Difference between revisions of "Prefetch"

Latest revision as of 00:24, 15 April 2014

Windows Prefetch files, introduced in Windows XP, are designed to speed up the application startup process. Prefetch files contain the name of the executable, a Unicode list of DLLs used by that executable, a count of how many times the executable has been run, and a timestamp indicating the last time the program was run. Although Prefetch is present in Windows 2003, by default it is only enabled for boot prefetching. The feature is also found in Windows Vista, where it has been augmented with SuperFetch, ReadyBoot, and ReadyBoost. For SSD drives Prefetch is disabled by default [1].

The Prefetch files are stored in the directory:

%SystemRoot%\Prefetch

The following files can be found in the Prefetch directory:

• *.pf, which are Prefetch files;
• Ag*.db and Ag*.db.trx, which are SuperFetch files;
• Layout.ini;
• PfPre_*.db;
• PfSvPerfStats.bin

A Prefetch file contains the name of the application, a dash, and then an eight character hash of the location from which that application was run, and a .pf extension. The filenames should be all uppercase except for the extension. E.g. a filename for md5deep would look like: MD5DEEP.EXE-4F89AB0C.pf. If an application is run from two different locations on the drive (i.e. the user runs C:\md5deep.exe and then C:\Apps\Hashing\md5deep.exe), there will be two different prefetch files in the Prefetch folder. According to MSDN up to 128 Prefetch files can be stored in the Prefetch directory [2].

File format

Each Prefetch file has a 4-byte signature (at offset 4) "SCCA" (or in hexadecimal notation 0x53 0x43 0x43 0x41). The signature is assumed to be preceded by a 4-byte format version indicator:

• 17 (0x00000011) for Windows XP and Windows 2003
• 23 (0x00000017) for Windows Vista, Windows 2008, Windows 7 and Windows 2012 (note Windows 2012 has not been confirmed)
• 26 (0x0000001a) for Windows 8.1 (note this could be Windows 8 as well but has not been confirmed)

For more information about the file format see: Windows Prefetch File Format

Metadata

The Prefetch file contains various metadata.

• The executable's name, up to 29 characters.
• The run count, or number of times the application has been run.
• Volume related information, like volume path and volume serial number.
• The size of the Prefetch file (sometimes referred to as end of file (EOF)).
• The files and directories that were used doing the application's start-up.

Timestamps

The Prefetch file contains 2 types of timestamps

• The time when the application was last ran (executed). Version 26 of the Prefetch format maintains 7 previous last run times.
• The volume creation time (part of the volume information) of the volume the Prefetch file was created on.

The file system creation time of the Prefetch file indicates the first time the application was executed. Both the file system modification time of the Prefetch file and the embedded last run time indicate the last time the application was executed.

Prefetch hash

There are multiple known hashing functions to be used for prefetch file filename hashing, namely:

• SCCA XP hash function; used on Windows XP and Windows 2003
• SCCA Vista hash function; used on Windows Vista
• SCCA 2008 hash function; used on Windows 2008, Windows 7, (possibly: Windows 2012) and Windows 8 (including 8.1)

SCCA XP hash function

A Python implementation of the SCCA XP hash function:

def ssca_xp_hash_function(filename):
    hash_value = 0
    for character in filename:
        hash_value = ((hash_value * 37) + ord(character)) % 0x100000000
        hash_value = (hash_value * 314159269) % 0x100000000
        if hash_value > 0x80000000:
            hash_value = 0x100000000 - hash_value

    return (abs(hash_value) % 1000000007) % 0x100000000

SCCA Vista hash function

A Python implementation of the SCCA Vista hash function:

def ssca_vista_hash_function(filename):
    hash_value = 314159
    for character in filename:
        hash_value = ((hash_value * 37) + ord(character)) % 0x100000000
    return hash_value

SCCA 2008 hash function

A Python implementation of the SCCA 2008 hash function:

def ssca_2008_hash_function(filename):
    hash_value = 314159
    filename_index = 0
    filename_length = len(filename)
    while filename_index + 8 < filename_length:
        character_value = ord(filename[filename_index + 1]) * 37
        character_value += ord(filename[filename_index + 2])
        character_value *= 37
        character_value += ord(filename[filename_index + 3])
        character_value *= 37
        character_value += ord(filename[filename_index + 4])
        character_value *= 37
        character_value += ord(filename[filename_index + 5])
        character_value *= 37
        character_value += ord(filename[filename_index + 6])
        character_value *= 37
        character_value += ord(filename[filename_index]) * 442596621
        character_value += ord(filename[filename_index + 7])
        hash_value = ((character_value - (hash_value * 803794207)) % 0x100000000)
        filename_index += 8

    while filename_index < filename_length: 
       hash_value = (((37 * hash_value) + ord(filename[filename_index])) % 0x100000000)
       filename_index += 1

    return hash_value 

Registry Keys

Key: HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Session Manager\Memory Management\PrefetchParameters

The EnablePrefetcher Registry value can be used to disable prefetch.

See Also

• Prefetch XML
• ReadyBoot
• SuperFetch
• Windows
• Windows Prefetch File Format

External Links

• More detail from Microsoft
• Wikipedia Prefetcher
• MSDN: Disabling Prefetch
• Kernel Enhancements for Windows XP, by Microsoft, January 13, 2003 (Microsoft's description of Prefetch when Windows XP was introduced)
• Misinformation and the The Prefetch Flag, MSDN Blogs, May 25, 2005
• Prefetch file metadata, by Harlan Carvey, July 13, 2005
• Prefetch files, revisited, by Harlan Carvey, April 13, 2006
• Support and Q&A for Solid-State Drives, by Steven Sinofsky, May 5, 2009
• De-mystifying Defrag: Identifying When Defrag Has Been Used for Anti-Forensics (Part 1 - Windows XP), by Chad Tilbury, August 5, 2009
• Windows Prefetch File (old blog entry from 42 LLC), by Yogesh Khatri, April 14, 2010
• Decoding Prefetch Files for Forensic Purposes: Part 1, by Mark Wade, December 8, 2010
• Prefetch Files at Face Value, by Mark Wade, April 11, 2011
• Windows Logical Prefetcher, TTS blog, July 30, 2011 (article is in Russian)
• Prefetch i niedokładny licznik by Paweł Hałdrzyński, August 20, 2011 (article in Polish; uncertain about the year of publication)
• Prefetch Analysis, Revisited, by Harlan Carvey, March 8, 2012
• Prefetch Analysis, Revisited...Again..., by Harlan Carvey, March 15, 2012
• Prefetch Hash Calculator + a hash lookup table xp/vista/w7/w2k3/w2k8, Hexacorn blog, June 13, 2012
• Prefetch file names and UNC paths, Hexacorn blog, October 29, 2012
• NTOSBOOT Prefetch File, by Corey Harrell, December 5, 2012
• What's New in the Prefetch for Windows 8??, by Jared Atkinson, September 21, 2013
• Windows Prefetch (.PF) files, by Yogesh Khatri, October 21, 2013
• Windows Systems and Artifacts in Digital Forensics: Part III: Prefetch Files, by Ivan Dimov, November 21, 2013
• Prefetch 101 - a Windows 8 Prefetch file walkthrough, by Jared Atkinson, 2014

Tools

Commercial

Free - Non Open Source

• PrefetchForensics, PrefetchForensics is an application to extract information from Windows Prefetch files
• Prefetch-Parser, Parse the prefetch files and display information
• Windows File Analyzer - Parses Prefetch files, thumbnail databases, shortcuts, index.dat files, and the recycle bin
• Windows Prefetch Parser (pf), Free tool that can be run on Windows, Linux or Mac OS-X

Open Source

• prefetch-tool, Script to extract information from windows prefetch folder
• prefetch-parser, Standalone Python tools that parses Windows prefetch files and extracts all known and forensically relevant artefacts contained.