If you ever wondered why when applying permissions on Windows Enviorment takes time to apply to the user the response is : the Kerberos ticket

The trick is simple just running the following command you should refresh the permissions without having to reboot

Open cmd.exe

run Klist purge

Now I would like to go through the Windows Actual AUTH processes and some security concerns and mitigations you should consider

Beware at the end of the article I have attached an official Microsoft PDF to mitigate pass the hash attacks

About Kerberos and NTLM

We can see some insights here on the Microsoft page and all together I’ve pulled out some info for you

https://docs.microsoft.com/en-us/windows-server/security/kerberos/kerberos-authentication-overview

The Kerberos ticket is used wide in Windows system domains , it had on the past questionable security due using DES ( data encryption standard )

Security concerns are still in the protocol as some protocols have still not been updated to other cyphers like AES

Things to consider for example in old enviorment , NTLM will always be used when a computer is not into the domain . ( ntl lan manager ) . This protocol was replaced with Kerberos in the year 2000 . NTLM was in part written by IBM and Microsoft.

Attacks Kerberos

Pass the ticket ( gaining local admin in the machine , would allow to use the ticket AUTH instead of password )

Mitigations for common Kerberos attacks

Use LAPS for local admin management

Use Tiered Adminstration

Do not allow Local admin cached logins

0 Trust Port configs in Servers –

Do not allow Domain admins to login remotely in servers , use tiered configs as a local admin for the server. If possible restrain in servers to allow Domain Admins to run commands over the network . Beware this configuration cannot always be applied. Specially in Remote desktop enviorment where the license server must be added to the group “Terminal License Servers”

Completely block domain controllers in a separated network where only with a trusted VM you can remote ( jump server ). If possible only accessible through a VPN with 2mfa .

Use AppLocker , and good antivirus defences in jump server. Shut down the servers when not being used during the day

Monitor any activity or changes in the Tiered accounts or Domain admin accounts ( use ATP , or a third party exploit anaylizer)

Do not allow the network to be opened in Wi-Fi enviorment if possible . Use always VPN

Implement 2mfa tokens for legacy authentications

Perform regular updates in clients and servers

Use Core isolation ( Windows 10 ) . Defender Guard policies

Migrate to Azure AD and if possible be a serverless company and use software as service in the cloud

Try to get away from Remote Desktop Legacy enviorment opened to the internet

If using azure Consider using Azure Bastion to remote in your servers

Update your systems regularly

Block internet outbound /Inbound when possible ( in every server )

Stay updated for security news

Limit your domain admins as much as possible

Implement access on demand

Bastion forest ( not always possible )

Change passwords regularly .

Control local admin users ( specially in servers )

Considerations of NTLM

Weak Cryptography ( hash md4)

When a client sends AUTH , there is not an implementation of a way to validate the identity of the server, wich opens up attacks of man in the middle

No multifactor ath

NTLM2 ( updated version ) has mitigated some securiy flows but as for today it is still a security issue

Attacks than can be used against NTLM

Pass the hash

Man in the middle ( relay attacks)

Mitigations against NTLM attacks

If possible you should get rid of NTLM protocols .

If that option is not on the table, make sure you update your operating systems regularly

Monitor NTLM usage ( either with scripts or third party solutions )

Enforce LDAP /SMB SINGING : https://docs.microsoft.com/en-us/troubleshoot/windows-server/identity/enable-ldap-signing-in-windows-server

Kerberos authentication flow

NTLM