
Media Reserved
Allocation Bit Map UP-Case Table
Root Directory
Offset 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
0000 F8 FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF
0010 FF FF FF FF 00 00 00 00 00 00 00 00 00 00 00 00
0020 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0040 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0060 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0080 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00A0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00C0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00E0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0100 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00

Allocation Bitmap
• Keeps track of cluster allocation status
• Zero – Free Cluster
• One – Allocated Cluster
• 1 Byte = Tracking of 8 Clusters
• Bit Zero – Byte Zero = Cluster 2
• Cluster 0 & Cluster 1 are not defined
• Addressed by Directory Entry
• With TexFAT – 2 of these (FAT Pairing)

Legacy FAT vs. exFAT Chains
• When deleting a file in a legacy FAT FS
the cells are wiped out
• When deleting a file in the exFAT FS the
cells are not touched, regardless whether
there is data in the cell
• If a file is fragmented, and is deleted,
then the FAT may be still have the chain
intact
*Some exFAT implementations might do it the legacy way

Data Hide Alert!
• The Allocation Bitmap and the UP-Case
Table are stored as files, and provide
hiding space in the metadata
• These files are static, typically won’t
move, and have slack space.
• Nothing prevents someone from moving
these files elsewhere in the cluster heap,
and actually making them larger

Directories in exFAT
• Root (VBR Pointer)
• Contains certain critical entries
• Almost unlimited in size
• Subdirectory (by File Entry)
• Contains file sets
• 256MiB Max size
• No physical “.” or “..” entries
• Uses 16 Bit Unicode for strings
• Every Entry 32 bytes in size
• Entry 0x00 is end of directory
• Has capabilities for user entries

Data Hide Alert!
• Manipulation of the Allocation Bitmap, and
creation of user directory entries provides
the capability of hiding file within the file
system
• It may also be possible to hide data within
the directory metadata itself

Entry Type
Type Field Offset (Bits) Size (Bits)
In Use 7 1
Category 6 1
Importance 5 1
Code 0 5

Entry Type
• In Use:
• 0 – Not in Use, 1- In Use
• Category:
• 0 – Primary, 1 – Secondary
• Importance:
• 0 – Critical, 1 – Benign
• Code: Identifies the entry

Volume Label Directory Entry
• 0x83 or 0x03 Entry
• Primary Entry
• Only resident in Root Directory
• Contains the Volume Label
• 16 bit Unicode
• 0x03 means no volume label (Blank Label)

Volume Label Directory Entry
Offset 0 1 2 3 4 5 6 7 8 9 A B C D E F
00000000 83 0A 65 00 78 00 46 00 41 00 54 00 2D 00 31 00 ƒ.e.x.F.A.T.-.1.
00000010 32 00 38 00 4B 00 00 00 00 00 00 00 00 00 00 00 2.8.K...........
Type
Volume Name Length (10)
Volume Label (exFAT-128K)

Allocation Bitmap Directory
Entry
• 0x81 Entry
• Primary Entry
• Points to the Allocation Bitmap
• If TexFAT, then 2 of these
• Flag bits says which FAT/Bitmap
• Cluster Address of Bitmap
• Size of Bitmap
• NO flag for INVALID FAT

Allocation Bitmap Directory
Entry
Offset 0 1 2 3 4 5 6 7 8 9 A B C D E F
0000 81 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0010 00 00 00 00
Type
Cluster Address (Cluster 2) Size (63 bytes)

UP-Case Table Directory Entry
• 0x82 Entry
• Primary Entry
• File names are case insensitive
• Used to fold file name
• Table has a checksum (32 bits)

UP-Case Table Directory Entry
Offset 0 1 2 3 4 5 6 7 8 9 A B C D E F
0000 82 00 00 00 0D D3 19 E6 00 00 00 00 00 00 00 00
0010 00 00 00 00
Type Cluster Address (3)
Length (0x16CC = 5,836)
Table Checksum

File Directory Entry Set
• Used to define a file
• May have 3 to 19 entries, or more
• 1 Primary, many Secondary
• Is considered an array
• Must be in order
• Must be contiguous (no gaps)
• Entire Set has Checksum

File Directory Entry
• 0x85 or 0x05 Entry
• Primary Entry
• Set Checksum (16 bits)
• Not modified on file delete
• Secondary Count
• # Secondary entries that follow
• File Attributes
• Timestamps

Timestamps & Time Zones
• 3 Timestamps (MAC)
• 32 bit DOS Date/Time
• Local Machine Time
• 10ms Offset (MC)
• TZ Offset (MAC)
• 15 minute increments
• 7 bit signed number
• ±16 hours
• Present with UTC support

Timestamp Accuracy
• FAT32 – Last Access – Date only
• exFAT – Last Access – Date/Time
• All DOS DATE/TIME Double Seconds
• 10ms adds 0-1990 ms to time
• 10ms only for Create/Modify

Timestamps
Timestamp EXFAT
CreationTime
Stored in UTC if available, else in
local time
10 millisecond granularity
LastAccessTime
local time
2 second granularity
ChangeTime Not Supported
LastWriteTime
local time
10 millisecond granularity

Timestamp Reliability
• Timestamps appear to be updated when
the file is created or modified.
• Last Accessed Timestamp appear to be
updated when file is created or modified.
• Last Accessed Timestamp appear NOT
modified on file read.
• Forensics Implication on MAC time
analysis

File Attributes
Attribute Offset Size Mask
Reserved2 6 10
Archive 5 1 0x20
Directory 4 1 0x10
Reserved1 3 1
System 2 1 0x04
Hidden 1 1 0x02
Read-Only 0 1 0x01

File Directory Entry
Type # Secondary Entries
Set Checksum (0x92D4)
Attributes (0x0020 = Archive)
Create
Offset 0 1 2 3 4 5 6 7 8 9 A B C D E F
0000 85 04 D4 92 20 00 00 00 F1 62 BA 3A
0010 A8 00 EC EC EC 00 00 00 00 00 00 00
Modified
TZ Offset CMA EC = GMT-5
Accessed
Modified 10ms
Create 10ms

Formatted File Directory Entry
Root Entry Type Read is: 85 Directory Entry Record
Checksum: 92D4
Calculated Checksum is: 92D4 Size Directory Set (bytes): 160
Secondary Count 004
File Attributes: 0020 Archive
Create Timestamp: 3B866244 12/06/2009 12:18:08
Last Modified Timestamp: 3ABA62F1 05/26/2009 12:23:34
Last Accessed Timestamp: 3B866244 12/06/2009 12:18:08
10 ms Offset Create A8 168
10 ms Offset Modified 00 0
Time Zone Create EC 236 Value of tz is: GMT -05:00
Time Zone Modified EC 236 Value of tz is: GMT -05:00
Time Zone Last Accessed EC 236 Value of tz is: GMT -05:00

Stream Extension Directory
Entry
• 0xC0 or 0x40 Entry
• Secondary Entry
• Length of Name
• Length of File (2 of them)
• Cluster address of first data block
• Name Search Hash value
• Secondary Flag
• FAT Invalid
• Allocation Possible

Stream Extension Directory
Entry
Entry Flags (Alloc Possible/Fat Invalid)
Length of File Name (0x28= 40)
Name Hash (0x3CAD)
Offset 0 1 2 3 4 5 6 7 8 9 A B C D E F
0000 C0 03 00 28 00 00 00 00 00 00
0010 00 00 00 00 00 00 00 00
Cluster (5)
Data Length 0x011d461f = 18,695,711

Parameters for Samples
Bytes Per Sector: 2 to the 09 power is: 512
Sectors Per Cluster: 2 to the 08 power is: 256
Bytes per Cluster: 131072 (128K)

Formatted Stream Extension
Root Entry Type Read is: C0 Directory Entry Record,
Stream Extension
Secondary Flags: 03
Flag Bit 0: Allocation Possible
Flag Bit 1: FAT Chain Invalid
Length of UniCode Filename is: 40
Name Hash Value is: AD3C
Stream Extension First Cluster 5
Cluster 5 is Allocated
Stream Extension Data Length 18695711 Bytes
Slack: 83487 Clusters Used: 143
Stream Extension Valid Data Length 18695711 Bytes
Slack: 83487 Clusters Used: 143

File Name Extension Directory Entry
• 0xC1 or 0x41 Entry
• Secondary Entry
• Secondary Flags
• Allocation not possible
• FAT Invalid
• 15 Characters (30 bytes) of Name
• Name in 16 Bit Unicode
• In order (FAT32 LFN was reversed)
• Up to 17 max, total 255 character

File Name Extension Directory Entry
Offset 0 1 2 3 4 5 6 7 8 9 A B C D E F
0000 C1 00 62 00 75 00 73 00 69 00 6E 00 65 00 73 00
Á.b.u.s.i.n.e.s.
0010 73 00 5F 00 6F 00 66 00 5F 00 73 00 65 00 63 00
s._.o.f._.s.e.c.
0000 C1 00 75 00 72 00 69 00 74 00 79 00 5F 00 5F 00
Á.u.r.i.t.y._._.
0010 62 00 75 00 73 00 2D 00 31 00 30 00 35 00 2D 00
b.u.s.-.1.0.5.-.
0000 C1 00 33 00 32 00 6B 00 62 00 70 00 73 00 2E 00
Á.3.2.k.b.p.s...
0010 6D 00 70 00 33 00 00 00 00 00 00 00 00 00 00 00
m.p.3...........
File Name = business_of_security__bus-105-32kbps.mp3

Significance of “not in use” flag
• 0x05, 0x40 & 0x41 Entries
• “Not in use” may mean deleted files
• May also be reallocated rename
• Set Checksum not changed when entries
marked “not in use”

Closing
Problems Observed
Summary
Q&A
Contact Information
References
EXFAT

Problems Observed
• Looking at Forum Posts
• Google Dork on “exFAT”
• People getting thrown into exFAT and Lost
• Conversion between exFAT & Fat32/NTFS, How-to
• Corruption between Windows and Mac
• Should File Defragmentation be done?
• Repartitioning
• Timestamp differences, and incompatibilities
• Vendor cross compatibility
• Chkdsk not cleaning disk
• Users want large files (>4GB) not Large Volumes

Summary
• exFAT is still a relatively new FS
• Need for exFAT support in forensics tools ↑
• Inconsistent Implementations of exFAT
• Compatibility across OS needed
• Tools & Utilities Need Improvement
• Need to Tool Up

Q&A

Contact Information
• E-mail: rshullic@earthlink.net
exFAT@mindspring.com
• Blog: rshullic.wordpress.com
• Blog: shullich.blogspot.com
• Linkedin: www.linkedin.com/in/RobertShullich
• Twitter: rshullic
Credit Cookie

NTFS 232-1 Clusters
Cluster size NTFS Max Size
512 bytes 2,199,023,255,040 (2TB)
1024 bytes 4,398,046,510,080 (4TB)
2048 bytes 8,796,093,020,160 (8TB)
4096 bytes 17,592,186,040,320 (16TB) (Default)
8192 bytes 35,184,372,080,640 (32TB)
16384 bytes 70,368,744,161,280 (64TB)
32768 bytes 140,737,488,322,560 (128TB)
65536 bytes 281,474,976,654,120 (256TB) (Maximum)

ReFS
Resilient File System
Coming to a Windows System soon
http://blogs.msdn.com/b/b8/archive/2012/0
1/16/building-the-next-generation-file-system-
for-windows-refs.aspx

References
Sans Reading Room:
http://www.sans.org/reading_room/whitepapers/forensics/rss/reverse_eng
ineering_the_microsoft_exfat_file_system_33274
SANS Summit ExFAT Presentation:
exFAT (Extended FAT) File System – Revealed & Dissected
Jeff Hamm & Robert Shullich, July 2010
https://digital-forensics.sans.org/summit-archives/2010/10-exfat-ham.pdf

References
Microsoft Patent US8583708, “Extensible File System”
Retrieved June 9, 2014 from
https://www.google.com/patents/us8583708
Microsoft Patent US8321439, “Quick Filename Lookup
Using Name Hash”. Retrieved 06/09/2014 from
https://www.google.com/patents/US8321439

References
Microsoft Patent US8606830, “Contiguous file
allocation in an extensible file system” retrieved
06/09/2014 from
http://www.google.com/patents/US8606830
Microsoft Patent US8024383, “Fat directory structure
for use in transaction safe file System” retrieved
06/09/2014 from
https://www.google.com/patents/US8024383

References
ExFAT overview
http://ntfs.com/exfat-overview.htm
Data Recovery Concept: Extended File System
(exFAT)
http://www.active-undelete.com/xfat_overview.htm
CIPA Standard DC-009-2010 (DCF)
http://www.cipa.jp/std/documents/e/DC-009-2010_E.pdf
CIPA Standard DC-008-2012 (Exif)
http://www.cipa.jp/std/documents/e/DC-008-2012_E.pdf
Comparison of File Systems
http://en.wikipedia.org/wiki/Comparison_of_file_systems

References
The Extended FAT file system - Differentiating with
FAT32 file system - Keshava Munegowda , Venkatraman
S, Dr. G T Raju
http://events.linuxfoundation.org/images/stories/pdf/lceu11_mune
gowda_s.pdf
File System Functionality Comparison
http://msdn.microsoft.com/en-us/
library/windows/desktop/ee681827(v=vs.85).aspx

Resume
http://jjcweb.jjay.cuny.edu/d4cs/faculty/Shu
llich Robert.pdf

Htcia an introduction to the microsoft ex fat file system 1.01 final

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