Analyzing Prefetch Files for Execution History

Analyzing Prefetch Files for Execution History

What Is This Skill?

The skill "Analyzing Prefetch Files for Execution History" focuses on extracting and interpreting information from Windows Prefetch files to reconstruct a program execution timeline on Windows systems. Prefetch files, with the .pf extension, are system-generated artifacts located in the C:\Windows\Prefetch\ directory. These files are created and updated by the Windows operating system each time an executable is run, with the goal of improving application startup performance. For forensic investigators, Prefetch files are valuable evidence, as they provide details about program execution history, including how many times a program ran, the last several execution times, and which files were referenced or loaded by the executable.

This skill allows practitioners to parse Prefetch files and extract critical forensic data such as:

• Program name and executable path
• Run count (how many times the program was executed)
• Multiple execution timestamps
• List of files and libraries the executable accessed during its run

By leveraging tools such as PECmd (by Eric Zimmerman), WinPrefetchView, or the python-prefetch parser, investigators can systematically analyze Prefetch files to support incident response, malware analysis, and timeline reconstruction.

Why Use It?

Analyzing Prefetch files plays a crucial role in digital forensics and incident response for several reasons:

• Verification of Program Execution: Prefetch files offer reliable evidence that a specific executable was run on a Windows system, even if the binary itself has been deleted.
• Timeline Reconstruction: Prefetch files store up to eight timestamps of the most recent executions, enabling the construction of detailed activity timelines.
• Detection of Malicious Activity: Identifying unauthorized or suspicious program executions, such as malware or anti-forensic tools, can help confirm or refute hypotheses during an investigation.
• Correlation With Other Artifacts: Prefetch data can be cross-referenced with other log sources (event logs, registry, $MFT) to strengthen the case and provide context.
• Evidence of Data Access: The list of referenced files within Prefetch files can potentially reveal which files or libraries an executable interacted with, providing leads on data exfiltration or lateral movement.

This skill is essential for forensic professionals who need to provide evidence of application usage, track attacker activity, or validate claims about what actions occurred on a compromised machine.

How to Use It

Prerequisites

• Access to Prefetch Files: Obtain the contents of the C:\Windows\Prefetch\ directory from a forensic image or live acquisition. Prefetch is enabled by default on Windows client editions (Windows XP and later), but usually disabled on server editions.
• Tools: Choose one of the following open-source utilities:
  • PECmd (Eric Zimmerman)
  • WinPrefetchView
  • python-prefetch or other Python-based parsers
• Familiarity With Prefetch File Versions: Windows uses several Prefetch file format versions (17, 23, 26, 30), depending on OS version.

Step-by-Step Guide

1. Extract Prefetch Files

If working from a forensic image, mount the image and copy the Prefetch directory:

## Example:

Mount image and copy Prefetch files
mount -o ro,loop,offset=$((2048*512)) /cases/case-2024-001/images/evidence.dd /mnt/evidence
cp /mnt/evidence/Windows/Prefetch/*.pf /analysis/prefetch/

2. Parse Prefetch Files

Using PECmd (recommended for comprehensive output):

## Download PECmd and run it against the extracted Prefetch files
PECmd.exe -d /analysis/prefetch/ -o /analysis/prefetch_results.csv

Or with WinPrefetchView (GUI/CLI):

WinPrefetchView.exe /prefetch_folder "C:\analysis\prefetch" /scomma "C:\analysis\prefetch_results.csv"

Or programmatically using python-prefetch:

from pypff import file as pff_file

with open('C:/analysis/prefetch/SUSPICIOUSAPP-ABCDE1234.pf', 'rb') as pf:
    pf_file = pff_file()
    pf_file.open_file_object(pf)
    print(pf_file.get_application_name())
    print(pf_file.get_run_count())
    print(pf_file.get_last_run_times())

3. Review and Interpret the Output

Typical Prefetch data includes:

• Executable Name: Name of the application.
• Run Count: Number of times the application was executed.
• Last Run Times: Up to eight most recent execution timestamps (in UTC).
• Referenced Files: File paths accessed by the executable.

Use this information to build an execution timeline or to confirm the use of suspicious or unauthorized software.

When to Use It

• During Malware or Intrusion Investigations: Confirm if a suspicious binary was executed and when.
• Timeline Analysis: Establish when specific actions occurred on a system relative to other forensic artifacts.
• Detection of Anti-Forensic Activity: Identify execution of tools commonly used to clear logs or cover attacker tracks.
• Correlating With Other Evidence: Use Prefetch data to validate or refute claims about application usage or incident timelines.
• Incident Response: Quickly ascertain what was run on a compromised machine before containment or remediation.

Important Notes

• Prefetch Is Not Always Available: Prefetch is enabled by default on Windows desktops, but usually disabled on Windows Server editions. Attackers may also clear or disable Prefetch to hinder investigations.
• Prefetch Naming Convention: Prefetch filenames follow the pattern APPNAME-HASH.pf, where the hash is derived from the executable path. Multiple versions of the same application in different locations will have different hashes and thus separate Prefetch files.
• Timestamps Are in UTC: Always convert timestamps to the correct time zone for accurate timeline analysis.
• File Format Versions: Windows 10 and later use newer Prefetch file versions, which may require up-to-date tools for parsing.
• Referenced Files Limitations: The list of referenced files may not include every file accessed, and is limited by the Prefetch format.
• Anti-Forensic Considerations: The absence of a Prefetch file does not prove that a program was never executed-files may have been deleted, or Prefetch may have been disabled.

By mastering this skill, forensic analysts can significantly enhance their ability to reconstruct execution histories and support a wide range of investigative scenarios on Windows systems.