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Analysis of hidden data in the NTFS file system

Analysis of hidden data in the NTFS file system

Page: 4/8


This hiding technique hides data in the additional clusters allocated to a file. For example, there is a file with file size 10752 bytes, which would be allocated 3 clusters in a NTFS with cluster size of 8 sectors. Suspects can allocate extra clusters to this file and hide data in the additional clusters allocated.

With this technique, the size of hidden data is unlimited as suspects are free to allocate as many additional clusters as they wish. At the point of view of suspects, one disadvantage of this hiding technique is that whenever the file increases in size, the hidden data would be replaced and lost. As a result, stable files are preferable targets of this technique.

Procedure to create test data

1) Run list information of the file is modified to allocate more clusters to the file
2) Last VCN of the file is modified to appropriate value
3) Allocated size of the file is modified to appropriate value
4) Allocation status of the additional clusters is set to 1

Analysis techniques

Figure 5 shows the flow to analyse hidden data in additional clusters allocated to a file.

Figure 5: Flow to analyse hidden data in additional clusters allocated to a file

Run chkdsk to check the file system. During the testing, it is discovered that if a suspect forgets to complete any of the first 3 steps in the procedure, error will occur as shown in figure 6 and if he forgets to perform step 4 after first 3 steps, error message will be displayed as figure 7. This would be an indication of file system manipulation.

Figure 6: Error message displayed when running chkdsk command

Figure 7: Error message displayed when running chkdsk command

Perform a recursive directory listing of the file system to identify all files
./fls -rFf ntfs /case1/image1

Analysis begins by retrieving the first file and comparing the allocated size and real size of the file. The fastest way to obtain these values is from the header of $DATA attribute of the file. However, only the real size is shown with istat command of Sleuth Kit but not the allocated size. As a result, you can either processing the MFT file record manually to get the value (which is time consuming), use other tools (DiskExplorer) or calculate the allocated size by multiplying cluster size and total clusters allocated to the file.

Number of allocated clusters and real size of the file can be retrieved with istat command:
./istat /case1/image1 29

Get the cluster size of the file system with fsstat
./fsstat -f ntfs /case1/image1

Calculate the additional space of the file

Allocated size = number of clusters * cluster size
Additional space = allocated size - real size

If additional space is larger than cluster size, additional/unnecessary clusters have been allocated to that file. This is uncommon and might contain hidden data. The additional clusters should be extracted and analyse with hex editor, foremost and comeforth similar to the analysis of faked bad clusters. Keyword search can also be performed. The analysis is then continues with other files until all files have been analysed. This process is time consuming if performed manually. However, there is no specific tool that automates this process at the moment.


File slack is the unused space between the end of file and the end of cluster. File slack appears because cluster is the smallest unit of disk space allocation in NTFS and whole cluster is used even the file does not fill the whole cluster (Mallery, 2001). This empty space can be used to hide data (Chuvakin, 2002)

There are 2 types of file slack, which are RAM slack and drive slack. RAM slack spans from end of a file to the end of sector while drive slack spans from the start of next sector to the end of cluster (NTI, 2004). For example, a 600 bytes file is stored in a NTFS with 2048-bytes cluster and 512-bytes sector as shown in figure 8. RAM slack is from the end of file to the end of sector 2 and drive slack is composed of sector 3 and 4.

Figure 8: Slack space of a 600 bytes file in 4096 bytes cluster (Carrer, 2005)

Analysis of hidden data in slack space is depending on operating system as it is the operating system that decides how to handle file slack and not the file system. For example, Microsoft Windows pads RAM slack with 0 and ignores drive slack when storing a file (Carrier, 2005). So any non 0 bit in RAM slack of a file is suspicious and worth further analysis.

The size of data that can be hid in slack space of single file is based on the file size and cluster size (Paladion Network, 2004). The smaller the file size and the larger the cluster size, the more data can be hid. The size of hidden data in file slack is actually huge as suspects can hide data in slack space of multiple files in stead of just a single file. Hidden data in file slack have the danger of being erased/ replaced when the file size increases. As a result, stable files are preferable for this hiding technique.

Procedure to create test data

Test data is created by hiding data in RAM slack, drive slack and both.

1) Locate the suitable file/files with sufficient slack to hide data
2) Paste the data to slack space of the files

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