NTP Pentesting Skill

This skill guides you through assessing Network Time Protocol (NTP) services for security vulnerabilities. NTP is critical infrastructure—compromised time sync can bypass authentication, break crypto-protocols, and obscure forensic trails.

When to Use This Skill

• Port 123/UDP is open in your scan results
• You need to enumerate NTP or chrony services
• You're assessing time synchronization security
• You found NTS-KE on port 4460/tcp
• You need hardening recommendations for NTP infrastructure

Quick Start

# Basic NTP discovery
nmap -sU -sV --script "ntp* and (discovery or vuln) and not (dos or brute)" -p 123 <TARGET_IP>

# Check for dangerous monlist
nmap -sU -p123 --script ntp-monlist <TARGET_IP>

Phase 1: Service Enumeration

Identify the NTP Implementation

First, determine which NTP daemon is running:

# ntpd (classic)
ntpq -c rv <IP>
ntpq -c readvar <IP>
ntpq -c peers <IP>
ntpq -c associations <IP>

# chrony (modern Linux)
chronyc -a -n tracking -h <IP>
chronyc -a -n sources -v -h <IP>
chronyc -a -n sourcestats -h <IP>

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

What to look for:

• Stratum level (1 = authoritative, higher = downstream)
• Number of peers configured
• Whether remote queries are allowed
• Version string (check against CVE list below)

Nmap Reconnaissance

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

# Explicit monlist check (amplification risk)
map -sU -p123 --script ntp-monlist <IP>

# NTS-KE TLS channel (port 4460)
nmap -sV -p 4460 --script ssl-enum-ciphers,ssl-cert <IP>

Mass Scanning (if you have many targets)

# zgrab2 for large-scale monlist detection
zgrab2 ntp --monlist --timeout 3 --output-file monlist.json -f "zmap_results.csv"

Phase 2: Vulnerability Assessment

Check for CVE-2023-26551 to 26555 (ntpd 4.2.8p15)

These are out-of-bounds write vulnerabilities in libntp reachable via ntpq responses.

Affected: ntp 4.2.8p15 and earlier Fix: Upgrade to 4.2.8p16 or later

# Check version
ntpq -c rv <IP> | grep -i version

# If version is 4.2.8p15 or earlier, flag as vulnerable

Check for CVE-2023-33192 (ntpd-rs NTS DoS)

Malformed NTS cookies cause remote DoS even when NTS is disabled.

Affected: ntpd-rs versions before 0.3.3 Impact: Port 123 DoS even with NTS disabled

Check for monlist Amplification Risk

The

monlist

# Test if monlist is enabled
ntpdc -c monlist <IP>

# Or with nmap
nmap -sU -p123 --script ntp-monlist <IP>

If monlist returns data:

• This is a DDoS amplification risk
• Recommend immediate disable via

  disable monitor

  in ntp.conf
• Rate-limit UDP/123 at the network edge

NTS-KE Security Assessment (Port 4460/tcp)

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

# Manual TLS inspection
openssl s_client -connect <IP>:4460 -alpn ntske/1 -tls1_3 -ign_eof

Check for:

• Self-signed or expired certificates
• Weak cipher suites (non-AEAD)
• TLS version < 1.3
• Certificate validity and chain

Phase 3: Configuration Review

If you have access to configuration files, review these:

ntpd Configuration (

/etc/ntp.conf

)

cat /etc/ntp.conf

Look for:

• restrict

  lines - should limit query access

• kod

  (Kiss-o'-Death) - should be enabled for rate limiting

• disable monitor

  - should be present to disable monlist

• nts enable

  - check if NTS is properly configured

• includefile

  entries - check for crypto configuration

Good restrict example:

restrict default kod nomodify notrap nopeer noquery
restrict 127.0.0.1
restrict ::1

chrony Configuration (

/etc/chrony/chrony.conf

)

cat /etc/chrony/chrony.conf

Look for:

• cmdallow

  - should be restricted to localhost or specific IPs

• maxdistance

  /

  maxjitter

  - sanity filters for time-shift attacks

• nts

  directives - NTS configuration

systemd-timesyncd (

/etc/systemd/timesyncd.conf

)

cat /etc/systemd/timesyncd.conf

Note: This is client-only, no server functionality.

Phase 4: Attack Testing (Authorized Only)

Time-Shift Attack Detection

On-path attackers can silently shift client clocks by dropping/delaying packets.

Detection:

• Monitor for step adjustments > 1000 seconds in daemon logs
• Check for

  panic

  events in NTP logs
• Use Khronos-style sanity checking (query diverse servers)

Modern chrony (4.4+) already implements:

• maxdistance

  - maximum acceptable time difference

• maxjitter

  - maximum acceptable jitter

NTP Amplification Testing

DO NOT perform amplification attacks against unauthorized targets. This is illegal and harmful.

Instead, document the risk:

Risk: NTP amplification attack vector
Impact: DDoS amplification factor up to 200x
Mitigation: Disable monlist, rate-limit UDP/123, enable BCP 38

Phase 5: Hardening Recommendations

Immediate Actions

1. Disable monlist (if ntpd):

   disable monitor
   

2. Enable rate limiting:

   restrict default kod nomodify notrap nopeer noquery
   

3. Upgrade ntpd to 4.2.8p16+ if running vulnerable version

4. Restrict chrony cmdallow to localhost only:

   cmdallow 127.0.0.1
   

Best Practices (BCP-233 / RFC 8633)

1. Use ≥4 independent time sources (public pools, GPS, PTP-bridges)
2. Enable NTS (Network Time Security) for authenticated time sync
3. Monitor logs for panic events or step adjustments > 1000s
4. Consider leap-smear to avoid leap-second outages
5. Keep polling ≤24 hours so leap-second flags aren't missed
6. Enable BCP 38 egress filtering to block source spoofing

NTS Configuration

# Check NTS status
chronyc -a -n tracking -h <IP>

# NTS-KE port should be 4460/tcp
nmap -sV -p 4460 <IP>

Reporting Template

## NTP Security Assessment

### Service Details
- Port: 123/UDP (NTP), 4460/TCP (NTS-KE if enabled)
- Daemon: [ntpd/chrony/timesyncd]
- Version: [version string]
- Stratum: [stratum level]

### Vulnerabilities Found
- [ ] CVE-2023-26551-26555 (ntpd OOB write) - [Status]
- [ ] CVE-2023-33192 (ntpd-rs NTS DoS) - [Status]
- [ ] monlist enabled (amplification risk) - [Status]
- [ ] Weak NTS TLS configuration - [Status]

### Configuration Issues
- [ ] Missing restrict directives
- [ ] cmdallow too permissive
- [ ] No NTS enabled
- [ ] Single time source

### Recommendations
1. [Priority 1 item]
2. [Priority 2 item]
3. [Priority 3 item]

Useful Tools Reference

ntpq

ntpq -c rv <IP>

ntpdc

ntpdc -c monlist <IP>

chronyc

chronyc -a -n sources -h <IP>

nmap

zgrab2

zgrab2 ntp --monlist

openssl

openssl s_client -connect <IP>:4460

Shodan/Censys Dorks (for reconnaissance)

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

References

• RFC 8915 – Network Time Security for NTP (port 4460)
• RFC 8633 – NTP Best Current Practice
• CVE-2023-26551 to 26555 – ntpd OOB write vulnerabilities
• CVE-2023-33192 – ntpd-rs NTS DoS
• Cloudflare DDoS Report 2024 Q4 (5.6 Tbps NTP amplification)
• Khronos/Chronos draft – Time-shift mitigation

Safety Notes

• Only test NTP services you have authorization to assess
• Do not perform amplification attacks - they are illegal and harmful
• monlist queries can trigger rate limiting - use responsibly
• Time-shift attacks can break systems - only test in controlled environments
• Document all findings - NTP is critical infrastructure