Investigating Phishing Email Incident

Investigating Phishing Email Incident

When to Use

Use this skill when:

• A user reports a suspicious email via the phishing report button or helpdesk ticket
• Email security gateway flags a message that bypassed initial filters
• Automated detection identifies credential harvesting URLs or malicious attachments
• A phishing campaign targeting the organization requires scope assessment

Do not use for spam or marketing emails without malicious intent — route those to email administration for filter tuning.

Detection Gaps & Validation

• Clean attachment, weaponized link: triagers detonate the attachment, get a benign verdict, and close — missing that the body's credential-harvest URL is the real payload. Always extract and analyze every URL (including those inside PDFs, QR codes, and HTML attachments) even when the file sandboxes clean.
• Delayed/cloaked payloads: the URL serves a benign page to sandboxes and the harvester only after a redirect, on a second visit, or when the geo/User-Agent matches the target. Re-scan from a realistic egress and follow the full redirect chain rather than trusting a single URLScan result.
• SPF/DKIM "pass" ≠ legitimate: a dmarc=pass from a lookalike or freshly registered domain, or a compromised-but-authenticated internal mailbox (thread hijacking), still phishes. Check domain age, display-name spoofing, and Reply-To/Return-Path mismatch, not just auth results.
• Scope undercounted: searching only the reported subject misses recipients who got the same campaign with a rotated subject/sender. Pivot on sending IP, URL domain, and message-body hash, and confirm "who clicked" with proxy POSTs (credentials submitted) — not just GETs.
• Validate the verdict: confirm a true positive by matching submitted-credential POSTs or sandbox C2 callbacks to the IOC, then audit each impacted mailbox for attacker-created inbox forwarding rules before declaring "no lateral movement". Tune FPs by routing confirmed marketing/bulk senders to gateway filtering instead of incident queues.

Prerequisites

• Access to email gateway logs (Proofpoint, Mimecast, or Microsoft Defender for Office 365)
• Splunk or SIEM with email log ingestion (O365 Message Trace, Exchange tracking logs)
• Sandbox access (Any.Run, Joe Sandbox, or Hybrid Analysis) for URL/attachment detonation
• Microsoft Graph API or Exchange Admin Center for email search and purge operations
• URLScan.io and VirusTotal API keys

Workflow

Step 1: Extract and Analyze Email Headers

Obtain the full email headers (.eml file) from the reported message:

import email
from email import policy

with open("phishing_sample.eml", "rb") as f:
    msg = email.message_from_binary_file(f, policy=policy.default)

# Extract key headers
print(f"From: {msg['From']}")
print(f"Return-Path: {msg['Return-Path']}")
print(f"Reply-To: {msg['Reply-To']}")
print(f"Subject: {msg['Subject']}")
print(f"Message-ID: {msg['Message-ID']}")
print(f"X-Originating-IP: {msg['X-Originating-IP']}")

# Parse Received headers (bottom-up for true origin)
for header in reversed(msg.get_all('Received', [])):
    print(f"Received: {header[:120]}")

# Check authentication results
print(f"Authentication-Results: {msg['Authentication-Results']}")
print(f"DKIM-Signature: {msg.get('DKIM-Signature', 'NONE')[:80]}")

Key checks:

• SPF: Does Return-Path domain match sending IP? Look for spf=pass or spf=fail
• DKIM: Is the signature valid? dkim=pass confirms the email was not modified in transit
• DMARC: Does the From domain align with SPF/DKIM domains? dmarc=fail indicates spoofing

Step 2: Analyze URLs and Attachments

URL Analysis:

import requests

# Submit URL to URLScan.io
url_to_scan = "https://evil-login.example.com/office365"
response = requests.post(
    "https://urlscan.io/api/v1/scan/",
    headers={"API-Key": "YOUR_KEY", "Content-Type": "application/json"},
    json={"url": url_to_scan, "visibility": "unlisted"}
)
scan_id = response.json()["uuid"]
print(f"Scan URL: https://urlscan.io/result/{scan_id}/")

# Check VirusTotal for URL reputation
import vt
client = vt.Client("YOUR_VT_API_KEY")
url_id = vt.url_id(url_to_scan)
url_obj = client.get_object(f"/urls/{url_id}")
print(f"VT Score: {url_obj.last_analysis_stats}")
client.close()

Attachment Analysis:

import hashlib

# Calculate file hashes
with open("attachment.docx", "rb") as f:
    content = f.read()
    md5 = hashlib.md5(content).hexdigest()
    sha256 = hashlib.sha256(content).hexdigest()

print(f"MD5: {md5}")
print(f"SHA256: {sha256}")

# Submit to MalwareBazaar for lookup
response = requests.post(
    "https://mb-api.abuse.ch/api/v1/",
    data={"query": "get_info", "hash": sha256}
)
print(response.json()["query_status"])

Submit to sandbox (Any.Run or Joe Sandbox) for dynamic analysis of macros, PowerShell execution, and C2 callbacks.

Step 3: Determine Campaign Scope

Search for all recipients of the same phishing email in Splunk:

index=email sourcetype="o365:messageTrace"
(SenderAddress="attacker@evil-domain.com" OR Subject="Urgent: Password Reset Required"
 OR MessageId="<phishing-message-id@evil.com>")
earliest=-7d
| stats count by RecipientAddress, DeliveryStatus, MessageTraceId
| sort - count

Alternatively, use Microsoft Graph API:

import requests

headers = {"Authorization": f"Bearer {access_token}"}
params = {
    "$filter": f"subject eq 'Urgent: Password Reset Required' and "
               f"receivedDateTime ge 2024-03-14T00:00:00Z",
    "$select": "sender,toRecipients,subject,receivedDateTime",
    "$top": 100
}
response = requests.get(
    "https://graph.microsoft.com/v1.0/users/admin@company.com/messages",
    headers=headers, params=params
)
messages = response.json()["value"]
print(f"Found {len(messages)} matching messages")

Step 4: Identify Impacted Users (Who Clicked)

Check proxy/web logs for users who visited the phishing URL:

index=proxy dest="evil-login.example.com" earliest=-7d
| stats count, values(action) AS actions, latest(_time) AS last_access
  by src_ip, user
| lookup asset_lookup_by_cidr ip AS src_ip OUTPUT owner, category
| sort - count
| table user, src_ip, owner, actions, count, last_access

Check if credentials were submitted (POST requests to phishing domain):

index=proxy dest="evil-login.example.com" http_method=POST earliest=-7d
| stats count by src_ip, user, url, status

Step 5: Containment Actions

Purge emails from all mailboxes:

# Microsoft 365 Compliance Search and Purge
New-ComplianceSearch -Name "Phishing_Purge_2024_0315" `
    -ExchangeLocation All `
    -ContentMatchQuery '(From:attacker@evil-domain.com) AND (Subject:"Urgent: Password Reset Required")'

Start-ComplianceSearch -Identity "Phishing_Purge_2024_0315"

# After search completes, execute purge
New-ComplianceSearchAction -SearchName "Phishing_Purge_2024_0315" -Purge -PurgeType SoftDelete

Block indicators:

• Add sender domain to email gateway block list
• Add phishing URL domain to web proxy block list
• Add attachment hash to endpoint detection block list
• Create DNS sinkhole entry for phishing domain

Reset compromised credentials:

# Force password reset for impacted users
$impactedUsers = @("user1@company.com", "user2@company.com")
foreach ($user in $impactedUsers) {
    Set-MsolUserPassword -UserPrincipalName $user -ForceChangePassword $true
    Revoke-AzureADUserAllRefreshToken -ObjectId (Get-AzureADUser -ObjectId $user).ObjectId
}

Step 6: Document and Report

Create incident report with full timeline, IOCs, impacted users, and remediation actions taken.

| makeresults
| eval incident_id="PHI-2024-0315",
       reported_time="2024-03-15 09:12:00",
       sender="attacker@evil-domain[.]com",
       subject="Urgent: Password Reset Required",
       url="hxxps://evil-login[.]example[.]com/office365",
       recipients_count=47,
       clicked_count=5,
       credentials_submitted=2,
       emails_purged=47,
       passwords_reset=2,
       domains_blocked=1,
       disposition="True Positive - Credential Phishing Campaign"
| table incident_id, reported_time, sender, subject, url, recipients_count,
        clicked_count, credentials_submitted, emails_purged, passwords_reset, disposition

Key Concepts

Tools & Systems

• Microsoft Defender for Office 365: Email security platform with Safe Links, Safe Attachments, and Threat Explorer for investigation
• URLScan.io: Free URL analysis service capturing screenshots, DOM, cookies, and network requests
• Any.Run: Interactive sandbox for detonating malicious files and URLs with real-time behavior analysis
• Proofpoint TAP: Targeted Attack Protection dashboard showing clicked URLs and delivered threats per user
• PhishTool: Dedicated phishing email analysis platform automating header parsing and IOC extraction

Common Scenarios

• Credential Phishing: Fake O365 login page — check proxy for POST requests, force password resets for submitters
• Macro-Enabled Document: Word doc with VBA macro — sandbox shows PowerShell download cradle, check endpoints for execution
• QR Code Phishing (Quishing): Email contains QR code linking to credential harvester — decode QR, submit URL to sandbox
• Thread Hijacking: Attacker uses compromised mailbox to reply in existing threads — check for impossible travel or new inbox rules
• Voicemail Phishing: Fake voicemail notification with HTML attachment — analyze attachment for redirect chains

Output Format

PHISHING INCIDENT REPORT — PHI-2024-0315
━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
Reported:     2024-03-15 09:12 UTC by jsmith (Finance)
Sender:       attacker@evil-domain[.]com (SPF: FAIL, DKIM: NONE, DMARC: FAIL)
Subject:      Urgent: Password Reset Required
Payload:      Credential harvesting URL

IOCs:
  URL:        hxxps://evil-login[.]example[.]com/office365
  Domain:     evil-login[.]example[.]com (registered 2024-03-14, Namecheap)
  IP:         185.234.xx.xx (VT: 12/90 malicious)

Scope:
  Recipients: 47 users across Finance and HR departments
  Clicked:    5 users visited phishing URL
  Submitted:  2 users entered credentials (confirmed via POST in proxy logs)

Containment:
  [DONE] 47 emails purged via Compliance Search
  [DONE] Domain blocked on proxy and DNS sinkhole
  [DONE] 2 user passwords reset, sessions revoked
  [DONE] MFA enforced for both compromised accounts
  [DONE] Inbox rules audited — no forwarding rules found

Status:       RESOLVED — No evidence of lateral movement post-compromise