Reversing Wdigest configuration in Windows Server 2012 R2 and Windows Server 2016

Reversing wdigest configuration in Windows Server 2012 R2 and Windows Server 2016. Wdigest is an authentication protocol used in Windows. It enables the transmission of credentials across a network in MD5 format or message digest. It was designed to increase security over basic authentication initially used back in Windows Server 2003 for LDAP and Web authentication. By today’s standards, since Windows Server 2012 and Windows 8.1 Wdigest is disabled by default, the functionality was also back ported to earlier versions of windows, such as Windows 7 for you to disable via reg key. The problem with wdigest is it stores usernames and passwords in clear text. Tools such as mimikatz/wce were/are able to dump clear text passwords out from LSA  and where wdigest was used , clear text passwords would be obtained.

The question is: As this is now disabled by default can wdigest simply be enabled by attackers adding the reg key and still used to dump clear text creds?… The short answer is yes it can. See below:

The reg key to disable wdigest in earlier operating systems is: HKEY_LOCAL_MACHINE\System\CurrentControlSet\Control\SecurityProviders\WDigest creating a DWORD ‘UseLogonCredential’ setting the vlaue to 0 disables it. 1 enables it.

Using a vanilla install of Windows Server 2012 R2, dumping creds we can see no clear text password is displayed in the wdigest field when running mimikatz.

Lets add the reg key and ‘Enable’ by adding a value of 1.

I immediately retried mimikatz and had the same result as before (no clear text password), I did kind of expected this however wanted to go through the motions to test. Next a log off and back on and retest. Then reboot and retest.

So, I log off and back on first:

Oh dear.

OK so what about Windows Server 2016? Lets try:

Now for the reg key, to enable :

As we know 2012 needed a log off and back on, presumably to cache the credentials, lets assume the same is needed with 2016.

Oh dear oh dear.

Ok so what can we do to prevent this. There are several methods such as, protecting the LSA and LSASS process in Windows 8.1 and Server 2012 R2 which I have talked about this here. Enabling Credential Guard, new for Windows 10 and Server 2016. Protected Users in AD. From here reducing the credential foot print of users on machines, ie blocking users from rdp’ing into other machines, disable Domain Admin’s from logging in to workstations, user a ‘Server Admins’ group instead. Having dedicated user privilege separation. Monitor for the above reg key changes. All the time we are trying to reduce the ability for an attacker to escalate privileges by dumping credentials, then moving on laterally through the network, reducing their ability to gain access to higher privilege accounts.

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Protected Users Group in Microsoft Active Directory

In this post we will be discussing the Protected Users group in Microsoft Active Directory. This is to try and raise awareness of this security feature built into Microsoft Windows Active Directory (AD) 2012 R2 Domain environments. The Protected Users group in AD gives its members additional security features and protection when logging into Windows Server 2012 R2, Windows 8.1 and above.

If an account is made a member of the group. The user account can only authenticate using the Kerberos protocol. We will demystify this shortly with an example, however in brief it means that this user will not be able to authenticate with NTLM, WDigest Authentication, or CredSSP. Which means using one of these Security Support Providers (SSP) will fail. This is a win,  as we know the NTLM hash is obtainable and is very crackable, Digest Auth stores credentials in clear text in memory, CredSSP also stores clear text credentials however is an interesting one as this may break applications that use it. However you might argue as you would probably only use this for high privilege accounts, (as Microsoft advises here) it might not be so much of an issue, and would probably be re-evaluated on a case by case basis after testing.

In addition to the above, the account will not use DES or RC4 in the pre-authentication stages. The domain therefore must support AES.

There are a couple of caveats that you should be aware (you can read about these here) of when using this as an option to help mitigate credential theft:

This will only take affect on Windows 8.1 and Windows Server 2012 R2, so no change for Windows 7 and Server 2008 I am afraid. So when logging into Windows 7 or Server 2008 variants your don’t get the protection from Protected Users membership.

Windows Server 2012 R2 Domain Functional Level is required.

Managed Service Accounts (MSAs) and group Managed Service Accounts (gMSAs) as well as computer objects can not be added to the group. (technically they can, however it will break things).

What does all this mean then in actual protection terms, what does it look like?

I feel an example coming on:

Lets test with Windows Server 2012 R2. If we dump creds on a vanilla install with mimikatz this is typically what we would see. So no wdigest as this is disabled by default however what we can see and retrieve is the NTLM hash for the account ‘Adam2’, which as we know can be cracked off-line.

Now if we add our account to the Protected Users group and test, lets see what happens. Lets log off and back on and re-run mimikatz:

No NTLM hash, this is because members of the protected users group will only authenticate with Kerberos and not with NTLM. Great. This seems like a win for high privilege accounts. More to come on this soon.

 

 

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Avoiding AV detection when running mimikatz with sed!

In this post I will be talking about avoiding AV detection when running mimikatz with sed! I came across this on the BlackHills Information Security Website, link here. Props to Carrie Roberts for sharing this. The classic Invoke-Mimikatz.ps1 from the PowerSploit suite located here, does get detected by many Anti-Virus vendors. This really is a great for the Enterprise. However whats not so great is the way in which AV vendors are detecting it and how it can be easily bypassed! Yes AV can easily be bypassed by modifying the powershell file. Using ‘sed’ in bash we can swap out various text in the ps1 file. For example swapping out mimikatz for mimidogz as in line 1 below.

I have talked about slowing down attackers who are using mimikatz in this post. This is where we can deny access to the clear text credentials in early versions of Windows (up to windows 7). In later versions of windows (8 and above) we can deny access to the hash and the clear text credentials. This will only slow attackers down though as the OS can be modified, however this will make noise on the network. This is really is a must for slowing down attackers. Harden your systems!

Below are the sed commands:

 

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Stopping mimikatz from dumping clear text credentials.

This post aims to demystify what can be done to restrict mimikatz from running successfully in Windows. Specifically how we can mitigate clear text credentials being dumped out of memory. My aim is to demonstrate what can be done to the operating system configuration to help reduce this from happening. In a follow up post perhaps we will look into how we might be able to identify if this has or is happening to a machine on the network.

Once a user logs on to a windows system the credentials are stored in the Local Security Authority Subsystem Service (LSASS). Once creds are cached, from here, with various tools, not just mimikatz, credentials in the form of clear text and hashed passwords can be extracted.

Mimikatz for a pen tester is a really great tool, like wise also unfortunately for hackers. Created by Benjamin Delphy ‘gentilkiwi’ allows one to dump clear text credentials out of memory. It can do all sorts of other pretty cool things like perform pass-the-hash, pass-the-ticket or build Golden tickets, among others. Some of which I may walk through in the future.

Mimikatz can be executed from different sources. Either directly from an executable and also from powershell. The later being the more desirable as no files are ever touching the hard disk drive. A typical screen shot of successful dumping would look like this, this is what we are trying to avoid:

Here you can see the local account ‘Adam’ and the clear text password being displayed. This is clearly bad news for organizations, and an IT teams worst nightmare. An adversary would have to initially compromise a workstation or server and gain admin or SYSTEM level privileges. This might be through a classic remote code execution vulnerability like MS08-069. Or through some other weakness in the software, or mis-configuration in the operating system. An attacker from here on would move laterally from system to system dumping creds. Ultimately with the aim of snagging a Domain Admin, or capturing that key piece of business data.

This post isn’t necessarily concerned with showing you how to use mimikatz, rather it’s, to better inform IT teams how they can prevent clear text credentials being dumped. Any tester worth their weight will already know how to use it.

I’m going to test by running  mimikatz natively on a couple of Windows operating systems in my test environment, make changes to the system then re-run.

I’ll run through executing Mimikatz and the preventative measures we can use to stop the software interacting with LSASS and dumping creds. Lets start with Windows Server 2012 R2.

Windows Server 2012 R2

When we run Mimikatz on a fully up to date system we get the following results:

Restricting Mimikatz

Note where the ‘Password’ field is for wdigest and kerberos , this would normally reveal the clear text password however in this case a ‘(null)’ value is displayed. This is default behavior in Windows Server 2012 R2 and Windows 8.1 and above, sadly for older operating systems such as Windows 7 the credentials are in displayed here in clear text. The NTLM hashes are still present here which still isn’t great and so can potentially be used in ‘pass the hash’ type attacks (PTH on the rid 500 account) or cracked offline using large password lists. Lets see what we can do to harden our systems further.

With Windows Server 2012 R2 and 8.1 there is a new feature called LSA (Local Security Authority) Protection according to the following Microsoft page we are able to run LSASS as a protected process. This essentially prevents mimikatz from working correctly out of the box. Although with some tweaking you can get mimikatz to run however a driver would need to added into the mix for it to work properly. The following registry change can be made at HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Lsa adding a new DWORD entry of “RunAsPPL”  with a value of 1. This does require a reboot however. Lets see this in action on the same Windows Server 2012 R2 Server. First the registry key:

Restricting Mimikatz WDigest

Lets reboot, and now lets see what happens when Mimikatz is executed:

Restricting mimikatz WDigest

Thats more like it! Now we are getting somewhere. This is only going to slow attackers down. It is however also going to generate noise on the network when the attacker reboots the server.

Windows 7 Professional

A patch was released by Microsoft which allowed you to control the clear text credentials from the wdigest for earlier systems. This same functionality can be utilized by tweaking the registry for earlier versions of Windows. This is available for Windows 7,8,2008 and is kb2871997. Once this patch is installed it allows you control how WDigest credentials are stored in memory. According to the following Microsoft Security Advisor;  to prevent WDigest credentials from being stored in memory. A reg key can be set via Group policy and thus applied to your estate. The registry key is new DWord with the name UseLogonCredential under the following subkey:

HKEY_LOCAL_MACHINE\System\CurrentControlSet\Control\SecurityProviders\WDigest

If the UseLogonCredential value is set to 0, WDigest will not store credentials in memory.

If the UseLogonCredential value is set to 1, WDigest will store credentials in memory.

Lets see how this works in Windows 7, first we will run Mimikatz to verify default behavior then we will add the WDigest registry key, reboot, test, then add the RunAsPPL registry key, reboot one last time and test again, lets do this!:

As with the standard behavior below we get clear text creds and hashes:

Restricting Mimikatz WDigest

Now lets add the HKEY_LOCAL_MACHINE\System\CurrentControlSet\Control\SecurityProviders\WDigest\ UseLogonCredential and set it 0:

Restricting Mimikatz WDigest

Lets reboot, and retry Mimikatz.

As we can see the password fields no longer contain our clear text password, and have the (null) value instead. This is a significantly better position to be in, this still does however, reveal the NTLM hash which is still not ideal.  Unfortunately the RunAsPPL registry key, cannot be utilized in Windows 7.

Windows Server 2008 R2

Lets try the same with Windows Server 2008 R2. When we run mimikatz natively this is the result:

As we can see the local administrators account password for the wdigest field is in displayed in clear text. Now lets see if we can apply the same  HKEY_LOCAL_MACHINE\System\CurrentControlSet\Control\SecurityProviders\WDigest\ UseLogonCredential registry key and at least mitigate the clear text password. First the registry key then a reboot.

After a reboot we rerun mimikatz and as we can see from the below screenshot we have successfully mitigated from the at least the clear text password being displayed, although the hash is still available:

However… this wouldn’t stop an attacker from changing the reg key back logging off and waiting for credentials to be cached.

Conclusion:

So far we have tested using mimikatz natively from the windows server 2012 R2 and successfully stopped mimikatz from executing the same should apply here for Windows 8.1 and above, however you should carry out your own due diligence and try this out to satisfy yourself. Windows 7 and Windows Server 2008 are slightly different and we can only go so far in stopping or slowing down this type of attack, to not displaying the clear text password only the hash.

Unfortunately with different versions of Windows there are different fixes and varying degrees of success in reducing the impact of mimikatz being run on a system.

As you will agree though this is significantly better than allowing the default configuration to remain. I hope this helps slow down your adversaries!

 

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