Agent skill

investigating-phishing-email-incident

Investigates phishing email incidents from initial user report through header analysis, URL/attachment detonation, impacted user identification, and containment actions using SOC tools like Splunk, Microsoft Defender, and sandbox analysis platforms. Use when a reported phishing email requires full incident investigation to determine scope and impact.

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Install this agent skill to your Project

npx add-skill https://github.com/mukul975/Anthropic-Cybersecurity-Skills/tree/main/skills/investigating-phishing-email-incident

SKILL.md

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.

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:

python
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:

python
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:

python
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:

spl
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:

python
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:

spl
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):

spl
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:

powershell
# 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:

powershell
# 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.

spl
| 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

Term Definition
SPF (Sender Policy Framework) DNS TXT record specifying which mail servers are authorized to send on behalf of a domain
DKIM DomainKeys Identified Mail — cryptographic signature proving email content was not altered in transit
DMARC Domain-based Message Authentication, Reporting and Conformance — policy combining SPF and DKIM alignment
Credential Harvesting Phishing technique using fake login pages to capture username/password combinations
Business Email Compromise (BEC) Social engineering attack using compromised or spoofed executive email for financial fraud
Message Trace O365/Exchange log showing email routing, delivery status, and filtering actions for forensic analysis

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

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