Kerberosting
Kerberosting Basics
Kerberos is a protocol for authentication used in Windows Active Directory environments (though it can be used for auth to Linux hosts as well). In 2014, Tim Medin presented an attack on Kerberos he called Kerberoasting. It’s worth reading through the presentation, as Tim uses good graphics to illustrate the process, but I’ll try to give a simple overview.
When you want to authenticate to some service using Kerberos, you contact the DC and tell it to which system service you want to authenticate. It encrypts a response to you with the service user’s password hash. You send that response to the service, which can decrypt it with it’s password, check who you are, and decide it if wants to let you in.
Kerberoasting is a lateral movement/privilege escalation technique in Active Directory environments. This attack targets Service Principal Names (SPN) accounts. SPNs are unique identifiers that Kerberos uses to map a service instance to a service account in whose context the service is running. Domain accounts are often used to run services to overcome the network authentication limitations of built-in accounts such as NT AUTHORITY\LOCAL SERVICE. Any domain user can request a Kerberos ticket for any service account in the same domain.
Domain accounts running services are often local administrators, if not highly privileged domain accounts. Due to the distributed nature of systems, interacting services, and associated data transfers, service accounts may be granted administrator privileges on multiple servers across the enterprise. Many services require elevated privileges on various systems, so service accounts are often added to privileged groups, such as Domain Admins, either directly or via nested membership. Finding SPNs associated with highly privileged accounts in a Windows environment is very common. Retrieving a Kerberos ticket for an account with an SPN does not by itself allow you to execute commands in the context of this account. However, the ticket (TGS-REP) is encrypted with the service account’s NTLM hash, so the cleartext password can potentially be obtained by subjecting it to an offline brute-force attack with a tool such as Hashcat.
Service accounts are often configured with weak or reused password to simplify administration, and sometimes the password is the same as the username. If the password for a domain SQL Server service account is cracked, you are likely to find yourself as a local admin on multiple servers, if not Domain Admin. Even if cracking a ticket obtained via a Kerberoasting attack gives a low-privilege user account, we can use it to craft service tickets for the service specified in the SPN. For example, if the SPN is set to MSSQL/SRV01, we can access the MSSQL service as sysadmin, enable the xp_cmdshell extended procedure and gain code execution on the target SQL server.
In a Kerberoasting attack, rather than sending the encrypted ticket from the DC to the service, you will use off-line brute force to crack the password associated with the service.
Most of the time you will need an active account on the domain in order to initial Kerberoast, but if the DC is configured with UserAccountControl setting “Do not require Kerberos preauthentication” enabled, it is possible to request and receive a ticket to crack without a valid account on the domain.
Several tools can be utilized to perform the attack:
Impacket’s GetUserSPNs.py from a non-domain joined Linux host.
A combination of the built-in setspn.exe Windows binary, PowerShell, and Mimikatz.
From Windows, utilizing tools such as PowerView, Rubeus, and other PowerShell scripts.
Method-1
We can start by just gathering a listing of SPNs in the domain. To do this, we will need a set of valid domain credentials and the IP address of a Domain Controller. We can authenticate to the Domain Controller with a cleartext password, NT password hash, or even a Kerberos ticket. For our purposes, we will use a password. Entering the below command will generate a credential prompt and then a nicely formatted listing of all SPN accounts. From the output below, we can see that several accounts are members of the Domain Admins group. If we can retrieve and crack one of these tickets, it could lead to domain compromise. It is always worth investigating the group membership of all accounts because we may find an account with an easy-to-crack ticket that can help us further our goal of moving laterally/vertically in the target domain.
Now let's get the SPNs. The script identified a user, Administrator:
So, we have a ticket. Now, lets crack it with john.
Step by Step Method
We can start by just gathering a listing of SPNs in the domain. To do this, we will need a set of valid domain credentials and the IP address of a Domain Controller. We can authenticate to the Domain Controller with a cleartext password, NT password hash, or even a Kerberos ticket. For our purposes, we will use a password. Entering the below command will generate a credential prompt and then a nicely formatted listing of all SPN accounts. From the output below, we can see that several accounts are members of the Domain Admins group. If we can retrieve and crack one of these tickets, it could lead to domain compromise. It is always worth investigating the group membership of all accounts because we may find an account with an easy-to-crack ticket that can help us further our goal of moving laterally/vertically in the target domain.
Listing SPN Accounts with GetUserSPNs.py
We can now pull all TGS tickets for offline processing using the -request flag. The TGS tickets will be output in a format that can be readily provided to Hashcat or John the Ripper for offline password cracking attempts.
Requesting all TGS Tickets
We can also be more targeted and request just the TGS ticket for a specific account. Let's try requesting one for just the sqldev account.
Requesting a Single TGS ticket
With this ticket in hand, we could attempt to crack the user's password offline using Hashcat. If we are successful, we may end up with Domain Admin rights.
To facilitate offline cracking, it is always good to use the -outputfile flag to write the TGS tickets to a file that can then be run using Hashcat on our attack system or moved to a GPU cracking rig.
Saving the TGS Ticket to an Output File
Kerberoasting - from Linux
Here we've written the TGS ticket for the sqldev user to a file named sqldev_tgs. Now we can attempt to crack the ticket offline using Hashcat hash mode 13100.
Cracking the Ticket Offline with Hashcat
We've successfully cracked the user's password as database!. As the last step, we can confirm our access and see that we indeed have Domain Admin rights as we can authenticate to the target DC in the INLANEFREIGHT.LOCAL domain. From here, we could perform post-exploitation and continue to enumerate the domain for other paths to compromise and other notable flaws and misconfigurations.
Testing Authentication against a Domain Controller
Kerberoasting - from Linux
Method-2
We also have another script in Impacket that we can directly run without providing username and we get the hash.
If we have potential usernames, we can also use that wordlist to fetch hash value.
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Kerberoasting with Mimikatz
Detecting Kerberosting Attack
We have prefetch files and event viewer for the sherlock.
Analyzing Domain Controller Security Logs
Open the event file of the domain controller.
Filter by the event ID
We still get a lot of events.
Understanding kerberos ticket request
Here we can see that Account Name “DC01$” requested a service ticket for service named DC01$. In Windows names ending with $ are typically service accounts and machine accounts. Similarly, the DC01$ service is related to that service account.
This all belongs to normal Active Directory operations. Below that we can see an option named “Ticket Encryption type” with the value of 0x12 which equals to “AES256-CTS-HMAC-SHA1-96 ”.
In legitimate use cases for Kerberos ticket operations, the encryption type would be 0x12 or 0x11.
But if we see an encryption type “0x17” which is RC4 encryption, that would be a clue to look into this further, as an attacker may request a ticket in this encryption type because it allows them to crack the password.
All major open-source tools, like Impacket and Rubeus, request tickets in RC4 encryption type.
To further reduce the chances of false positives, we can filter out requests from other service accounts and machine accounts.
Service accounts request service tickets from domain controllers all the time; that’s the nature of how service accounts work. To further reduce the events to investigate, we can filter out requests from service names starting with “$”—they are computer accounts or other service account-related services that Windows uses as part of its operations.
We can search for the 0x17 encryption type and events.
We can see that a domain Account “alonzo.spire” requested a ticket for a service name “MSSQLService” with an encryption type of 0x17 from a workstation with IP Address 172.17.79.129.
Notice that both the account name and service name do not end with $.
For SIEMS
SOC analysts can query the logs in SIEMs to create a filter for all the things mentioned.
With the filters discussed above we’re snooping for a 4769 event where:
1. Account name that is NOT a service or machine account (ending with $), so any normal domain user account (this would be the account which is compromised and from which the attacker performed this attack.)
2. Service Names that do NOT end with $.
3. Ticket encryption type will be 0x17 which is RC4 encryption, allowing attackers to easily crack the hash.
The follow-up to this detection would be to:
Create a timeline of when this event was generated.
Do a forensic analysis of the machine with IP Address 172.17.79.129, and find out how the “alonzo.spire” user account got compromised.
We can use artifacts like Process Logs from Sysmon if available, prefetch, lnk files, Managed File Transfer (MFT), or registry to gain insights on what occurred around the time when Kerberoasting activity was noticed.
Analyzing Power shell logs
we can see executed commands/scripts by filtering for event ID 4104.
we can look into the events and see that powerview has been used.
Exploring Timeline
Now we can use Timeline Explorer to look into the actual timeline of the event. we can use the csv file generated from prefetch files.
We should look for any execution around the timeline we established so far. Let's filter for the date of the incident to reduce the noise. We add the filter for the "Last Run" field
Looking at the last run timestamps, we find an exe was executed just a second before our malicious events were logged on the Domain Controller.
This is a Kerberos abuse tool. One that's common in Active Directory pentesting or offensive operations. To get the full path of the file, go to the files loaded and double-click to see all files loaded by this tool at execution.
You can check the last run column to see when was the tool actually run.
NPUsers vs UsersNP
Target:
GetUserSPNs.pytargets service accounts associated with SPNs (Kerberoasting).GetNPUsers.pytargets user accounts with pre-authentication disabled (AS-REP Roasting).
Objective:
GetUserSPNs.pyaims to retrieve service tickets that can be cracked to reveal service account passwords.GetNPUsers.pyaims to retrieve AS-REP responses that can be cracked to reveal user passwords.
Prerequisites:
GetUserSPNs.pyrequires knowledge of the domain and typically valid credentials to request service tickets.
Efficacy of the Attack
While it can be a great way to move laterally or escalate privileges in a domain, Kerberoasting and the presence of SPNs do not guarantee us any level of access. We might be in an environment where we crack a TGS ticket and obtain Domain Admin access straightway or obtain credentials that help us move down the path to domain compromise. Other times we may perform the attack and retrieve many TGS tickets, some of which we can crack, but none of the ones that crack are for privileged users, and the attack does not gain us any additional access. I would likely write up the finding as high-risk in my report in the first two cases. In the third case, we may Kerberoast and end up unable to crack a single TGS ticket, even after days of cracking attempts with Hashcat on a powerful GPU password cracking rig. In this scenario, I would still write up the finding, but I would drop it down to a medium-risk issue to make the client aware of the risk of SPNs in the domain (these strong passwords could always be changed to something weaker or a very determined attacker may be able to crack the tickets using Hashcat), but take into account the fact that I was unable to take control of any domain accounts using the attack. It is vital to make these types of distinctions in our reports and know when it's ok to lower the risk of a finding when mitigating controls (such as very strong passwords) are in place.
Kerberoasting - from Windows
Kerberoasting - Semi Manual method
Before tools such as Rubeus existed, stealing or forging Kerberos tickets was a complex, manual process. As the tactic and defenses have evolved, we can now perform Kerberoasting from Windows in multiple ways. To start down this path, we will explore the manual route and then move into more automated tooling. Let's begin with the built-in setspn binary to enumerate SPNs in the domain.
Enumerating SPNs with setspn.exe
We will notice many different SPNs returned for the various hosts in the domain. We will focus on user accounts and ignore the computer accounts returned by the tool. Next, using PowerShell, we can request TGS tickets for an account in the shell above and load them into memory. Once they are loaded into memory, we can extract them using Mimikatz. Let's try this by targeting a single user:
Targeting a Single User
Before moving on, let's break down the commands above to see what we are doing (which is essentially what is used by Rubeus when using the default Kerberoasting method):
The Add-Type cmdlet is used to add a .NET framework class to our PowerShell session, which can then be instantiated like any .NET framework object
The
-AssemblyNameparameter allows us to specify an assembly that contains types that we are interested in usingSystem.IdentityModel is a namespace that contains different classes for building security token services
We'll then use the New-Object cmdlet to create an instance of a .NET Framework object
We'll use the System.IdentityModel.Tokens namespace with the KerberosRequestorSecurityToken class to create a security token and pass the SPN name to the class to request a Kerberos TGS ticket for the target account in our current logon session
We can also choose to retrieve all tickets using the same method, but this will also pull all computer accounts, so it is not optimal.
Retrieving All Tickets Using setspn.exe
The above command combines the previous command with setspn.exe to request tickets for all accounts with SPNs set.
Now that the tickets are loaded, we can use Mimikatz to extract the ticket(s) from memory.
Extracting Tickets from Memory with Mimikatz
If we do not specify the base64 /out:true command, Mimikatz will extract the tickets and write them to .kirbi files. Depending on our position on the network and if we can easily move files to our attack host, this can be easier when we go to crack the tickets. Let's take the base64 blob retrieved above and prepare it for cracking.
Next, we can take the base64 blob and remove new lines and white spaces since the output is column wrapped, and we need it all on one line for the next step.
Preparing the Base64 Blob for Cracking
We can place the above single line of output into a file and convert it back to a .kirbi file using the base64 utility.
Placing the Output into a File as .kirbi
Next, we can use this version of the kirbi2john.py tool to extract the Kerberos ticket from the TGS file.
Extracting the Kerberos Ticket using kirbi2john.py
This will create a file called crack_file. We then must modify the file a bit to be able to use Hashcat against the hash.
Modifiying crack_file for Hashcat
Now we can check and confirm that we have a hash that can be fed to Hashcat.
Viewing the Prepared Hash
We can then run the ticket through Hashcat again and get the cleartext password database!.
Cracking the Hash with Hashcat
If we decide to skip the base64 output with Mimikatz and type mimikatz # kerberos::list /export, the .kirbi file (or files) will be written to disk. In this case, we can download the file(s) and run kirbi2john.py against them directly, skipping the base64 decoding step.
Now that we have seen the older, more manual way to perform Kerberoasting from a Windows machine and offline processing, let's look at some quicker ways. Most assessments are time-boxed, and we often need to work as quickly and efficiently as possible, so the above method will likely not be our go-to every time. That being said, it can be useful for us to have other tricks up our sleeves and methodologies in case our automated tools fail or are blocked.
Automated / Tool Based Route
Next, we'll cover two much quicker ways to perform Kerberoasting from a Windows host. First, let's use PowerView to extract the TGS tickets and convert them to Hashcat format. We can start by enumerating SPN accounts.
Using PowerView to Extract TGS Tickets
From here, we could target a specific user and retrieve the TGS ticket in Hashcat format.
Using PowerView to Target a Specific User
Finally, we can export all tickets to a CSV file for offline processing.
Exporting All Tickets to a CSV File
Viewing the Contents of the .CSV File
We can also use Rubeus from GhostPack to perform Kerberoasting even faster and easier. Rubeus provides us with a variety of options for performing Kerberoasting.
Using Rubeus
As we can see from scrolling the Rubeus help menu, the tool has a vast number of options for interacting with Kerberos, most of which are out of the scope of this module and will be covered in-depth in later modules on advanced Kerberos attacks. It is worth scrolling through the menu, familiarizing yourself with the options, and reading up on the various other possible tasks. Some options include:
Performing Kerberoasting and outputting hashes to a file
Using alternate credentials
Performing Kerberoasting combined with a pass-the-ticket attack
Performing "opsec" Kerberoasting to filter out AES-enabled accounts
Requesting tickets for accounts passwords set between a specific date range
Placing a limit on the number of tickets requested
Performing AES Kerberoasting
Viewing Rubeus's Capabilities
We can first use Rubeus to gather some stats. From the output below, we can see that there are nine Kerberoastable users, seven of which support RC4 encryption for ticket requests and two of which support AES 128/256. More on encryption types later. We also see that all nine accounts had their password set this year (2022 at the time of writing). If we saw any SPN accounts with their passwords set 5 or more years ago, they could be promising targets as they could have a weak password that was set and never changed when the organization was less mature.
Using the /stats Flag
Let's use Rubeus to request tickets for accounts with the admincount attribute set to 1. These would likely be high-value targets and worth our initial focus for offline cracking efforts with Hashcat. Be sure to specify the /nowrap flag so that the hash can be more easily copied down for offline cracking using Hashcat. Per the documentation, the ""/nowrap" flag prevents any base64 ticket blobs from being column wrapped for any function"; therefore, we won't have to worry about trimming white space or newlines before cracking with Hashcat.
Using the /nowrap Flag
A Note on Encryption Types
The below examples on encryption types are not reproducible in the module lab because the target Domain Controller is running Windows Server 2019. More on that later in the section.
Kerberoasting tools typically request RC4 encryption when performing the attack and initiating TGS-REQ requests. This is because RC4 is weaker and easier to crack offline using tools such as Hashcat than other encryption algorithms such as AES-128 and AES-256. When performing Kerberoasting in most environments, we will retrieve hashes that begin with $krb5tgs$23$*, an RC4 (type 23) encrypted ticket. Sometimes we will receive an AES-256 (type 18) encrypted hash or hash that begins with $krb5tgs$18$*. While it is possible to crack AES-128 (type 17) and AES-256 (type 18) TGS tickets using Hashcat, it will typically be significantly more time consuming than cracking an RC4 (type 23) encrypted ticket, but still possible especially if a weak password is chosen. Let's walk through an example.
Let's start by creating an SPN account named testspn and using Rubeus to Kerberoast this specific user to test this out. As we can see, we received the TGS ticket RC4 (type 23) encrypted.
Checking with PowerView, we can see that the msDS-SupportedEncryptionTypes attribute is set to 0. The chart here tells us that a decimal value of 0 means that a specific encryption type is not defined and set to the default of RC4_HMAC_MD5.
Next, let's crack this ticket using Hashcat and note how long it took. The account is set with a weak password found in the rockyou.txt wordlist for our purposes. Running this through Hashcat, we see that it took four seconds to crack on a CPU, and therefore it would crack almost instantly on a powerful GPU cracking rig and probably even on a single GPU.
Cracking the Ticket with Hashcat & rockyou.txt
Let's assume that our client has set SPN accounts to support AES 128/256 encryption.
If we check this with PowerView, we'll see that the msDS-SupportedEncryptionTypes attribute is set to 24, meaning that AES 128/256 encryption types are the only ones supported.
Checking Supported Encryption Types
Requesting a new ticket with Rubeus will show us that the account name is using AES-256 (type 18) encryption.
Requesting a New Ticket
To run this through Hashcat, we need to use hash mode 19700, which is Kerberos 5, etype 18, TGS-REP (AES256-CTS-HMAC-SHA1-96) per the handy Hashcat example_hashes table. We run the AES hash as follows and check the status, which shows it should take over 23 minutes to run through the entire rockyou.txt wordlist by typing s to see the status of the cracking job.
Running Hashcat & Checking the Status of the Cracking Job
When the hash finally cracks, we see that it took 4 minutes 36 seconds for a relatively simple password on a CPU. This would be greatly magnified with a stronger/longer password.
Viewing the Length of Time it Took to Crack
We can use Rubeus with the /tgtdeleg flag to specify that we want only RC4 encryption when requesting a new service ticket. The tool does this by specifying RC4 encryption as the only algorithm we support in the body of the TGS request. This may be a failsafe built-in to Active Directory for backward compatibility. By using this flag, we can request an RC4 (type 23) encrypted ticket that can be cracked much faster.
Using the /tgtdeleg Flag
In the above image, we can see that when supplying the /tgtdeleg flag, the tool requested an RC4 ticket even though the supported encryption types are listed as AES 128/256. This simple example shows the importance of detailed enumeration and digging deeper when performing attacks such as Kerberoasting. Here we could downgrade from AES to RC4 and cut cracking time down by over 4 minutes and 30 seconds. In a real-world engagement where we have a strong GPU password cracking rig at our disposal, this type of downgrade could result in a hash cracking in a few hours instead of a few days and could make and break our assessment.
Note: This does not work against a Windows Server 2019 Domain Controller, regardless of the domain functional level. It will always return a service ticket encrypted with the highest level of encryption supported by the target account. This being said, if we find ourselves in a domain with Domain Controllers running on Server 2016 or earlier (which is quite common), enabling AES will not partially mitigate Kerberoasting by only returning AES encrypted tickets, which are much more difficult to crack, but rather will allow an attacker to request an RC4 encrypted service ticket. In Windows Server 2019 DCs, enabling AES encryption on an SPN account will result in us receiving an AES-256 (type 18) service ticket, which is substantially more difficult (but not impossible) to crack, especially if a relatively weak dictionary password is in use.
It is possible to edit the encryption types used by Kerberos. This can be done by opening Group Policy, editing the Default Domain Policy, and choosing: Computer Configuration > Policies > Windows Settings > Security Settings > Local Policies > Security Options, then double-clicking on Network security: Configure encryption types allowed for Kerberos and selecting the desired encryption type allowed for Kerberos. Removing all other encryption types except for RC4_HMAC_MD5 would allow for the above downgrade example to occur in 2019. Removing support for AES would introduce a security flaw into AD and should likely never be done. Furthermore, removing support for RC4 regardless of the Domain Controller Windows Server version or domain functional level could have operational impacts and should be thoroughly tested before implementation.
Mitigation & Detection
An important mitigation for non-managed service accounts is to set a long and complex password or passphrase that does not appear in any word list and would take far too long to crack. However, it is recommended to use Managed Service Accounts (MSA), and Group Managed Service Accounts (gMSA), which use very complex passwords, and automatically rotate on a set interval (like machine accounts) or accounts set up with LAPS.
Kerberoasting requests Kerberos TGS tickets with RC4 encryption, which should not be the majority of Kerberos activity within a domain. When Kerberoasting is occurring in the environment, we will see an abnormal number of TGS-REQ and TGS-REP requests and responses, signaling the use of automated Kerberoasting tools. Domain controllers can be configured to log Kerberos TGS ticket requests by selecting Audit Kerberos Service Ticket Operations within Group Policy.
Doing so will generate two separate event IDs: 4769: A Kerberos service ticket was requested, and 4770: A Kerberos service ticket was renewed. 10-20 Kerberos TGS requests for a given account can be considered normal in a given environment. A large amount of 4769 event IDs from one account within a short period may indicate an attack.
Below we can see an example of a Kerberoasting attack being logged. We see many event ID 4769 being logged in succession, which appears to be anomalous behavior. Clicking into one, we can see that a Kerberos service ticket was requested by the htb-student user (attacker) for the sqldev account (target). We can also see that the ticket encryption type is 0x17, which is the hex value for 23 (DES_CBC_CRC, DES_CBC_MD5, RC4, AES 256), meaning that the requested ticket was RC4, so if the password was weak, there is a good chance that the attacker would be able to crack it and gain control of the sqldev account.
Some other remediation steps include restricting the use of the RC4 algorithm, particularly for Kerberos requests by service accounts. This must be tested to make sure nothing breaks within the environment. Furthermore, Domain Admins and other highly privileged accounts should not be used as SPN accounts (if SPN accounts must exist in the environment).
This excellent post by Sean Metcalf highlights some mitigation and detection strategies for Kerberoasting.
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