smtp-open-relay: Linux SMTP Open Relay Misconfigured

id: smtp-open-relay

info:
  name: Linux SMTP Open Relay Misconfigured
  author: songyaeji
  severity: high
  description: |
    Sendmail and Postfix were checked for proper restrictions against unauthorized relay attempts. Improper configurations allowed attackers to abuse the server for spam or denial-of-service (DoS) attacks.
  reference:
    - https://isms.kisa.or.kr
  tags: linux,local,kisa,audit,compliance

self-contained: true

code:
  - engine:
      - bash
    source: |
      if ps -ef | grep -v grep | grep -q sendmail; then
        if grep -q 'R\$.\* \$#error \$@ 5.7.1 \$: "550 Relaying denied"' /etc/mail/sendmail.cf 2>/dev/null \
          || (test -f /etc/mail/access.db && echo "[INFO] Sendmail access.db exists (relay likely restricted)"); then
          echo "[SAFE] Sendmail relay restriction is configured."
        else
          echo "[VULNERABLE] Sendmail relay restriction is missing."
        fi
      else
        echo "[INFO] Sendmail is not active."
      fi

  - engine:
      - bash
    source: |
      if systemctl is-active postfix | grep -q running; then
        mynetworks=$(grep -E '^\s*mynetworks\s*=' /etc/postfix/main.cf 2>/dev/null)
        relay_restrict=$(grep -E '^\s*smtpd_relay_restrictions\s*=.*permit_mynetworks' /etc/postfix/main.cf 2>/dev/null)
        reject_unauth=$(grep -E '^\s*smtpd_relay_restrictions\s*=.*reject_unauth_destination' /etc/postfix/main.cf 2>/dev/null)

        if [[ -z "$mynetworks" || -z "$relay_restrict" || -z "$reject_unauth" ]]; then
          echo "[VULNERABLE] Postfix relay restrictions are missing or misconfigured."
        else
          echo "[SAFE] Postfix relay restrictions are properly configured."
        fi
      else
        echo "[INFO] Postfix is not active."
      fi

    matchers:
      - type: word
        name: sendmail
        part: code_1_response
        words:
          - "[VULNERABLE]"

      - type: word
        name: postfix
        part: code_2_response
        words:
          - "[VULNERABLE]"
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