Breaching a Windows environment by capturing and cracking NTLM challenge/response hashes

In this scenario we have limited physical access on a client’s network. We have been provided with an empty cubicle and only the ability to plug into the network. We have been granted no logical access and the goal is to see if take our limited physical access, gain an initial foothold within the network, and ultimately escalate our privileges throughout the environment.

The tools we’ll be using are:

Responder is a LLMNR, NBT-NS and MDNS poisoner, with built-in HTTP/SMB/MSSQL/FTP/LDAP rogue authentication server supporting NTLMv1/NTLMv2/LMv2, Extended Security NTLMSSP and Basic HTTP authentication. Link-Local Multicast Name Resolution (LLMNR) and Netbios Name Service (NBT-NS) are two components of nearly all Microsoft Windows networks. LLMNR and NBT-NS allow Windows operating systems on the same subnet help each other identify hosts if/when DNS resolution fails. If DNS resolution fails for a particular host, that computer will typically attempt to query other machines on the LAN for the correct address via LLMNR or NBT-NS.

This default behavior can be leveraged into a foothold for an attacker within the network. An attacker can impersonate the servers/services being requested by other workstations and convince the workstation to provide authentication information to it – instead of the actual server. Consider the following example:

  1. A workstation attempts to access the UNC path \\SERVER01, but mistakenly types in \\SRVER01.
  2. That workstation’s DNS server(s) responds to the query saying that it doesn’t know any host by that name.
  3. The workstation then query’s all other devices on the LAN asking if they know of the location of \\SRVER01
  4. An attacker responds to the workstation, informing it that it is SRVER01
  5. The workstation then goes through the typical challenge/response procedure to validate the domain user’s credentials. During this step, the attacker able to capture the domain user’s NTLMv2 hash.
  6. The attacker can now attempt to crack the hash to discover the domain user’s password in order to gain additional access to the environment.

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To start, we need our Kali Linux box to exist on the same subnet as all the other workstations. After plugging into the network, can attempt to discover hosts on our subnet by looking at the IP address assigned to us via DHCP:

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Using the above information, we can scan our subnet for workstations with the following command: nbtscan 192.168.23.0/24

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This should give us enough information and insight about the network to fire up Responder and attempt to capture NTLM challenge/response hashes. If our nbtscan returned no (or limited) results, running Responder on that particular subnet would probably be ineffective.

Responder is a very powerful tool and it has the potential to break stuff on a network. I don’t plan to explain all of the capabilities and nuances of Responder – so use it at your own risk. An effective command for capturing hashes within a Windows network is: ./Responder.py -I eth0 -wrdfbF

After letting Responder run for a few minutes, we are able to capture the following Challenge/Response handshake hash when a host on the network attempts to access the below network resource:

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We are then able to take this captured hash and feed it into John The Ripper and attempt to crack it against a dictionary file of known passwords. In this particular instance, we’re using the Rockyou file. This is seen below:

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We can see  (above) that this particular domain user has a very weak password that was able to be cracked within 23 seconds. This was/is an actual domain user account within a production environment. With this information, we are now able to use other tools to enumerate where this user has privileges on the network and further escalate our privileges within the environment.

This topic will be covered in a future post.

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