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37. Enumeration: NetBIOS, SMB, SNMP, LDAP, SMTP, NFS and DNS

37.5 SMTP Enumeration

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SMTP (port 25) mail pathavto – pan junya servers VRFY / EXPN ne usernames confirm kartat. Attacker valid mailbox list banavto, mag phishing/password spray.

# Manual enum via nc / telnet (Metasploitable often has SMTP)
nc 192.168.56.20 25
# After banner, try (type carefully; quit with QUIT):
# HELO lab.local
# VRFY root
# VRFY nosuchuser
# EXPN root
# RCPT TO:<root@localhost>   # sometimes works after MAIL FROM
# QUIT

Nmap scripts (lab):

nmap -p 25 --script smtp-enum-users --script-args smtp-enum-users.methods=VRFY,EXPN,RCPT 192.168.56.20

Defence: VRFY ani EXPN disable (Postfix / Sendmail config); rate-limit RCPT; require auth for relay; do not expose SMTP submission to the whole internet without controls; monitor failed RCPT floods. Aata pudhe jaauya.

Ravindra Bagale's Tip

Students see VRFY succeed and think "hacked". It only confirms a username – it gives no access. But a list of valid users is gold for phishing. Defender: disable VRFY/EXPN + anti-spam. In interviews, explain both the attack and the steps to disable it.

Lab

nc 192.168.56.20 25 ne banner grab kara. VRFY root ani ek random naav try kara – response code fark note kara (250 vs 550). Mag Postfix/Sendmail hardening note liha: "disable VRFY/EXPN". Real mail servers var permission shivay try karu naka.

Extended teaching notes

Why this lesson matters

Enumeration maps what a network exposes. Defenders use the same ideas to close shares, fix SNMP community strings, restrict LDAP binds and stop zone transfers.

Hardening related services

[('SSH', 'Key auth, no root login, Fail2ban or equivalent, SG limited to your IP/VPN.', '', ''), ('SMB / NFS / LDAP / SNMP', 'Do not expose to the internet; patch; strong auth; disable unused versions (e.g. SMBv1).', '', ''), ('SMTP', 'Authenticated relay only; SPF/DKIM/DMARC for your domains.', '', ''), ('DNS', 'Restrict zone transfers; monitor for unexpected records (crt.sh for certs).', '', ''), ('Firewalls', 'Host firewall + cloud SG/NSG; default deny; document every allow.', '', '')]

Lab rule

Run enumeration only against host-only lab VMs you own. Then write the defensive fix for each open service you found.

Mini case study

Suppose we are on a purple-team exercise for an Indian mid-size company (sample data). Red notes a technique from this lesson against a lab clone. Blue must show the control that stops it, the log that would have fired, and the ticket that hardens production. That is the standard this course trains you for.

Defender's worksheet (do this today)

  1. Write the lesson title, date and the exact lab target you are allowed to use.
  2. List three owned systems (VMs, apps, or cloud accounts) where this lesson applies.
  3. Run one safe check from the lesson on a lab system only. Paste the command and output.
  4. Write a two-sentence fix you would give a teammate.
  5. Name one related public advisory (CERT-In, CISA, NVD, or a vendor bulletin) with its date.

Indian workplace (sample data)

Suppose we are a junior analyst at Infosys, TCS, Wipro or a fintech SOC in Bengaluru/Pune. Turn this lesson into one checklist item with an owner, a due date and a place to store evidence (ticket + screenshot).

Global practice

Compare your notes with OWASP, CIS Benchmarks, NIST CSF/IR guidance or a cloud Well-Architected security question. Write five lines on what overlaps.

Everyday analogy

Explain the risk and the fix to a family member without jargon. If they can repeat the warning, you understand it well enough to teach juniors.

Key terms

  • Control: a safeguard (patch, config, process, detection).
  • Finding: a weakness backed by evidence.
  • Remediation: the fix plus how you verified it.
  • Scope: systems you have written permission to touch.
  • Least privilege: only the access the task needs.
  • Defence in depth: several layers so one failure does not end the story.

Common mistakes

  • Skipping written permission "just for a quick scan".
  • Practising on hostel, office or public targets without authorisation.
  • Collecting tool output without writing the fix.
  • Copying attack steps onto production to "verify". Defenders verify with versions, configs and patches.
  • Publishing screenshots that contain secrets, cookies or internal hostnames.

Recap

  • Evidence, fix, owner, date.
  • Own lab only.
  • Fix > spectacle.