123/udp - Pentesting NTP

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Basic Information

The Network Time Protocol (NTP) ensures computers and network devices across variable-latency networks sync their clocks accurately. It's vital for maintaining precise timekeeping in IT operations, security, and logging. Because time is used in nearly every authentication, crypto-protocol and forensic process, an attacker that can influence NTP can often bypass security controls or make attacks harder to investigate.

Summary & Security Tips

• Purpose: Syncs device clocks over networks.
• Importance: Critical for security, logging, crypto-protocols and distributed systems.
• Security Measures:
  • Use trusted NTP or NTS (Network Time Security) sources with authentication.
  • Restrict who can query/command the daemon (restrict default noquery, kod etc.).
  • Disable legacy Mode-6/7 control queries (monlist, ntpdc) or rate-limit them.
  • Monitor synchronization drift/leap-second state for tampering.
  • Keep the daemon updated (see recent CVEs below).

Default ports

123/udp   NTP            (data + legacy control)
4460/tcp  NTS-KE (RFC 8915) – TLS key-establishment for NTP

PORT    STATE SERVICE REASON
123/udp open  ntp     udp-response

Enumeration

Classic ntpd / ntpq / ntpdc

# Information & variables
ntpq -c rv <IP>
ntpq -c readvar <IP>
ntpq -c peers <IP>
ntpq -c associations <IP>

# Legacy mode-7 (often disabled >=4.2.8p9)
ntpdc -c monlist <IP>
ntpdc -c listpeers <IP>
ntpdc -c sysinfo  <IP>

chrony / chronyc (in most modern Linux distros)

Only a handful of monitoring commands are accepted from remote IPs when cmdallow is enabled:

chronyc -a -n tracking   -h <IP>
chronyc -a -n sources -v -h <IP>
chronyc -a -n sourcestats -h <IP>

See the chronyc man page for the meaning of the M/S flags and other fields (stratum, reach, jitter, etc.).

Nmap

# Safe discovery & vuln detection
nmap -sU -sV --script "ntp* and (discovery or vuln) and not (dos or brute)" -p 123 <IP>

# Explicit monlist check
nmap -sU -p123 --script ntp-monlist <IP>

Mass/Internet scanning

# Check if MONLIST is enabled (zgrab2 module)
zgrab2 ntp --monlist --timeout 3 --output-file monlist.json -f "zmap_results.csv"

Examine configuration files

• /etc/ntp.conf (ntpd)
• /etc/chrony/chrony.conf (chrony)
• /etc/systemd/timesyncd.conf (timesyncd – client only)

Pay special attention to restrict lines, kod (Kiss-o'-Death) settings, disable monitor/includefile /etc/ntp/crypto and whether NTS is enabled (nts enable).

Recent Vulnerabilities (2023-2025)

Exploit kits: Proof-of-concept payloads for the 2023 ntpq OOB-write series are on GitHub (see Meinberg write-up) and can be weaponised for client-side phishing of sysadmins.

Advanced Attacks

1. NTP Amplification / Reflection

The legacy Mode-7 monlist query returns up to 600 host addresses and is still present on thousands of Internet hosts. Because the reply (428-468 bytes/entry) is ~ 200× larger than the 8-byte request, an attacker can reach triple-digit amplification factors. Mitigations:

• Upgrade to ntp 4.2.8p15+ and add disable monitor.
• Rate-limit UDP/123 on the edge or enable sessions-required on DDoS appliances.
• Enable BCP 38 egress filtering to block source spoofing.

See Cloudflare’s learning-center article for a step-by-step breakdown.

2. Time-Shift / Delay attacks (Khronos / Chronos research)

Even with authentication, an on-path attacker can silently shift the client clock by dropping/delaying packets. The IETF Khronos (formerly Chronos) draft proposes querying a diverse set of servers in the background and sanity-checking the result to detect a shift > 𝚡 ms. Modern chrony (4.4+) already implements a similar sanity filter (maxdistance / maxjitter).

3. NTS abuse & 4460/tcp exposure

NTS moves the heavy crypto to a separate TLS 1.3 channel on 4460/tcp (ntske/1). Poor implementations (see CVE-2023-33192) crash when parsing cookies or allow weak ciphers. Pentesters should:

# TLS reconnaissance
nmap -sV -p 4460 --script ssl-enum-ciphers,ssl-cert <IP>

# Grab banner & ALPN
openssl s_client -connect <IP>:4460 -alpn ntske/1 -tls1_3 -ign_eof

Look for self-signed or expired certificates and weak cipher-suites (non-AEAD). Reference: RFC 8915 §4.

Hardening / Best-Current-Practice (BCP-233 / RFC 8633)

Operators SHOULD:

1. Use ≥ 4 independent, diverse time sources (public pools, GPS, PTP-bridges) to avoid single-source poisoning.
2. Enable kod and limited/nomodify restrictions so abusive clients receive Kiss-o'-Death rate-limit packets instead of full responses.
3. Monitor daemon logs for panic events or step adjustments > 1000 s. (Signatures of attack per RFC 8633 §5.3.)
4. Consider leap-smear to avoid leap-second outages, but ensure all downstream clients use the same smear window.
5. Keep polling ≤24 h so leap-second flags are not missed.

See RFC 8633 for a comprehensive checklist.

Shodan / Censys Dorks

port:123 "ntpd"          # Version banner
udp port:123 monlist:true # Censys tag for vulnerable servers
port:4460 "ntske"         # NTS-KE

Useful Tools

HackTricks Automatic Commands

Protocol_Name: NTP
Port_Number: 123
Protocol_Description: Network Time Protocol

Entry_1:
  Name: Notes
  Description: Notes for NTP
  Note: |
    The Network Time Protocol (NTP) ensures computers and network devices across variable-latency networks sync their clocks accurately. It's vital for maintaining precise timekeeping in IT operations, security, and logging. NTP's accuracy is essential, but it also poses security risks if not properly managed.

    https://book.hacktricks.wiki/en/network-services-pentesting/pentesting-ntp.html

Entry_2:
  Name: Nmap
  Description: Enumerate NTP
  Command: nmap -sU -sV --script "ntp* and (discovery or vuln) and not (dos or brute)" -p 123 {IP}

References

• RFC 8915 – Network Time Security for the Network Time Protocol (port 4460)
• RFC 8633 – Network Time Protocol BCP
• Cloudflare DDoS report 2024 Q4 (5.6 Tbps)
• Cloudflare NTP Amplification Attack article
• NTP 4.2.8p15 CVE series 2023-04
• NVD entries CVE-2023-26551–55, CVE-2023-33192
• SUSE chrony security update 2024 (chrony 4.5)
• Khronos/Chronos draft (time-shift mitigation)
• chronyc manual/examples for remote monitoring
• zgrab2 ntp module docs