Analyzing Outlook PST for Email Forensics
Overview
Microsoft Outlook PST (Personal Storage Table) and OST (Offline Storage Table) files are critical evidence sources in digital forensics investigations. PST files store email messages, calendar events, contacts, tasks, and notes in a proprietary binary format based on the MAPI (Messaging Application Programming Interface) property system. Forensic analysis of these files enables recovery of deleted emails (from the Recoverable Items folder), extraction of email headers for tracing message routes, analysis of attachments for malware or exfiltrated data, and reconstruction of communication patterns. Modern PST files use Unicode format with 4KB pages and can grow up to 50GB, while legacy ANSI format is limited to 2GB.
When to Use
- When investigating security incidents that require analyzing outlook pst for email forensics
- When building detection rules or threat hunting queries for this domain
- When SOC analysts need structured procedures for this analysis type
- When validating security monitoring coverage for related attack techniques
Prerequisites
- libpff/pffexport (open-source PST parser)
- Python 3.8+ with pypff or libratom libraries
- MailXaminer, Forensic Email Collector, or SysTools PST Forensics (commercial)
- Microsoft Outlook (optional, for native PST access)
- Sufficient disk space for extracted content
PST File Locations
| Source | Path |
|---|
| Outlook 2016+ Default | %USERPROFILE%\Documents\Outlook Files*.pst |
| Outlook Legacy | %LOCALAPPDATA%\Microsoft\Outlook*.pst |
| OST Cache | %LOCALAPPDATA%\Microsoft\Outlook*.ost |
| Archive | %USERPROFILE%\Documents\Outlook Files\archive.pst |
Analysis with Open-Source Tools
libpff / pffexport
# Export all items from PST file
pffexport -m all evidence.pst -t exported_pst
# Export only email messages
pffexport -m items evidence.pst -t exported_emails
# Export recovered/deleted items
pffexport -m recovered evidence.pst -t recovered_items
# Get PST file information
pffinfo evidence.pst
Python PST Analysis
import pypff
import os
import json
import hashlib
import email
import sys
from datetime import datetime
from collections import defaultdict
class PSTForensicAnalyzer:
"""Forensic analysis of Outlook PST/OST files."""
def __init__(self, pst_path: str, output_dir: str):
self.pst_path = pst_path
self.output_dir = output_dir
os.makedirs(output_dir, exist_ok=True)
self.pst = pypff.file()
self.pst.open(pst_path)
self.messages = []
self.attachments = []
self.stats = defaultdict(int)
def process_folder(self, folder, folder_path: str = ""):
"""Recursively process PST folders and extract messages."""
folder_name = folder.name or "Root"
current_path = f"{folder_path}/{folder_name}" if folder_path else folder_name
for i in range(folder.number_of_sub_messages):
try:
message = folder.get_sub_message(i)
msg_data = self.extract_message(message, current_path)
if msg_data:
self.messages.append(msg_data)
self.stats["total_messages"] += 1
except Exception as e:
self.stats["parse_errors"] += 1
for i in range(folder.number_of_sub_folders):
try:
subfolder = folder.get_sub_folder(i)
self.process_folder(subfolder, current_path)
except Exception:
continue
def extract_message(self, message, folder_path: str) -> dict:
"""Extract forensic metadata from a single email message."""
msg_data = {
"folder": folder_path,
"subject": message.subject or "",
"sender": message.sender_name or "",
"sender_email": "",
"creation_time": str(message.creation_time) if message.creation_time else None,
"delivery_time": str(message.delivery_time) if message.delivery_time else None,
"modification_time": str(message.modification_time) if message.modification_time else None,
"has_attachments": message.number_of_attachments > 0,
"attachment_count": message.number_of_attachments,
"body_size": len(message.plain_text_body or b""),
"html_size": len(message.html_body or b""),
}
# Extract transport headers for routing analysis
headers = message.transport_headers
if headers:
msg_data["headers_present"] = True
msg_data["headers_size"] = len(headers)
# Parse key headers
parsed = email.message_from_string(headers)
msg_data["from_header"] = parsed.get("From", "")
msg_data["to_header"] = parsed.get("To", "")
msg_data["date_header"] = parsed.get("Date", "")
msg_data["message_id"] = parsed.get("Message-ID", "")
msg_data["x_originating_ip"] = parsed.get("X-Originating-IP", "")
msg_data["received_headers"] = parsed.get_all("Received", [])
# Process attachments
for j in range(message.number_of_attachments):
try:
attachment = message.get_attachment(j)
att_data = {
"message_subject": msg_data["subject"],
"name": attachment.name or f"attachment_{j}",
"size": attachment.size,
"content_type": "",
}
self.attachments.append(att_data)
self.stats["total_attachments"] += 1
except Exception:
continue
return msg_data
def save_attachments(self, max_size_mb: int = 100):
"""Export attachments to disk for analysis."""
att_dir = os.path.join(self.output_dir, "attachments")
os.makedirs(att_dir, exist_ok=True)
root = self.pst.get_root_folder()
self._save_attachments_recursive(root, att_dir, max_size_mb)
def _save_attachments_recursive(self, folder, att_dir, max_size_mb):
for i in range(folder.number_of_sub_messages):
try:
message = folder.get_sub_message(i)
for j in range(message.number_of_attachments):
att = message.get_attachment(j)
if att.size and att.size < max_size_mb * 1024 * 1024:
name = att.name or f"unknown_{i}_{j}"
safe_name = "".join(c if c.isalnum() or c in ".-_" else "_" for c in name)
path = os.path.join(att_dir, safe_name)
try:
data = att.read_buffer(att.size)
with open(path, "wb") as f:
f.write(data)
except Exception:
continue
except Exception:
continue
for i in range(folder.number_of_sub_folders):
try:
self._save_attachments_recursive(folder.get_sub_folder(i), att_dir, max_size_mb)
except Exception:
continue
def generate_report(self) -> str:
"""Generate comprehensive PST forensic analysis report."""
root = self.pst.get_root_folder()
self.process_folder(root)
report = {
"analysis_timestamp": datetime.now().isoformat(),
"pst_file": self.pst_path,
"pst_size_bytes": os.path.getsize(self.pst_path),
"statistics": dict(self.stats),
"messages": self.messages[:500],
"attachments": self.attachments[:200],
}
report_path = os.path.join(self.output_dir, "pst_forensic_report.json")
with open(report_path, "w") as f:
json.dump(report, f, indent=2, default=str)
print(f"[*] Total messages: {self.stats['total_messages']}")
print(f"[*] Total attachments: {self.stats['total_attachments']}")
print(f"[*] Parse errors: {self.stats['parse_errors']}")
return report_path
def close(self):
self.pst.close()
def main():
if len(sys.argv) < 3:
print("Usage: python process.py <pst_file> <output_dir>")
sys.exit(1)
analyzer = PSTForensicAnalyzer(sys.argv[1], sys.argv[2])
analyzer.generate_report()
analyzer.close()
if __name__ == "__main__":
main()
Email Header Analysis
Key headers for forensic investigation:
| Header | Forensic Value |
|---|
| Received | Message routing chain (read bottom to top) |
| X-Originating-IP | Sender's actual IP address |
| Message-ID | Unique identifier for correlation |
| Date | Send timestamp |
| Return-Path | Bounce address (may differ from From) |
| DKIM-Signature | Domain authentication signature |
| Authentication-Results | SPF, DKIM, DMARC verification results |
| X-Mailer | Email client used |
References
Example Output
$ pffexport /evidence/jsmith_archive.pst -t /analysis/pst_output
pffexport 20231205 - libpff PST/OST Export Tool
=================================================
Input: /evidence/jsmith_archive.pst (2.3 GB)
Exporting PST contents...
Folders: 45
Messages: 12,456
Attachments: 3,234
Contacts: 567
Calendar: 234
Tasks: 89
Export completed in 3m 42s.
$ python3 pst_analyzer.py /analysis/pst_output /analysis/email_report
PST Forensic Analysis Report
==============================
Source: jsmith_archive.pst (john.smith@corporate.com)
Date Range: 2023-06-01 to 2024-01-18
--- Mailbox Statistics ---
Total Emails: 12,456
Sent: 4,567
Received: 7,889
With Attachments: 3,234
Deleted (recovered): 234
--- Phishing / Suspicious Emails ---
Email #8923
Date: 2024-01-15 14:30:22 UTC
From: "IT Support" <it-support@c0rporate-help.com>
To: john.smith@corporate.com
Subject: Urgent: Password Reset Required
Headers:
Return-Path: bounce@mail-relay.c0rporate-help.com
X-Originating-IP: 203.0.113.55
Received: from mail-relay.c0rporate-help.com (203.0.113.55)
SPF: FAIL (domain c0rporate-help.com)
DKIM: NONE
DMARC: FAIL
Attachments:
- Password_Reset_Form.xlsm (245 KB) SHA-256: 7a3b8c9d...e1f2a3b4
Body Preview: "Dear Employee, Your password will expire in 24 hours.
Please open the attached form to reset your credentials..."
--- Data Exfiltration Indicators ---
Email #9102
Date: 2024-01-16 03:15:45 UTC
From: john.smith@corporate.com
To: j.smith.personal8842@protonmail.com
Subject: (no subject)
Attachments:
- archive_part1.7z (24.5 MB) - encrypted
- archive_part2.7z (24.5 MB) - encrypted
Email #9103
Date: 2024-01-16 03:18:22 UTC
From: john.smith@corporate.com
To: j.smith.personal8842@protonmail.com
Subject: Re:
Attachments:
- archive_part3.7z (18.2 MB) - encrypted
--- Keyword Hits ---
"confidential": 45 emails
"password": 23 emails
"transfer": 12 emails
"resign": 3 emails
"delete evidence": 1 email (Email #9200, 2024-01-17 22:30:00 UTC)
Summary:
Phishing emails detected: 1 (initial compromise vector)
Suspicious sent emails: 5 (to personal accounts with attachments)
Encrypted attachments: 3 (67.2 MB total - possible exfiltration)
Report: /analysis/email_report/pst_forensic_report.json