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Windows Sysinternals Administrator's Reference: Security Utilities

SDelete

Object reuse protection is a fundamental policy of the Windows security model. This means that when an application allocates file space or virtual memory it is unable to view data that was previously stored in that space. Windows zero-fills memory and zeroes the sectors on disk where a file is placed before it presents either type of resource to an application. Object reuse protection does not dictate that the space that a file occupies be zeroed when it is deleted, though. This is because Windows is designed with the assumption that the operating system alone controls access to system resources. However, when the operating system is not running it is possible to use raw disk editors and recovery tools to view and recover data that the operating system has deallocated. Even when you encrypt files with Windows’ Encrypting File System (EFS), a file’s original unencrypted file data might be left on the disk after a new encrypted version of the file is created. Space used for temporary file storage might also not be encrypted.

The only way to ensure that deleted files, as well as files that you encrypt with EFS, are safe from recovery is to use a secure delete application. Secure delete applications overwrite a deleted file’s on-disk data using techniques that are shown to make disk data unrecoverable, even if someone is using recovery technology that can read patterns in magnetic media that reveal weakly deleted files. SDelete (Secure Delete) is such an application. You can use SDelete both to securely delete existing files, as well as to securely erase any file data that exists in the unallocated portions of a disk (including files you have already deleted or encrypted). SDelete implements the U.S. Department of Defense clearing and sanitizing standard DOD 5220.22-M, to give you confidence that after it is deleted with SDelete, your file data is gone forever. Note that SDelete securely deletes file data, but not file names located in free disk space.

Using SDelete

SDelete is a command-line utility. It works on Windows XP and newer and does not require administrative rights. It uses a different command-line syntax for secure file deletion and for erasing content in unallocated disk space. To securely delete one or more files or folder hierarchies, use this syntax:

sdelete [-p passes] [-a] [-s] [-q] file_spec

The file_spec can be a file or folder name, and it can contain wildcard characters. The –p option specifies the number of times to overwrite each file object. The default is one pass. The –a option is needed to delete read-only files. The –s option recurses subfolders to delete files matching the specification or to delete a folder hierarchy. The –q option (quiet) suppresses the listing of per-file results. Here are some examples:

REM  Securely deletes secret.txt in the current folder
sdelete secret.txt

REM  Securely deletes all *.docx files in the current folder and subfolders
sdelete -s *.docx

REM  Securely deletes the C:\Users\Bob folder hierarchy
sdelete -s C:\Users\Bob

To securely delete unallocated disk space on a volume, use this syntax:

sdelete [-p passes] [-z|-c] [d:]

There are two ways to overwrite unallocated space: the –c option overwrites it with random data, while the –z option overwrites it with zeros. The –c option supports DoD compliance; the –z option makes it easier to compress and optimize virtual hard disks. The –p option specifies the number of times to overwrite the disk areas. If the drive letter is not specified, the current volume’s unallocated space is cleansed. Note that the colon must be included in the drive specification.

Note that during free-space cleaning, Windows might display a warning that disk space is running low. This is normal, and the warning can be ignored. (The reason this happens will be explained in the next section.)

How SDelete Works

Securely deleting a file that has no special attributes is relatively straightforward: the secure delete program simply overwrites the file with the secure delete pattern. What is trickier is to securely delete compressed, encrypted, or sparse files, and securely cleansing disk free spaces.

Compressed, encrypted and sparse files are managed by NTFS in 16-cluster blocks. If a program writes to an existing portion of such a file, NTFS allocates new space on the disk to store the new data, and after the new data has been written NTFS deallocates the clusters previously occupied by the file. NTFS takes this conservative approach for reasons related to data integrity, and (for compressed and sparse files) in case a new allocation is larger than what exists (for example, the new compressed data is larger than the old compressed data). Thus, overwriting such a file will not succeed in deleting the file’s contents from the disk.

To handle these types of files SDelete relies on the defragmentation API. Using the defragmentation API, SDelete can determine precisely which clusters on a disk are occupied by data belonging to compressed, sparse and encrypted files. When SDelete knows which clusters contain the file’s data, it can open the disk for raw access and overwrite those clusters.

Cleaning free space presents another challenge. Because FAT and NTFS provide no means for an application to directly address free space, SDelete has one of two options. The first is that—like it does for compressed, sparse and encrypted files—it can open the disk for raw access and overwrite the free space. This approach suffers from a big problem: even if SDelete were coded to be fully capable of calculating the free space portions of NTFS and FAT drives (something that’s not trivial), it would run the risk of collision with active file operations taking place on the system. For example, say SDelete determines that a cluster is free, and just at that moment the file system driver (FAT, NTFS) decides to allocate the cluster for a file that another application is modifying. The file system driver writes the new data to the cluster, and then SDelete comes along and overwrites the freshly written data: the file’s new data is gone. The problem is even worse if the cluster is allocated for file system metadata because SDelete will corrupt the file system’s on-disk structures.

The second approach, and the one SDelete takes, is to indirectly overwrite free space. First, SDelete allocates the largest file it can. SDelete does this using noncached file I/O so that the contents of the NT file system cache will not be thrown out and replaced with useless data associated with SDelete’s space-hogging file. Because noncached file I/O must be sector (512-byte) aligned, there might be some left over space that isn’t allocated for the SDelete file even when SDelete cannot further grow the file. To grab any remaining space, SDelete next allocates the largest cached file it can. For both of these files, SDelete performs a secure overwrite, ensuring that all the disk space that was previously free becomes securely cleansed.

On NTFS drives, SDelete’s job isn’t necessarily through after it allocates and overwrites the two files. SDelete must also fill any existing free portions of the NTFS MFT (Master File Table) with files that fit within an MFT record. An MFT record is typically 1 KB in size, and every file or directory on a disk requires at least one MFT record. Small files are stored entirely within their MFT record, while files that don’t fit within a record are allocated clusters outside the MFT. All SDelete has to do to take care of the free MFT space is allocate the largest file it can; when the file occupies all the available space in an MFT record, NTFS will prevent the file from getting larger, because there are no free clusters left on the disk (they are being held by the two files SDelete previously allocated). SDelete then repeats the process. When SDelete can no longer even create a new file, it knows that all the previously free records in the MFT have been completely filled with securely overwritten files.

To overwrite the file name of a file that you delete, SDelete renames the file 26 times, each time replacing each character of the file’s name with a successive alphabetic character. For instance, the first rename of sample.txt would be to AAAAAA.AAA.

The reason that SDelete does not securely delete file names when cleaning disk free space is that deleting them would require direct manipulation of directory structures. Directory structures can have free space containing deleted file names, but the free directory space is not available for allocation to other files. Hence, SDelete has no way of allocating this free space so that it can securely overwrite it.