AS-REP roasting detection

Welcome to part two of a special series on detecting Active Directory attacks & misconfigurations. Each blog post dives deep into identifying, detecting, and mitigating a dangerous AD vulnerability. 

And the best part? 

You’ll get a free AD-focused Sherlock to practice the defensive techniques you learn!

The attack methods and misconfigurations we cover will include:

• Kerberoasting (part one)

• AS-REP Roasting (part two)

• LLMNR poisoning (part three)

• NTLM relay (part four)

• NTDS dumping (part five) 

Let’s get started! 

When authentication occurs within Kerberos, the first thing that happens is an authentication request to the domain controller so the identity trying to authenticate can be verified. 

That request is known as Authentication Server Request (AS-REQ.) 

This process is commonly referred to as Kerberos preauthentication.

After the client's authentication is validated, the domain controller sends an Authentication Server Reply (AS-REP) to the client, containing a session key and a Ticket Granting Ticket (TGT).

The session key is encrypted using the user’s password hash so that only that user can decrypt and reuse it.

Now check this out! Users within the domain can be configured to skip the preauthentication process, and that means attackers can send the AS-REQ to the domain controller on behalf of any user configured that way. (Since the AS-REP contains the session keys encrypted with the user's password hash.)

As a result, since the AS-REP contains the session keys encrypted with the user's password hash, they can obtain the password hash of any user. 

(Tools like GetNPUsers from the Impacket tool suite can simplify this process.)

Attackers can then try to brute force passwords to decrypt the session key. If the key decrypts, that means the attacker successfully guessed it and now has the user’s password.

People in information security will often refer to this encrypted session key as a hash, but it’s really not a hash at all. Still, tools like Hashcat and John the Ripper can brute force many passwords against this “hash” in a short time to try to recover the user’s plaintext password.

By default, the AD User Account Control (UAC) setting: “Do not require Kerberos preauthentication” is disabled. This means that Kerberos preauthentication is performed for all users.

But here's the kicker: This account option can be enabled manually and is seen from time to time during real-world engagements.

I recently supervised an internal networking penetration test where all user accounts had the "do not require Kerberos preauthentication" option enabled. The reason for this, as later stated by the client, was due to an internal application malfunction.

 

The administrators decided to enable this option "momentarily" while they fixed the issue. However, what was supposed to be temporary ended up lasting almost 2 years.

 

It's worth noting this particular misconfiguration was only identifiable due to this particular penetration test.

Detecting AS-REP Roasting

We'll cover how you can detect AS-REP Roasting activity using domain controller logs. 

Spotting this type of attack is easier than Kerberoasting attack detection. However, it’s still complex because you need knowledge of AD and event logs to properly filter down to malicious activity.

As we mentioned in part one of this series, regular AD operations in corporate environments make it harder to detect malicious activity because there are thousands of Kerberos events going on per minute. However, if we know what to look for, we can still find this needle in the haystack. 

Filter logs by Event ID 4768 

Event ID 4768 is an event ID recorded in Security Logs on the domain controller whenever a Kerberos Authentication ticket is requested. 

Depending on the Active directory size and assets, this can be well over thousands of tickets per minute by different accounts in the network.

Let's view one of the many events to understand this event’s fields.

• Account Name: The user account that requested an authentication ticket from the domain controller.

• Service Name: Name of the service that handled the ticket.

• Ticket Encryption type: Depicts the Ticket encryption algorithm used (For example aes, RC4, etc). 

• Pre-Authentication Type: The status code shows whether pre-authentication was disabled or enabled for the said object (The Account Name).

We can see that the administrator user requested an authentication ticket and the service name is krbtgt. This is regular operations and whenever an account logs in to a workstation, krbtgt is a universal AD service that handles Kerberos authentication.

Now let's discuss a few of the filters or conditions that would indicate a possible attack. 

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. 

Note💡: All major open-source tools, like Impacket and Rubeus, request tickets in RC4 encryption type. 

User accounts request authentication tickets from domain controllers all the time; that’s the nature of how Active Directory Kerberos authentication works. 

To further reduce the events to investigate, we can filter out requests from all service names other than “krbtgt”. 

This is because during this attack, the attacker retrieves the authentication ticket just like a legitimate user account would, and krbtgt is a default AD Service that handles the authentication flow in Active Directory.

The major indicator that the AS-REP attack has been successful (the attacker managed to get the ticket, whether they cracked it or not is another case) is the pre-authentication type value in the resultant logs. 

Note:💡A great way to threat hunt for this attack is to just look for pre-authentication type = 0, which means it is disabled. This would already remove 90 percent of the noise in the logs, leaving more granular results to go through.

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 4768 event where:

1. Pre-Authentication Type is 0, which means it is disabled. This is a major condition to be fulfilled as without this condition, the attack can’t happen.

2. Service Name should always be krbtgt. This is also straightforward. As only krbtgt can perform authentication-related processes in AD.

3. Ticket encryption type will be 0x17 which is RC4 encryption, allowing attackers to easily crack the hash.

Here’s an example of identifying an actual event that was the result of a AS-REP  attack using the detection tips above:

This event fulfills all the conditions we set which would highly indicate AS-REP Roasting activity. We can see that a domain Account “arthur.kyle” requested an authentication ticket for a user whose pre-authentication is disabled, with an encryption type of 0x17 from a workstation with IP Address 172.17.79.129.

Correlating events to detect a compromised account 

So far we found out that the user arthur.kyle got compromised due to pre-authentication being disabled. 

What we don't know is which user account was used to perform the attack. 

It's important to note that while the “arthur.kyle” account is the victim here, the bad actor used another account to perform the attack. 

We need to find that account, too, because it’s also been compromised! And our single AS-REP incident may expand into an incident with a wider scope as we keep more compromised assets.

We have the machine’s IP address from which the request originated. We will look for Kerberos service ticket events, as every domain user account requests those either during login/authentication or normal domain usage. 

Filter for Event ID 4769 and look for events around the time of the malicious event. 

We spotted an event about a minute later after the malicious event and it originated from User account of the “172.17.79.129” machine.

Now nothing in this event is malicious by itself. It’s purely a regular operation and is not a result of any attack or exploit. 

But since we already found AS-REP activity in the previous section, and we know it originated from this machine, this event caught our eye. 

Here we can see that happy.grunwald was the user account logged in around the time of AS-REP Roasting attack on the source machine (machine that performed the attack). 

It can be safe to assume now that this user account is compromised hence expanding the scope of the incident.

💡Note: you can AS-REP Roast with just a user list (i.e., if you gather it from an SMB NULL SESSION). But if you're just running a tool like GetNPUsers.py or Rubeus you need a valid user account to query the user list (which all happens in the background when you run the attack).

That means this attack can be executed without any authentication if an attacker has a user list through some means (null session, an SQLi on an AD login form, successful username enumeration using Kerbrute, etc.

Detecting AS-REP roasting with Splunk

Let's see a Splunk query for this as well

Query : Event.EventData.TicketEncryptionType="0x17" Event.System.EventID="4768" Event.EventData.PreAuthType="0" Event.EventData.ServiceName="krbtgt"

| table Event.EventData.TargetUserName,Event.EventData.IpAddress

(Please note that in Splunk query, your field name might differ a little as it depends on the configurations. For example instead of  Event.System.EventID it can be “EventID” or “Windows.EventID”. It all depends on your Splunk configuration and deployment.)

In the above query, we are hunting for Event Log 4768 where the encryption type is 0x17, the authentication type is 0 (it's disabled), and the service name is krbtgt. So this fulfills our criteria. 

Note💡: AS-REP Roasting is mapped to the sub-technique T1558.004 on the MITRE ATT&CK framework

The follow-up to this detection would be to:

1. Create a timeline of when this event was generated.

2. Do a forensic analysis of the machine with IP Address 172.17.79.129, and find out how the “happy.grunwald” user account got compromised. 

3. 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 AS-REP activity was noticed.

Remediation

To prevent AS-REP Roasting attacks, it is crucial to start by identifying all user accounts that do not require Kerberos pre-authentication. If not requiring pre-authentication is not necessary, ensure that pre-authentication is enabled for every account.

Always implement a robust password policy with long, complex passwords that are changed regularly. If possible, enhance security by enabling 2FA on authenticated services.