Spoofing Attacks - Learning Module

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Email Spoofing: Forging Digital Correspondence

The Trust Crisis in Email

When you receive an email claiming to be from your bank, your CEO, or a government agency, how do you know it's genuine? The uncomfortable truth is that, at the protocol level, you often can't.

Email spoofing is the creation of email messages with forged sender addresses, exploiting the fundamental design of the Simple Mail Transfer Protocol (SMTP) which includes no native sender authentication. Attackers can make emails appear to originate from any address they choose—your company's CEO, a trusted vendor, a financial institution, or even your own email address.

This vulnerability has made email the primary vector for phishing attacks, business email compromise (BEC), malware distribution, and social engineering campaigns that cost organizations billions of dollars annually.

What You Will Learn

By the end of this page, you will understand SMTP's architecture and why it enables spoofing, the different levels at which email addresses can be forged, how spoofed emails enable sophisticated attacks, and the comprehensive authentication framework (SPF, DKIM, DMARC) that modern email systems use to combat sender forgery.

SMTP Protocol and the Spoofing Opportunity

To understand why email spoofing is possible, we must examine the Simple Mail Transfer Protocol (SMTP) and its historical design choices.

SMTP's Trust Model:

SMTP was designed in 1982 (RFC 821) for a small, trusted Internet of academic and research institutions. Like many early protocols, it assumed good faith among participants. The protocol allows any sending server to claim any sender address with no verification—the receiving server simply trusts the claimed identity.

The Two-Layer Address System:

Critically, email has two different sender addresses that can be manipulated independently:

Envelope Sender (MAIL FROM): Used in SMTP transaction; invisible to recipients (usually). This is the address used for delivery failures (bounces) and is processed by mail servers.
Header Sender (From:): Displayed to recipients in email clients. This is what users see and trust. It's just text in the email body that can be set to anything.

smtp_transaction_example.txt
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# SMTP Transaction demonstrating the separation of envelope and headers
# Connect to mail server on port 25
 
$ telnet mail.example.com 25
 
220 mail.example.com ESMTP ready
EHLO attacker.com
250-mail.example.com Hello attacker.com
250-SIZE 50000000
250-AUTH PLAIN LOGIN
250-STARTTLS
250 HELP
 
# ENVELOPE SENDER - Used for routing and bounces
# This can be spoofed but may be checked by SPF
MAIL FROM:<realaddress@attacker.com>
250 OK
 
# ENVELOPE RECIPIENT - Where to deliver
RCPT TO:<victim@company.com>
250 OK
 
# Begin message data
DATA
354 Start mail input
 
# EMAIL HEADERS - What recipient sees in email client
# These are completely unverified by basic SMTP!
 
From: CEO Name <ceo@company.com>
To: victim@company.com
Subject: Urgent Wire Transfer Required
Reply-To: attacker@gmail.com
Date: Mon, 15 Jan 2024 10:00:00 -0500
Message-ID: <fake-message-id@attacker.com>
MIME-Version: 1.0
Content-Type: text/plain; charset="UTF-8"
 
This is an urgent request. I need you to wire $50,000 to the 
following account immediately. I'm in a meeting and cannot talk.
 
Account: XXXXXX
Routing: YYYYYY
 
Please confirm when done.
 
- CEO Name
 
.
250 OK
QUIT
221 Bye
 
# Result: Victim receives email appearing to be from ceo@company.com
# But it was sent by attacker with different MAIL FROM

Key Observation:

In the SMTP transaction above, there are actually three different addresses involved:

Envelope FROM: realaddress@attacker.com (MAIL FROM command)
Header From: ceo@company.com (From: header)
Reply-To: attacker@gmail.com (where responses are directed)

Traditional SMTP checks none of these for authenticity. The receiving server has no way to verify that:

The connecting server is authorized to send for the envelope domain
The header From matches the envelope From
Any of these addresses are legitimate

This separation is the root cause of email spoofing vulnerabilities.

Visual Deception

Email clients typically only show the Header From address to users. A recipient sees 'From: CEO Name ceo@company.com' with no indication that the actual sending server was unauthorized. Even examining message headers requires technical knowledge most users lack, and headers themselves can be partially forged.

Categories of Email Spoofing

Email spoofing manifests in several forms, each with different technical requirements and detection difficulty.

Email Spoofing Categories
Type	Mechanism	Detection Difficulty	Use Case
Display Name Spoofing	Set From: "CEO Name" attacker@gmail.com	Easy (address visible)	Low-effort phishing, relies on display name only
Lookalike Domain	Send from ceo@c0mpany.com (zero instead of 'o')	Medium (domain looks similar)	Sophisticated phishing, bypasses simple filters
Legitimate Domain Spoofing	Forge From: header to exact victim domain	Hard without SPF/DKIM/DMARC	Most dangerous—exact impersonation
Subdomain Spoofing	Use real.company.com.attacker.com or uncontrolled subdomain	Medium	Exploits subdomain confusion
Cousin Domain	Register companyy.com or company-secure.com	Medium-Hard	Typosquatting with email capability
Reply-To Manipulation	From: legitimate, Reply-To: attacker	Medium (hidden attack path)	Capture responses to legitimate-appearing mail
Free Email Impersonation	ceo.company@gmail.com	Easy	Social engineering via recognizable name

Display Name Spoofing

•Simplest attack, requires no special access
•Set From: "CEO Name" random@gmail.com
•Many email clients prominently show name, hide address
•Mobile clients especially vulnerable (small screens)
•Often passes spam filters (legitimate sending domain)
•Defense: Train users to check actual email address

Domain Spoofing

•Most dangerous—exact domain impersonation
•From: header claims victim's actual domain
•MAIL FROM may differ (envelope vs header)
•Blocked by properly configured SPF/DKIM/DMARC
•Many domains still lack proper email authentication
•Defense: Implement and enforce DMARC with reject policy

Lookalike Domain Techniques:

When exact domain spoofing is blocked by email authentication, attackers register convincing lookalike domains:

Typosquatting: exampel.com, examle.com (common typos)
Homoglyphs: exаmple.com (Cyrillic 'а' instead of Latin 'a')
Character Addition/Removal: examplee.com, exampe.com
TLD Variants: example.co, example.net, example.io
Dash/Underscore Addition: example-support.com, ex-ample.com
Word Additions: example-security.com, myexample.com, example-online.com

These domains can actually be registered by attackers and configured with proper SPF/DKIM, making them appear legitimate to authentication systems. Defense requires domain monitoring and user awareness.

Unicode/IDN Attacks

Internationalized Domain Names (IDN) enable homoglyph attacks using characters from different scripts that appear identical: Latin 'a' vs Cyrillic 'а', Latin 'o' vs Greek omicron 'ο'. Modern email clients and browsers display IDN domains in punycode (xn--...) format to reveal these attacks, but protection varies by software and settings.

Attack Scenarios: Business Email Compromise

Email spoofing enables some of the most financially devastating cyber attacks. Business Email Compromise (BEC) has become a billion-dollar crime category where attackers use spoofed or compromised email to manipulate business processes.

FBI Statistics (IC3 2022 Report):

$2.7 billion in reported BEC losses in 2022
Average loss per incident: $120,000+
BEC represents highest dollar loss of any crime category
Affected businesses of all sizes and industries

Common BEC Attack Patterns

•CEO Fraud: Spoof executive email to request urgent wire transfers from finance staff. Creates pressure and authority to bypass normal procedures. 'I'm in a confidential meeting, need this done now, don't call.'
•Invoice Manipulation: Intercept or spoof vendor communications to provide fraudulent banking details. Legitimate invoice amounts go to attacker accounts. Often undetected until vendor follows up.
•Account Compromise: Actual email account takeover (phishing credentials first), then use legitimate access to manipulate transactions. Most sophisticated but most convincing.
•Attorney Impersonation: Spoof law firm communications during M&A, real estate closings, or litigation. Time pressure and confidentiality of legal matters reduce verification.
•W-2/PII Requests: Spoof HR leadership to request employee tax forms or personal information. Leads to identity theft and tax fraud.
•Supply Chain Attacks: Insert into vendor communication chains to redirect payments or deliver malicious payloads claiming to be software updates.

bec_email_example.txt
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# Example BEC Email (for awareness training purposes)
# Note the psychological manipulation techniques
 
From: "John Smith - CEO" <jsmith@company-exec.com>
      # Note: Lookalike domain, not company.com
To: accountspayable@company.com
Subject: Confidential - Urgent Wire Transfer
 
Hi Sarah,
 
I need you to process an urgent wire transfer today. I'm finalizing 
a confidential acquisition and cannot discuss this over the phone.
 
Amount: $47,500
Bank: First National Bank
Routing: 021000089
Account: 4829501837
Beneficiary: Strategic Partners LLC
 
This is time-sensitive and must be completed before 3 PM today.
Please confirm by email once done. Do not discuss this with others
as it's confidential until the deal closes.
 
Thanks for your help on this.
John
 
---
John Smith
Chief Executive Officer
Company Inc.
 
# Red Flags in this email:
# 1. Urgency and time pressure
# 2. Request to not verify through normal channels
# 3. Unusual payment request bypassing procedure
# 4. "Confidential" framing to prevent questions
# 5. Slight domain difference (company-exec.com vs company.com)
# 6. Generic wording that could apply to any business

Real-World Impact

In 2020, a Puerto Rico government agency lost $2.6 million to a BEC attack using spoofed vendor emails. In 2019, Toyota's European subsidiary lost $37 million to a BEC scam. Norfund, Norway's sovereign wealth fund, lost $10 million in 2020. These aren't small businesses—sophisticated organizations fall victim when email authentication and procedural controls are lacking.

SPF: Sender Policy Framework

Sender Policy Framework (SPF) is the first layer of email authentication, allowing domain owners to specify which mail servers are authorized to send email for their domain.

How SPF Works:

Domain owner publishes SPF record in DNS (TXT record at domain apex)
Receiving server extracts domain from MAIL FROM (envelope sender)
Server queries DNS for SPF record of that domain
Server checks if connecting IP is authorized by the SPF record
Result (pass/fail/softfail/neutral) informs filtering decision

spf_record_examples.txt
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# SPF Record Structure and Examples
 
# Basic SPF syntax
v=spf1 [mechanisms] [modifiers]
 
# Mechanisms (in order of checking):
# ip4:x.x.x.x/yy    - Allow this IPv4 address/range
# ip6:xxxx::/yy     - Allow this IPv6 address/range
# a                 - Allow IPs from domain's A record
# mx                - Allow domain's mail exchangers
# include:domain    - Include another domain's SPF
# exists:domain     - Advanced: check domain exists
# all               - Match all (usually at end)
 
# Qualifiers:
# + Pass (default)
# - Fail (reject)
# ~ SoftFail (accept but mark)
# ? Neutral (no assertion)
 
# Example 1: Small company using Office 365
v=spf1 include:spf.protection.outlook.com -all
 
# Example 2: Google Workspace with additional servers
v=spf1 include:_spf.google.com ip4:203.0.113.10 -all
 
# Example 3: Complex enterprise setup
v=spf1 mx ip4:192.0.2.0/24 include:_spf.google.com include:sendgrid.net -all
 
# Example 4: Publishing domain (no email should be sent)
v=spf1 -all
 
# Viewing SPF records
$ dig +short TXT company.com | grep spf
"v=spf1 include:_spf.google.com ~all"
 
# SPF record limitations:
# - 10 DNS lookup limit (include, a, mx, exists, redirect)
# - Only checks envelope sender (MAIL FROM), not header From
# - Forwards break SPF (forwarding server's IP fails check)
# - Must be combined with DKIM/DMARC for header From protection

SPF Result Codes
Result	Meaning	Recommended Action
Pass	Sender IP is authorized	Accept message
Fail (-all)	Sender IP explicitly not authorized	Reject message
SoftFail (~all)	IP probably not authorized, testing mode	Accept but flag/quarantine
Neutral (?all)	No assertion made	Accept, no conclusion
None	No SPF record exists	Accept, domain doesn't use SPF
TempError	DNS lookup failed temporarily	Accept or defer
PermError	SPF record syntax error	Accept, log error for domain owner

SPF Limitations

SPF has critical limitations: it only validates the envelope sender (MAIL FROM), not the header From that users see. An attacker can pass SPF by using their own domain in MAIL FROM while spoofing the header From. SPF also breaks with email forwarding because the forwarding server's IP isn't in the original domain's SPF. DKIM and DMARC address these limitations.

DKIM: DomainKeys Identified Mail

DomainKeys Identified Mail (DKIM) adds cryptographic authentication to emails, allowing the sending domain to digitally sign messages and proving they haven't been modified in transit.

How DKIM Works:

Domain owner generates public/private key pair
Public key published in DNS (TXT record at selector._domainkey.domain)
Sending server signs outgoing email headers and body with private key
Signature added as DKIM-Signature header
Receiving server retrieves public key from DNS
Server verifies signature—failure indicates tampering or forgery

dkim_example.txt
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# DKIM Signature Header (found in email headers)
 
DKIM-Signature: v=1; 
    a=rsa-sha256;           # Signing algorithm
    c=relaxed/relaxed;      # Canonicalization (header/body)
    d=company.com;          # Signing domain
    s=selector1;            # Selector (key identifier)
    h=from:to:subject:date:message-id;  # Headers included in signature
    bh=abc123...;           # Body hash (base64)
    b=xyz789...;            # Signature (base64)
 
# Breaking down the components:
# v=1         : DKIM version
# a=          : Algorithm (rsa-sha256 recommended)
# c=          : Canonicalization (how to normalize before signing)
#               relaxed = ignore minor formatting differences
#               simple = exact match required
# d=          : Domain that signed (critical for authentication)
# s=          : Selector - allows multiple keys per domain
# h=          : List of headers included in signature
#               'from' header MUST be included
# bh=         : Hash of message body
# b=          : The actual signature value
 
# Viewing DKIM public key in DNS
$ dig +short TXT selector1._domainkey.company.com
"v=DKIM1; k=rsa; p=MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQ..."
 
# DKIM key record fields:
# v=DKIM1     : DKIM version
# k=rsa       : Key type (rsa is standard)
# p=          : Public key (base64 encoded)
# t=y         : Testing mode (optional)
# t=s         : Strict mode - 'd=' must exactly match From: domain
 
# Verifying DKIM with OpenDKIM tools
$ opendkim-testkey -d company.com -s selector1 -vvv
 
# Multiple selectors enable key rotation:
# selector1._domainkey.company.com  (current key)
# selector2._domainkey.company.com  (new key during rotation)
# google._domainkey.company.com     (Google Workspace)

DKIM Advantages Over SPF:

Survives Forwarding: Signature travels with message, so forwarded email still verifies (if not modified)
Proves Integrity: Any modification to signed headers or body invalidates signature
Domain Identification: The signing domain (d=) is recorded and can be compared to header From

DKIM Limitations:

Doesn't specify what to do on failure (that's DMARC's job)
Attackers can sign their own domain's spoofed mail
Mailing lists that modify messages break DKIM
Only as secure as DNS (DNS spoofing could serve fake keys)

DKIM Alignment

DKIM by itself doesn't prevent header From spoofing—an attacker could sign mail with their own DKIM key (d=attacker.com) while spoofing the From header (ceo@company.com). The signature validates, but the domains don't match. DMARC addresses this by requiring 'alignment' between the signing domain and the header From domain.

DMARC: Domain-based Message Authentication

Domain-based Message Authentication, Reporting, and Conformance (DMARC) is the policy layer that ties SPF and DKIM together, specifying how receivers should handle authentication failures for the domain that users see (the header From domain).

The Critical Innovation:

DMARC introduces identifier alignment—requiring that the domain in the header From (what users see) matches either:

The domain in MAIL FROM (for SPF alignment), OR
The domain in DKIM signature (d=) (for DKIM alignment)

This closes the loophole where SPF and DKIM could pass for different domains than the visible From header.

dmarc_record_examples.txt
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# DMARC Record Published at _dmarc.domain.com
 
# DMARC syntax
v=DMARC1; p=[policy]; [optional tags]
 
# Policies:
# p=none      : Monitor mode - no action, just reporting
# p=quarantine: Mark as spam/suspicious
# p=reject    : Reject failing messages outright
 
# Example 1: Monitoring mode (start here)
v=DMARC1; p=none; rua=mailto:dmarc-reports@company.com; ruf=mailto:dmarc-forensics@company.com
 
# Example 2: Quarantine mode (intermediate step)  
v=DMARC1; p=quarantine; pct=50; rua=mailto:dmarc@company.com
 
# Example 3: Full enforcement (goal)
v=DMARC1; p=reject; rua=mailto:dmarc@company.com; adkim=s; aspf=s
 
# Tag explanations:
# v=DMARC1     : Version (required, must be first)
# p=           : Policy for domain (none/quarantine/reject)
# sp=          : Subdomain policy (if different from p=)
# pct=         : Percentage of messages to apply policy (0-100)
# rua=         : Aggregate report destination (XML reports)
# ruf=         : Forensic report destination (individual failures)
# adkim=       : DKIM alignment mode (r=relaxed, s=strict)
# aspf=        : SPF alignment mode (r=relaxed, s=strict)
# fo=          : Failure reporting options
# rf=          : Report format
# ri=          : Reporting interval (seconds)
 
# Strict vs Relaxed alignment:
# Relaxed (r): Subdomains align (mail.company.com aligns with company.com)
# Strict (s) : Exact match required (mail.company.com does NOT align)
 
# View DMARC record
$ dig +short TXT _dmarc.company.com
"v=DMARC1; p=reject; rua=mailto:dmarc@company.com; pct=100; adkim=s; aspf=s"
 
# DMARC deployment progression:
# Phase 1: p=none (monitoring) - 2-4 weeks
# Phase 2: p=quarantine pct=10 - Gradual increase to 100%
# Phase 3: p=reject pct=10 - Gradual increase to 100%
# Goal: p=reject pct=100 - Full protection

DMARC Authentication Flow
Step	Check	Requirement for DMARC Pass
1	SPF Check	SPF passes AND domain aligns with header From
OR
1	DKIM Check	DKIM passes AND d= domain aligns with header From
2	Policy Lookup	Receiving server queries _dmarc.headerfromdomain.com
3	Policy Application	If Step 1 fails, apply policy (none/quarantine/reject)
4	Reporting	Send aggregate/forensic reports to domain owner

DMARC Reporting:

DMARC provides unprecedented visibility into email authentication:

Aggregate Reports (rua=): XML files sent daily by receiving servers, showing authentication results for all messages claiming your domain. Reveals unauthorized senders, misconfigured services, and attack attempts.
Forensic Reports (ruf=): Detailed reports on individual authentication failures, including message headers. Privacy concerns limit adoption—many receivers don't send these.

Third-party DMARC report analyzers (dmarcian, Valimail, Agari) help parse aggregate reports into actionable insights, essential for organizations with complex email infrastructure.

DMARC Deployment Strategy

Never deploy DMARC at p=reject without monitoring first. Start with p=none and analyze reports for weeks to identify all legitimate email sources (marketing platforms, CRM, etc.). Add them to SPF and configure DKIM. Gradually increase enforcement (p=quarantine with increasing pct=) before reaching p=reject. Rushing enforcement breaks legitimate email.

Comprehensive Email Security Strategy

Defending against email spoofing requires implementing email authentication, but goes beyond just SPF/DKIM/DMARC to include user awareness, procedural controls, and technical safeguards.

Complete Email Protection Strategy

•Implement SPF — Publish SPF records specifying all authorized sending sources. Use -all (fail) for enforcement. Keep within 10 DNS lookup limit.
•Implement DKIM — Sign all outgoing email. Use 2048-bit RSA keys minimum. Maintain key rotation schedule. Configure for all sending sources.
•Implement DMARC — Start with p=none for monitoring. Analyze reports to find all legitimate sources. Progress to p=reject over weeks/months.
•Configure Inbound Validation — Ensure your email gateway validates SPF, DKIM, and DMARC for incoming mail. Reject or quarantine failures.
•External Email Warnings — Banner on emails from outside organization: '[EXTERNAL]' subject prefix or visible warning in body.
•Lookalike Domain Monitoring — Use services that detect registration of domains similar to yours. Register common typos and variations yourself.
•User Awareness Training — Regular phishing simulations. Train users to recognize BEC patterns. Establish out-of-band verification procedures.
•Payment Verification Procedures — Require phone verification (to known number, not from email) for payment changes, wire transfers, and vendor modifications.
•Account Security — MFA on all email accounts. Monitor for unauthorized access/rules. Regularly audit mailbox delegations.
•Email Security Gateway — Deploy advanced threat protection that analyzes content, links, and attachments beyond authentication.

email_auth_checklist.txt
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# Email Authentication Verification Checklist
 
# Check SPF
$ dig +short TXT company.com | grep spf
# Expected: "v=spf1 ... -all" (not ~all)
 
# Check DKIM (need to know selector)
$ dig +short TXT selector._domainkey.company.com
# Expected: "v=DKIM1; k=rsa; p=..."
 
# Check DMARC
$ dig +short TXT _dmarc.company.com
# Expected: "v=DMARC1; p=reject; ..." (eventually)
 
# Online verification tools:
# - mxtoolbox.com/SuperTool.aspx
# - mail-tester.com (send test email)
# - dmarcian.com/dmarc-inspector/
 
# Send test email and verify headers:
# Look for these headers in received message:
# Authentication-Results: dkim=pass; spf=pass; dmarc=pass
# Received-SPF: pass
# DKIM-Signature: v=1; ...
 
# DMARC Aggregate Report Analysis:
# Reports arrive as XML, typically gzip compressed
# Contain authentication results from receiving servers
# Key data points:
#   - Source IPs sending as your domain
#   - SPF/DKIM pass/fail rates  
#   - Disposition (delivered/quarantined/rejected)
 
# Sample aggregate report structure:
<feedback>
  <policy_published>
    <domain>company.com</domain>
    <p>reject</p>
  </policy_published>
  <record>
    <row>
      <source_ip>192.0.2.1</source_ip>
      <count>1542</count>
      <policy_evaluated>
        <disposition>none</disposition>
        <dkim>pass</dkim>
        <spf>pass</spf>
      </policy_evaluated>
    </row>
  </record>
</feedback>

Authentication ≠ Trust

Even perfectly authenticated email can be malicious. An attacker's domain with valid SPF/DKIM/DMARC is still an attacker's domain. Email authentication prevents domain spoofing but not lookalike domains or social engineering. Defense must combine authentication with content analysis, user training, and procedural controls.

Summary: Email Spoofing Defense

Email spoofing exploits SMTP's lack of authentication to enable phishing, business email compromise, and social engineering attacks that cost organizations billions annually. Modern email authentication provides strong defenses when properly implemented.

Key Takeaways

•Protocol weakness — SMTP has no native sender authentication; any server can claim any sender address, enabling trivial forgery
•Two-layer addressing — Envelope sender (MAIL FROM) and header From can differ; users only see header From, which is easily spoofed
•Business impact — Business Email Compromise causes billions in losses; CEO fraud, invoice manipulation, and credential phishing are common patterns
•SPF validates sending servers — Domain publishes authorized sending IPs; receiving servers check if connecting IP is authorized for envelope domain
•DKIM provides cryptographic proof — Messages are digitally signed; receiving servers verify signature using DNS-published public keys
•DMARC ties it together — Requires alignment between visible From header and authenticated domains; specifies policy for failures; provides reporting
•Defense in depth — Combine authentication with user training, procedural controls, lookalike monitoring, and content analysis for comprehensive protection

What's Next:

Having explored the major spoofing attack categories—IP, ARP, DNS, and email—the next page synthesizes this knowledge into comprehensive countermeasures. We'll examine how organizations build layered defenses against spoofing attacks at every network layer.

Page Complete

You now understand email spoofing comprehensively: SMTP vulnerabilities, attack categories, business email compromise patterns, and the SPF/DKIM/DMARC authentication framework. This knowledge is essential for securing organizational email infrastructure against increasingly sophisticated impersonation attacks.