Friday, August 11, 2017

Week of Evading Microsoft ATA - Day 4 - Silver ticket, Kerberoast and SQL Servers

This is Day 4 of Week of Evading Microsoft ATA. The week has been split in the following days:
Day 4 - Bypasses/avoidance by reducing conversation with the DC
Day 5 - Attacking ATA deployment, limitations of research and mitigation

Day 4 is dedicated to those attacks which need minimum and normal communication with the domain controller and thus ATA. That is, after bypassing ATA on first three days, today we will discuss how ATA can be avoided.

Silver Ticket

If get our hands on NTLM hash of a service account, it is possible to create a TGS (Ticket Granting Service) and present it to the service to get access. As we would be creating a TGS, there is no communication with DC. Read more about silver ticket attack in this post. Now, if we are not talking to the DC, ATA can't read the traffic and thus, there is no detection. As simple as that!

What intrigued me during testing was even if a silver ticket is used for a service running on the DC, ATA still won't detect it.  We can use the NTLM/RC4 hash of the DC's machine account for some very interesting services like CIFS, WMI, PowerShell Remoting. LDAP etc. Since we already have DA access on Day 2, the ability to create a silver ticket for the DC is a decent persistence mechanism.

Why a silver ticket is not detected even for the DC? I think ATA is currently interested only in authentication requests.


Kerberoast attack involves requesting a TGS from the DC for a service, save the TGS (which is encrypted using the NTLM hash of the target service account) to a file and brute-force the NTLM hash. Read this and this article for more information.

The only communication with the DC is when the TGS is requested from the DC. The DC gets TGS requests all day and spotting an anomaly in such a regular request is, well, not easy.

Actually, there is a chance for detection based on anomaly. Remember the encryption type we discussed on Day 3? A TGS request with encryption type 0x17 (RC4) may be used as an indicator of a ticket request for Kerberoasting. How? Let's use the below code to request a TGS for a service:
This is how it looks like in logs:
But there are many legit uses (legacy applications, service accounts, trusts etc.) which still use RC4, we can only hope that ATA starts detecting TGS requests using RC4 as malicious in a future release for those environments where AES is prevalent.

Kerberoast Variants

There are two Kerberoast variants which are helpful in active directory dominance.

Request a TGT for an account with Kerberos Pre-Authentication disabled (or force disable if we have sufficient rights), DC replies with TGT (AS-REP) which has a piece of information (emcrypted part) encrypted with the account's NTLM hash, save it to the disk and brute force offline. Please read this and this post for a detailed explanation.

ATA doesn't detect this. Is there any anomaly which can be seen? Yes!

After enumerating users with Pre-Auth disabled, send AS-REQ and receive AS-REP with encrypted part:
And this is how it looks like in Wireshark:
And this is how it is logged:
Hello RC4 we met again :)

As Will explains in this post, cracking AS-REP encrypted part which uses RC4 is much easier then the AES one so chances of an attacker using AES are not common. An anomaly, if ATA would like to spot it :)

Force SPN
If we have enough privileges, it is possible to set a user's SPN to anything, request a TGS for that made up SPN and then, brute-force the ticket offline.

Like in other TGS related attacks, a simple anomaly in this case could be use of RC4:
ATA doesn't detect it. ATA team commented that abnormal behaviour detection will catch this and AS-REP method when the brute-forced passwords are used to access a resource.

SQL Server

Targeting SQL servers and staying within the database server makes sure that there is no communication with the DC and therefore, avoids ATA. I wrote a post couple of months back on lateral movement within the database layer.

That is all for Day 4. Hope you enjoyed it!

1 comment:

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