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**IPsec** is widely recognized as the principal technology for securing communications between networks (LAN-to-LAN) and from remote users to the network gateway (remote access), serving as the backbone for enterprise VPN solutions.
The establishment of a **security association (SA)** between two points is managed by **IKE**, which operates under the umbrella of ISAKMP, a protocol designed for the authentication and key exchange. This process unfolds in several phases:
* **Phase 1:** A secure channel is created between two endpoints. This is achieved through the use of a Pre-Shared Key (PSK) or certificates, employing either main mode, which involves three pairs of messages, or **aggressive mode**.
* **Phase 1.5:** Though not mandatory, this phase, known as the Extended Authentication Phase, verifies the identity of the user attempting to connect by requiring a username and password.
* **Phase 2:** This phase is dedicated to negotiating the parameters for securing data with **ESP** and **AH**. It allows for the use of algorithms different from those in Phase 1 to ensure **Perfect Forward Secrecy (PFS)**, enhancing security.
The IPSec configuration can be prepared only to accept one or a few transformations. A transformation is a combination of values. **Each transform** contains a number of attributes like DES or 3DES as the **encryption algorithm**, SHA or MD5 as the **integrity algorithm**, a pre-shared key as the **authentication type**, Diffie-Hellman 1 or 2 as the key **distribution algorithm** and 28800 seconds as the **lifetime**.
Then, the first thing that you have to do is to **find a valid transformation**, so the server will talk to you. To do so, you can use the tool **ike-scan**. By default, Ike-scan works in main mode, and sends a packet to the gateway with an ISAKMP header and a single proposal with **eight transforms inside it**.
As you can see in the previous response, there is a field called **AUTH** with the value **PSK**. This means that the vpn is configured using a preshared key (and this is really good for a pentester).\
* _**1 returned handshake; 0 returned notify:**_ This means the **target is configured for IPsec and is willing to perform IKE negotiation, and either one or more of the transforms you proposed are acceptable** (a valid transform will be shown in the output).
* _0 returned handshake; 1 returned notify:_ VPN gateways respond with a notify message when **none of the transforms are acceptable** (though some gateways do not, in which case further analysis and a revised proposal should be tried).
Then, in this case we already have a valid transformation but if you are in the 3rd case, then you need to **brute-force a little bit to find a valid transformation:**
First of all you need to create all the possible transformations:
```bash
for ENC in 1 2 3 4 5 6 7/128 7/192 7/256 8; do for HASH in 1 2 3 4 5 6; do for AUTH in 1 2 3 4 5 6 7 8 64221 64222 64223 64224 65001 65002 65003 65004 65005 65006 65007 65008 65009 65010; do for GROUP in 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18; do echo "--trans=$ENC,$HASH,$AUTH,$GROUP" >> ike-dict.txt ;done ;done ;done ;done
while read line; do (echo "Valid trans found: $line" && sudo ike-scan -M $line <IP>) | grep -B14 "1 returned handshake" | grep "Valid trans found" ; done <ike-dict.txt
```
If the brute-force didn't work, maybe the server is responding without handshakes even to valid transforms. Then, you could try the same brute-force but using aggressive mode:
```bash
while read line; do (echo "Valid trans found: $line" && ike-scan -M --aggressive -P handshake.txt $line <IP>) | grep -B7 "SA=" | grep "Valid trans found" ; done <ike-dict.txt
Cisco indicates to avoid using DH groups 1 and 2 because they're not strong enough. Experts believe that **countries with a lot of resources can easily break the encryption** of data that uses these weak groups. This is done by using a special method that prepares them to crack the codes quickly. Even though it costs a lot of money to set up this method, it allows these powerful countries to read the encrypted data in real time if it's using a group that's not strong (like 1,024-bit or smaller).
Then, you can use ike-scan to try to **discover the vendor** of the device. The tool send an initial proposal and stops replaying. Then, it will **analyze** the **time** difference **between** the received **messages** from the server and the matching response pattern, the pentester can successfully fingerprint the VPN gateway vendor. More over, some VPN servers will use the optional **Vendor ID (VID) payload** with IKE.
For being allowed to capture the hash you need a valid transformation supporting Aggressive mode and the correct ID (group name). You probably won't know the valid group name, so you will have to brute-force it.\
If **no hash is returned**, then probably this method of brute forcing will work. **If some hash is returned, this means that a fake hash is going to be sent back for a fake ID, so this method won't be reliable** to brute-force the ID. For example, a fake hash could be returned (this happens in modern versions):
If you have discovered an specific transformation add it in the ike-scan command. And if you have discovered several transformations feel free to add a new loop to try them all (you should try them all until one of them is working properly).
You can use the[ dictionary of ikeforce](https://github.com/SpiderLabs/ikeforce/blob/master/wordlists/groupnames.dic) or [the one in seclists](https://github.com/danielmiessler/SecLists/blob/master/Miscellaneous/ike-groupid.txt) of common group names to brute-force them:
[**iker.py**](https://github.com/isaudits/scripts/blob/master/iker.py) also uses **ike-scan** to bruteforce possible group names. It follows it's own method to **find a valid ID based on the output of ike-scan**.
[**ikeforce.py**](https://github.com/SpiderLabs/ikeforce) is a tool that can be used to **brute force IDs also**. This tool will **try to exploit different vulnerabilities** that could be used to **distinguish between a valid and a non-valid ID** (could have false positives and false negatives, that is why I prefer to use the ike-scan method if possible).
* The **first method** is to brute-force the group names by **searching** for the information **Dead Peer Detection DPD** of Cisco systems (this info is only replayed by the server if the group name is correct).
* The **second method** available is to **checks the number of responses sent to each try** because sometimes more packets are sent when the correct id is used.
* The **third method** consist on **searching for "INVALID-ID-INFORMATION" in response to incorrect ID**.
* Finally, if the server does not replay anything to the checks, **ikeforce** will try to brute force the server and check if when the correct id is sent the server replay with some packet.\
Obviously, the goal of brute forcing the id is to get the **PSK** when you have a valid id. Then, with the **id** and **PSK** you will have to bruteforce the XAUTH (if it is enabled).
If you have discovered an specific transformation add it in the ikeforce command. And if you have discovered several transformations feel free to add a new loop to try them all (you should try them all until one of them is working properly).
(From the book **Network Security Assessment: Know Your Network**): It is also possible to obtain valid usernames by sniffing the connection between the VPN client and server, as the first aggressive mode packet containing the client ID is sent in the clear
Finally, If you have found a **valid transformation** and the **group name** and if the **aggressive mode is allowed**, then you can very easily grab the crackable hash:
You can use **psk-crack**, **john** (using [**ikescan2john.py**](https://github.com/truongkma/ctf-tools/blob/master/John/run/ikescan2john.py)) and **hashcat** to **crack** the hash:
**Aggressive mode IKE** combined with a **Pre-Shared Key (PSK)** is commonly employed for **group authentication** purposes. This method is augmented by **XAuth (Extended Authentication)**, which serves to introduce an additional layer of **user authentication**. Such authentication typically leverages services like **Microsoft Active Directory**, **RADIUS**, or comparable systems.
Transitioning to **IKEv2**, a notable shift is observed where **EAP (Extensible Authentication Protocol)** is utilized in lieu of **XAuth** for the purpose of authenticating users. This change underscores an evolution in authentication practices within secure communication protocols.
So you can capture the data of the login using _fiked_ and see if there is any default username (You need to redirect IKE traffic to `fiked` for sniffing, which can be done with the help of ARP spoofing, [more info](https://opensourceforu.com/2012/01/ipsec-vpn-penetration-testing-backtrack-tools/)). Fiked will act as a VPN endpoint and will capture the XAuth credentials:
Also, using IPSec try to make a MitM attack and block all traffic to port 500, if the IPSec tunnel cannot be established maybe the traffic will be sent in clear.
To brute force the **XAUTH** (when you know a valid group name **id** and the **psk**) you can use a username or list of usernames and a list o passwords:
In Kali, **VPNC** is utilized to establish IPsec tunnels. The **profiles** must be located in the directory `/etc/vpnc/`. You can initiate these profiles using the command _**vpnc**_.
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