Kerberos is a network authentication protocol used to provide secure authentication over a non-secure network. While it is an essential component of network security, it can also be exploited by hackers to gain unauthorized access to sensitive information. In this article, we will take a deep dive into one such exploitation technique, AS-REP Roasting.
We will explore the basics of Kerberos, the difference between AS-REP Roasting, Kerberoasting, and Golden Ticket attacks, and the tools required to perform an AS-REP Roasting attack.
Before we delve into the specifics of AS-REP Roasting, it is important to understand the basics of Kerberos. Kerberos is a protocol that enables secure authentication between a client and a server over a non-secure network. The protocol relies on a trusted third-party service called the Key Distribution Center (KDC) to authenticate users and services on the network.
When a user logs in to a Kerberos-enabled network, the user’s computer sends a request to the KDC for a ticket-granting ticket (TGT). The KDC verifies the user’s credentials and sends a TGT back to the user’s computer. The TGT contains a session key that can be used to authenticate the user to other services on the network.
AS-REP Roasting is an exploitation technique that targets the Kerberos protocol. It allows an attacker to retrieve password hashes for users that do not require pre-authentication. Pre-authentication is an initial stage in Kerberos authentication that prevents brute-force attacks. If a user has “Do not use Kerberos pre-authentication” enabled, an attacker can recover a Kerberos AS-REP encrypted with the user’s RC4-HMAC’d password and attempt to crack this ticket offline.
To better understand AS-REP Roasting, it is important to differentiate it from other Kerberos attacks, including Kerberoasting and Golden Ticket attacks.
AS-REP Roasting
AS-REP Roasting is an attack that retrieves user hashes that can be brute-forced offline. If the user has “Do not use Kerberos pre-authentication” enabled, an attacker can recover a Kerberos AS-REP encrypted with the user’s RC4-HMAC’d password and attempt to crack this ticket offline.
Kerberoasting
Kerberoasting is an attack that retrieves application service hashes that can be brute-forced offline. This attack targets Kerberos service tickets rather than TGTs.
Golden Ticket
If an attacker possesses the password hash for the KRBTGT account, they can create a golden ticket. This type of ticket allows the attacker to produce authentication material for any account located in the Active Directory. Once they have a golden ticket, they can use it to request ticket granting services (TGS) tickets that will grant them access to specific resources. To obtain TGS, attackers must communicate with the Key Distribution Center (KDC), which operates on domain controllers within the Active Directory domain.
To perform an AS-REP Roasting attack, you will need the following tools:
Rubeus.exe
Rubeus.exe is a tool that can be used to perform various Kerberos-related attacks, including AS-REP Roasting. To use Rubeus.exe, simply run the command Rubeus.exe asreproast to dump the user account hashes used to encrypt the timestamp. These hashes can then be saved in a text document for offline password cracking.
ASREPRoast PowerShell Script
The ASREPRoast PowerShell script is another tool that can be used to perform an AS-REP Roasting attack. To use the script, download it from the official repository on Github and import the module in PowerShell. Then, run the command Invoke-ASREPRoast to extract the user hash with the AS_REP message.
Impacket
Impacket is a collection of Python classes that can be used to craft and decode network packets. The GetNPUsers.py script, which is part of the Impacket collection, can be used to attempt to list and get TGTs for those users that have the property ‘Do not require Kerberos pre-authentication’ set (UFDONTREQUIRE_PREAUTH).
Now that we have a basic understanding of AS-REP Roasting and the tools required to perform an attack, let’s dive into the specifics of performing an AS-REP Roasting attack.
Enabling “Do not require pre-authentication” for a user
By default, “Do not require pre-authentication” is disabled for domain users. To test an AS-REP Roasting attack, we will need to enable this setting for a user. Once all prerequisites required to perform this attack are in place, we can enable “Do not require pre-authentication” for a user.
Using Rubeus.exe for AS-REP Roasting
To use Rubeus.exe for AS-REP Roasting, simply run the command Rubeus.exe asreproast. This will dump the user account hashes used to encrypt the timestamp. These hashes can then be saved in a text document for offline password cracking.
.\Rubeus.exe asreproast /format:<hashcat/john> /outfile:<output>
Using the ASREPRoast PowerShell Script for AS-REP Roasting
To use the ASREPRoast PowerShell script for AS-REP Roasting, download it from the official repository on Github and import the module in PowerShell. Then, run the command Invoke-ASREPRoast to extract the user hash with the AS_REP message.
Import-Module .\ASREPRoast.ps1
Invoke-ASREPRoast
Invoke-ASREPRoast | select -ExpandProperty Hash
Using Impacket for AS-REP Roasting
To use Impacket for AS-REP Roasting, run the GetNPUsers.py script with the appropriate parameters. This will extract the user hash with the AS_REP message, which can then be used for offline password cracking.
python GetNPUsers.py <domain_name>/<domain_user>:<domain_user_password> -request -format <hashcat/john> -outputfile <output>
Cracking with a Dictionary of Passwords
hashcat -m 18200 -a 0 <output> <passwords_file>
john --wordlist=<passwords_file> <output>
AS-REP Roasting is a potent technique that can be used to exploit the Kerberos protocol. By enabling “Do not require pre-authentication” for a user, an attacker can retrieve password hashes that can be cracked offline. The tools required to perform an AS-REP Roasting attack are readily available, making it an accessible technique for both hackers and security professionals. It is essential to understand the basics of Kerberos and the difference between AS-REP Roasting, Kerberoasting, and Golden Ticket attacks to protect against these types of attacks.
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