Prefetch

Overview

Windows Prefetch is a performance enhancement feature that enables prefetching of applications to make system boots or applications startups faster. Prefetch files are created whenever a program is executed from a specific path. If the same binary is executed from different locations, separate Prefetch files will be created for each different location. A Prefetch file can be created even if the executable did not successfully run.

Whether the Prefect feature is enabled is configured by the EnablePrefetcher registry key:

• 0/undefined: disabled (default on Windows Server operating systems). Prefetch is thus disabled by default on Windows Server operating systems.
• 0x1: Partially enabled (application prefetching only).
• 0x2: Partially enabled (boot prefetching only).
• 0x3: Enabled (application and boot prefetching).

Prefetch files are not automatically deleted if the related executable is deleted and can thus be a source of historical information. However, as the Prefetch directory is limited to 128 entries on Windows XP to Windows 7 and 1024 entries starting from Windows 8, Prefetch files may be overwritten and information lost.

Information of interest

The Prefecth filenames are based on the executed program name and a hash, computed using a proprietary algorithm and based on the full path (and for some binaries, such as dllhost.exe or svchost.exe, command line parameters) of the executed program.

The Prefecth files can yield the following information of forensic interest:

• The file name and size of the binary executed.

• The first and, starting from Windows 8, last eight executions timestamps.

• The Prefecth file NTFS created and last modified timestamps also indicate the first and last time the program was executed.

• Run count (number of time the binary was executed).

• List of files and directories accessed during the first ten seconds of execution (including the eventual DLL loaded). The full path to executable file can often be determined from the list of files accessed (duplicate possible if a given binary access another binary with the same name).

Note that the Prefecth files can be easily deleted, potentially invalidating the trace of execution and timestamps (notably of first execution).

Prefecth files indirect information

The creation or modification of Prefecth files observed in others artefacts ($MFT, UsnJrnl, etc.) reflect an execution of the binary linked to the Prefecth file (and whose name can be deducted from the Prefecth filename).

Prefecth information related to PowerShell execution

The POWERSHELL.EXE-[...].pf Prefetch file may contain references to recently executed PowerShell scripts. For an entry to be created in the Prefetch file, the given script must be executed within the first ten seconds of the powershell.exe execution.

The accessed file list does retain entries from previous instances of a program execution. Accessed files information may thus persist through powershell.exe subsequent runs.

Tool(s)

PECmd (KAPE associated PECmd module) can be used to parse Prefecth file(s).

# Parses the specified Prefecth file.
PECmd.exe [-q --csv <CSV_DIRECTORY_OUTPUT>] -f <PF_FILE>

# Recursively retrieves and parses the Prefecth files in the specified directory.
PECmd.exe [-q --csv <CSV_DIRECTORY_OUTPUT>] -d <C:\Windows\Prefetch | C:\ | DIRECTORY>

References

• Forensic Focus - Oleg Skulkin - Hunting For Attackers’ Tactics And Techniques With Prefetch Files

• 13Cubed - Prefetch Deep Dive