In this article, it’s time to explore the scenario where the attacker wants to extract the hash or credentials of the target user but cannot use Mimikatz or any other noisy tool. This attack is called a Downgrade Attack because the attacker downgrades the version of NTLM to extract the hash.

Table of Content

• Introduction
• Exploitation
• PowerShell Empire Exploitation
• Decryption of Hash
• Conclusion

Introduction

While performing Red Team Operations, it is possible to come across a scenario where the attacker cannot use Mimikatz. This can be because almost all the Antivirus or Malware software are going to detect the presence of Mimikatz as soon as it lands on the Target Machine. This is the scenario where an attacker can perform Internal Monologue Attack. To perform this, attack a tool is required that was developed by Elad Shamir from Missing Link Security.

Being in touch with the Windows Security Mechanism, you will be familiar with the NetNTLM. It is a challenge response-based protocol that is used wherever Windows cannot apply Kerberos-based Authentication. In this method, the server sends an 8-byte challenge with the NTLM hash as the key to the user. The hash is an MD4 hash of the user’s password. There are two versions of NetNTLM. Both are vulnerable. Version 1 of the NetNTLM has introduced quite a while ago and it is disabled by default currently.

In a general sense, the downgrade attack was performed with the Mimikatz itself. After the exploitation of the target machine. The attacker then either using Mimikatz or manually can edit Registry Keys such as the LMCompatibilityLevel with values such as 0,1,2 that can make the compromised device use the NTLM downgraded or older version to interact with other SMB servers and can lead to pivoting to other users and servers.

However, in this attack that is described in the demonstration, the Mimikatz is not used and the attacker instead invokes a local procedure call from a user-mode application to the NTLM authentication package through the SSPI. This calculates the NetNTLM response that we discussed earlier in the context of the logged-on user. The attack inherently disables the NetNTLMv1 preventive controls, then it moves on to extract all non-network logon tokens from currently running processes and impersonate the associated users. For each impersonated user, NTLM SSP locally invokes an NTLMv1 response to the chosen challenge and then restores the original values of the Registry keys discussed earlier. Now the captured hash can be cracked with the tool of your preference such as John the Ripper or Hash Cat.

GitHub: Internal Monologue

Exploitation

You have the option to compile the executable by yourself by getting the binaries from GitHub. However, for this demonstration, we will be downloading the executable itself.

Download InternalMonologue.exe

After downloading the executable, assuming the attacker holds the initial foothold of the target machine. It is required to transfer the executable to the target machine and run it with certain parameters. The Downgrade parameter should have the value true to downgrade the version. Then the Threads parameter should also hold the true value and finally to perform the Impersonation, the Impersonate parameter value should also be true. Upon successfully running the executable, the attacker is successfully able to extract the downgraded v1 hash of the target user as demonstrated.

PowerShell Empire Exploitation

In case, the attacker decided to compromise the target machine through the PowerShell Empire and has an agent active, then they can perform a downgrade attack directly from PowerShell Empire. Inside the Credentials, the PowerShell Empire has a module by the name of invoke_internal_monologue that essentially performs the same attacker as the executable that was discussed earlier. This method doesn’t involve transferring an executable and running it on the target machine is much stealthier.

Decryption of Hash

In both variants of attacks that were performed earlier. The hash for the raj user was found to be the same and now there are two ways in which this has can be used. Firstly, the attacker can directly use the hash to log in by performing a Pass the Hash. But if the attacker wants, they can crack the hash using John the Ripper. Store the extracted hash of the raj user in a file on the Desktop of our Kali Linux and named its hash. Then using John the Ripper with describing the format to be NetNTLM as demonstrated below. It can be observed that the hash can be cracked. The hash was found to be the password “123”.

Conclusion

Sometimes, ideas as simple as downgrading the version of the authentication mechanism can serve to be dangerous. As this attack doesn’t require any tools that are on the target of various Defensive Mechanisms it can fly under the radar and get those credentials. This is a testament that Security is ever-evolving and the only way to get ahead of an attacker is to think like one.

Author: Pavandeep Singh is a Technical Writer, Researcher, and Penetration Tester. Can be Contacted on Twitter and LinkedIn