1. FAT Concepts and Analysis

2. Acknowledgments Dr. David Dampier and the
Center for Computer Security Research (CCSR)

3. Basic Concepts
The FAT file system is one of the most simple file systems and does not clear follow the five category model. It consists of two main data structures:
File Allocation Table
Directory Entries

4. Basic Concepts
Each file and directory is allocated a directory entry, that contains:
File name
File size
Starting address of file content
Other metadata
File and directory content is stored in clusters
If a file or directory needs more than one cluster, those clusters are found in the FAT structure
Versions of FAT: FAT12, FAT 16, and FAT32
Difference is the size of entries in the FAT structure

5. Versions of FAT FAT12 FAT16 FAT32
Designed as a file system for floppy diskettes
12-bit cluster addresses
FAT16
16-bit cluster addresses
FAT32
32-bit cluster addresses (28 bits used) => 228 clusters
Drive size up to 8TB with 32KB clusters
Can become slow and inefficient
Video applications and large databases often exceed FAT32 limitations

6. Layout of a FAT file system
The layout of the FAT file system consists of 3 physical sections:
Reserved area – for file system category
FAT area – primary and backup FAT structures
Data area – clusters used for storing file and directory content
Reserved
Sector 0
FAT area
Data Area

7. FAT File System Data
In order to analyze the FAT file system, it is necessary to locate the three physical layout areas.
The reserved area starts at sector 0, and its size is given in the boot sector.
In FAT12/16, the reserved area is typically only 1 sector, but FAT32 will typically reserve many sectors
The FAT area begins in the sector after the reserved area.
Its size is calculated by multiplying the number of FAT structures by the size of each FAT, both of which can be found in the boot sector
The data area begin in the sector after the FAT area.
Its size can be found by subtracting the starting address of the data area from the total number of sectors in the file system, which can be found in the boot sector.

8. FAT System Layout FAT 32 FAT 12/16
Reserved
Area
FAT Area
Root
Directory
Data
Area
FAT 32
Reserved
Area
FAT Area
Root
Directory
Data
Area
The main difference between these layouts is that FAT 12/16’s root directory is at the beginning of the data sector, while in the FAT 32’s root directory can be anywhere in the data area. The first 36 bytes are the same in all.

9. File System Category
The data in the File System Category describes the general file system and can be found in the boot sector data structure. The boot sector is located in the first sector of the volume and is part of the reserved area of the file system.

10. Boot Sector The Boot Sector is contained in the first 512 bytes.
The first 36 bytes of all FAT Boot Sectors contain:
0-2 jump to boot code
3-10 name in ASCII
bytes per sector
13 sectors per cluster (powers of 2 < 32KB)
size in sectors of reserved area
16 number of FATs, 2 if backup
max # of root directory entries
bit value of number of sectors in file system
21 media type: 0xf8 fixed disks, 0xf0 removable
bit size in sectors of each FAT
24-25 sectors per track
26-27 number of heads
28-31 number of sectors before start of partition
bit value of # of sectors in file system, > 0
Bytes 510 and 511 have signature 0x55 and 0xAA

11. Example Image FAT32 (See pp. 216 and 217)
# fsstat –f fat fat-4.dd
FILE SYSTEM INFORMATION
File system type: FAT
OEM Name: MSDOS5.0
Volume ID: 0x4c194603
Volume Label (Boot Sector): NO NAME
Volume Label (Root Directory): FAT DISK
File System Type Label: FAT32
Backup Boot Sector Location: 6
FS Info Sector Location: 1
Next Free Sector (FS Info): 1778
Free Sector Count (FS Info):
File System Layout (in sectors)
Total Range: 0 –
* Reserved:
** Boot Sector: 0
** FS Info Sector: 1
* FAT 0:
* FAT 1:
* Data Area:
*** Root Directory:
CONTENT DATA INFORMATION
Sector Size: 512
Cluster Size:
38 reserved sectors
2 FAT structures

12. Analysis
In order to analyze a disk or find hidden data, it is necessary to know the layout of file systems and know which OS formatted the disk
Several places not used by the file system could contain hidden data
In the reserved area, at the end of the boot sector data and the final signature
Between the end of the file system and the end of the volume
FAT32 systems have a backup boot sector in sector 6
The primary and backup copies could be compared to find inconsistencies
If values in the primary have been changed, the backup may contain original data

13. Analysis Scenario
Imagine that the first 32 sectors of a disk are damaged and cannot be read. What do you do?
First, find the start of the file system.
The signature for a fat file system is 0x55 and 0xAA in the final two bytes of the boot sector. The sigfind tool can be used to look for the signature.
When the tool find the signature, additional test can be conducted on a range of values that are valid for a given data structure. For example, byte 13 of the boot sector identifies how many sectors in a cluster, and must have a value that is a power of 2. Any other value would indicate that the sector was not part of a FAT file system boot sector, even though it contained the signature.

14. Content Category
The Content category comprises of the file and directory content.
FAT file systems use the term cluster for its data units in the Data Area
A cluster is a group of consecutive sectors, the number of sectors must be a power of 2 (1, 2, 4, ..., 64) Each cluster has an address and the address of the first cluster is 2 (there are no clusters with address 0 or 1)
Reserved
Sector 0
FAT area
Data Area

15. Finding the First Cluster
Finding cluster 2 is not easy, because it is not at the beginning of the file system. Depending on whether it is a FAT 12/16 or FAT 32 system, the procedure is different.
FAT 12/16
Reserved
Area
FAT Area
Root
Directory
Data
Area
Sector 1224
Sector 1256
Cluster 2
FAT 32
Reserved
Area
FAT Area
Root
Directory
Data
Area
Sector 1224
Cluster 2

16. Finding the First Cluster
In a FAT 12/16 system, the number of root directory entries are given in the boot sector, and cluster 2 starts in the next sector.
For example, consider a FAT16 file system with 32 sectors allocated for the root directory. If the data area starts in sector 1224, then the root directory spans from sector 1224 to If we have 2048 byte clusters, then cluster 2 would start at 1256 and cluster 3 would start at 1260.

17. Cluster Allocation Status
The status of a cluster (whether allocated or not) is found in the FAT structure.
The basic concept of the FAT is that it has one entry for each cluster in the file system. If the table entry is 0, then the cluster corresponding to that table entry is not allocated to a file. All other values mean that the cluster is allocated.
Reserved
Sector 0
FAT area
Data Area

18. Allocation Algorithms
To find an unallocated cluster the OS scans the FAT for an entry with a 0 in it
Most operating systems do not clear cluster contents when unallocated 61 62 63 64 65 66 67 X X X X
X – allocated
0 - available
Last allocated

19. Analysis Techniques
When analyzing the content category, there are several places where data could be hidden.
Clusters can be marked as ‘bad’, and bad clusters should be examined, because the OS does not look at them.
The size of the data are might not be a multiple of the cluster size, so there could be a few sectors at the end of the data area that are not part of a cluster.

20. Analysis Scenario
Imagine a FAT 16 file system in which you need to locate cluster 812. The only tool available is a hex editor.
First, view the boot sector, which is located at sector 0 of the file system and process it
Processing this indicates that there are 6 reserved sectors, two FATS, each FAT is 249 sectors, each cluster is 32 sectors and there are 512 directory entries in the root directory.
Sector 6
Sector
255
Sector
504
Sector
536
Sector
568
Sector
26456
Reserved
FAT1
FAT2
Root
Directory
Cluster2
Cluster812

21. Metadata Category
This category includes data that describe files and directories in directory entries
Where content is stored, dates and times, and permissions
In an FAT file system, this information is stored in a directory entry structure.
Every file or directory is allocated a directory entry.
Exists anywhere in the Data area.
Each directory entry is 32 bytes: file attributes, size, starting cluster, dates and times
When a new file or directory is created, a directory entry in the parent directory (..) is allocated for it
Searched by using full name
FAT structure is used to find remaining clusters

22. Directory Entry Structures
Clusters
FAT Structure
Cluster 34
File1.dat
4,000 bytes
Cluster 34 35 34
EOF 35
Cluster 35

23. Directory Entry Data Structure
0 first character of file name in ASCII
1-10 characters 2 to 11 of file name in ASCII
11 file attributes (read only, hidden, volume label etc. )
13 creation time (tenths of seconds)
14-15 creation time (hours, minutes, seconds)
16-17 creation day
18-19 last accessed day
20-21 high 2 bytes of first cluster address
22-23 written time (hours, minutes, seconds)
24-25 written day
26-27 low 2 bytes of first cluster address
28-31 size of file (0 for directories), max file size 4GB
See Table 10.5 and 10.6

24. Directory Entries
Directory entries can exist anywhere in the data area. When a new file or directory is created, a directory entry in the parent directory is allocated for it.
The 11th byte in the directory entry has an attribute field that can contain 7 different attributes.
Directory attribute
Long file name attribute
Volume label – only one directory entry should have this label
Read-only attribute
Hidden attribute
System attribute
Archive attribute
The allocation status of a directory entry is determined by using the first byte. With an allocated entry, the first byte stores the first character in the file name, but it is replaced by 0xe5 when the entry becomes unallocated.

25. Directory Entries ... ... Sector 520 Sector 1,376 Each 512 byte sector
can store 16 directory entry structures
...
...
FAT Area
Data Area
Carrier Figure 9.12

26. Cluster Chains
If a FAT entry is non-zero, it contains the address of the next cluster, an EOF, or a bad sector indicator.
FAT
Directory Entry 39
File1.dat
Start: 40
Size: 6,013 41 40 44 41
EOF 42
We know from the file size how
many clusters are needed 43
EOF 44

27. Creation times in directory entries
Cluster 110
Cluster 256
Name
Created
Cluster
Name
Created
Cluster
Dir2
3/30/08 01:02:03
128 .
4/01/08 05:14:00
256
Dir1
4/03/08 11:12:13
256 ..
4/04/08 05:14:00
110
File8.dat
5/24/08 12:12:12
512
File1.dat
4/03/08 12:12:12
208
The created time in the directory entry for the
directory does not match the . and .. entries
Carrier Figure 9.10

28. Directories
When a new directory is created a cluster is allocated and wiped with zeros
The size field is always zero
To find the size of the directory, go to the starting cluster and follow the cluster chain until EOF
The first two directory entries in a directory entry are:
Current directory (.)
Parent directory (..)
Time fields may be used to verify creation time of a directory
However we cannot confirm the last written date because . and .. entries are not updated for each directory modification

29. Directory Entry Allocation
Unallocated
Directory Entry #2
Allocated
Directory Entry #3
Directory Entry #4
Last Allocated ... Entry 3 was unallocated after entry 4
was allocated
Directory Entry #5
Directory Entry #6
Carrier’s Observations:
Windows 98 uses a first-available allocation strategy and starts from the beginning
Windows XP uses next-available and starts from the last allocated directory entry

30. Searching for deleted directories
When unallocated, the first letter of a file name is changed to _ (0xe5)
Thus, if two files had similar names: A-1.dat and B-1.dat, they would now both be _-1.dat
When a directory is deleted and its entry is reallocated, the cluster for that directory is orphaned
To find orphan files, every sector of the data area needs to be examined... See figure 9.11

31. Analysis Scenario
Imagine that there is a FAT file system has been recently formatted and we need to recover the directories from before the format.
That means, we need to look at all the unallocated space and see if there is any directory information in there. Using TSK, we can extract the unallocated space using dls.

32. Analysis Contd. - Search for Unallocated Space and then Search for Directories
dls allows us to extract the unallocated space.
#dls –f fat Fat-10.dd > fat-10.dls
sigfind can be used to search for a signature.
Example: the first 4 bytes of a directory is always “. “ (period followed by 3 spaces – current directory) which has the hex code of 0x2e202020
#sigfind –b 512 2e fat-10.dls
Block size: Offset: 0
Block: 180 (-)
Block 2004 (+1824)
Block 3092 (+1088)
Block 3188 (+96)
Block (+15840)
...says that this signature occurred in sector 180 and others

33. Viewing the contents of Sector 180
#dd if=fat-10.dls skip=180 count=1 | xxd
: 2e daf ].
: 3c23 3c daf 3c23 4f <#<#..].<#0....
: 2e2e daf ].
: 3c23 3c daf 3c23 dc0d <#<#..].<#0....
: e549 4c e ILE1.DAT
: d u!u!.....u!V.....
Three entries are shown here. The first two are for the . and .. entries.
The . entry points to cluster 6,479 (0x194f)
The .. entry points to cluster 3,548 (0x0ddc)
The third entry is for a file that starts in cluster 6,486 (0x1956) with a size of 53, 248
bytes (0xd000).
File recovery could be performed on this file now that we know its starting
address and size.

34. File Name Category
FAT does not differentiate between a file name address and metadata address, and this is the same as what was there in the metadata category.
So far, what we saw were filenames with 8 characters plus a 3-character extension (SFN)
SFN entry contains time, size, and starting cluster information
A file may also have a longer, more descriptive file name, LFN
If there are > 13 characters, more LFN entries are used...see figure 9.15

35. Finding Hidden Data Unused sectors in the reserved area
Between the end of the file system and the end of the volume
Compare the number of sectors in the file system (given in boot sector) with the number of sectors in the volume to find volume slack
The total number of sectors value can be easily changed in the boot sector

36. Finding more hidden data...
Between the last entry in the primary FAT and the start of the backup copy or between the last valid entry in the backup FAT and the start of the data area
Compare the size of each FAT with the size needed for the number of clusters in the file system
Someone could create a directory with only a few files and use the rest of the directory space for hiding data
Compare the allocated size of the directory to the number of allocated files

37. The Big Picture Boot Sector Data Area FAT Root Directory Cluster 90
This is the content of a file that I just created
File1.txt
200
dir1 90
201
200
EOF
201
Cluster 201
This is the content from the rest of the file that didn’t fit in the cluster