Conducting Phishing Incident Response

Conducting Phishing Incident Response

When to Use

• A user reports receiving a suspicious email via the phishing report button or abuse mailbox
• Email gateway detects a malicious email that bypassed initial filtering
• Threat intelligence indicates an active phishing campaign targeting the organization
• A user confirms they clicked a link or opened an attachment from a suspicious email
• Credentials have been entered on a suspected phishing page

Do not use for business email compromise (BEC) involving compromised internal accounts; use BEC response procedures which focus on account takeover investigation.

Prerequisites

• Email security gateway with message trace and quarantine capabilities (Microsoft Defender for Office 365, Proofpoint, Mimecast)
• Microsoft 365 admin access or Google Workspace admin for mailbox search and purge
• Malware sandbox for attachment and URL analysis (ANY.RUN, Joe Sandbox, Hybrid Analysis)
• Email header analysis tools (MXToolbox Header Analyzer, Google Admin Toolbox)
• Identity provider access for account remediation (Azure AD, Okta, Duo)
• Phishing report intake process (dedicated mailbox or integrated report button)

Workflow

Step 1: Receive and Triage the Phishing Report

Evaluate the reported email to determine if it is malicious:

• Extract the email as an .EML or .MSG file (preserves headers)
• Analyze email headers to determine the true sender, relay path, and authentication results

Email Header Analysis Checklist:
━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
Return-Path:     billing@spoofed-domain[.]com
From:            "IT Support" <support@corp-lookalike[.]com>
Reply-To:        attacker@gmail[.]com (different from From)
SPF:             FAIL (sender IP not authorized for domain)
DKIM:            FAIL (signature invalid)
DMARC:           FAIL (policy: none - no enforcement)
Received:        from mail.attacker-infra[.]net [45.33.x.x]
X-Originating-IP: 45.33.x.x
Message-ID:      <random@attacker-infra.net>

Classification criteria:

• Confirmed Phishing: Malicious URL/attachment, spoofed sender, credential harvesting page
• Suspicious: Anomalous headers but no confirmed malicious content
• Spam/Marketing: Unwanted but not malicious
• Legitimate: Not a phishing email (false report)

Step 2: Analyze Malicious Content

Examine URLs and attachments in a safe environment:

URL Analysis:

• Check URL against VirusTotal, URLscan.io, and Google Safe Browsing
• Open URL in a sandbox browser to capture the landing page
• Check if the URL redirects to a credential harvesting page
• Identify the phishing kit type (Microsoft 365 login clone, Okta clone, generic)
• Determine if the phishing page is still active

Attachment Analysis:

• Calculate file hash (SHA-256) and check against VirusTotal
• Detonate in sandbox (ANY.RUN, Joe Sandbox)
• Analyze document for macros (olevba for Office files)
• Check for embedded exploits (CVE exploitation in document parsers)

Step 3: Determine Scope of Impact

Identify all recipients and assess who interacted with the phishing email:

Scope Assessment:
━━━━━━━━━━━━━━━━
Total Recipients:     47 users
Delivered to Inbox:   38 users (9 caught by email gateway)
Opened Email:         24 users (email tracking pixel data)
Clicked Link:         8 users (proxy/firewall logs)
Entered Credentials:  3 users (phishing page submitted form data)
Opened Attachment:    2 users (EDR process execution telemetry)

Search methods:

• Microsoft 365: Use Threat Explorer or Content Search to find all instances of the email
• Google Workspace: Use Admin Console > Investigation tool for message search
• Proxy logs: Search for connections to the phishing URL from internal IPs
• EDR: Search for attachment file hash execution across all endpoints

Step 4: Contain the Threat

Execute containment actions based on impact assessment:

Email Containment:

• Purge the phishing email from all mailboxes using Microsoft 365 Content Search and Purge or Google Workspace Admin delete
• Block the sender domain at the email gateway
• Add the phishing URL to the web proxy blocklist
• Add attachment hash to email gateway and EDR blocklists

Account Containment (for users who entered credentials):

• Force password reset immediately
• Revoke all active sessions and OAuth tokens
• Enable or re-verify MFA enrollment
• Review mailbox rules for attacker-created forwarding rules
• Check for unauthorized OAuth application grants
• Review recent sign-in activity for suspicious locations

# Microsoft 365: Revoke sessions and reset password
Connect-AzureAD
Revoke-AzureADUserAllRefreshToken -ObjectId "user@corp.com"
Set-AzureADUserPassword -ObjectId "user@corp.com" -ForceChangePasswordNextLogin $true

# Check for mailbox forwarding rules
Get-InboxRule -Mailbox "user@corp.com" | Where-Object {$_.ForwardTo -or $_.RedirectTo}

# Remove suspicious forwarding rules
Remove-InboxRule -Mailbox "user@corp.com" -Identity "Rule Name"

Step 5: Eradicate and Recover

Remove all traces of the phishing attack:

• Confirm email purge completed successfully across all mailboxes
• Verify compromised accounts have been secured (password changed, sessions revoked, MFA verified)
• Remove any malware installed via phishing attachments from affected endpoints
• Monitor compromised accounts for 72 hours for signs of continued unauthorized access
• Check for data exfiltration from compromised accounts during the exposure window

Step 6: Post-Incident Actions

Strengthen defenses against similar phishing attacks:

• Report the phishing URL to Google Safe Browsing and Microsoft SmartScreen
• Submit the phishing domain for takedown via the domain registrar abuse contact
• Update email gateway filtering rules based on observed evasion techniques
• Send targeted security awareness notification to affected users
• Update phishing simulation program to include the observed technique