rbcd-attack: Kerberos Resource-Based Constrained Delegation Attack

by do son · August 31, 2020

Abusing Kerberos Resource-Based Constrained Delegation

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 usually defaults, 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:

Create a fake computer
Abuse msDS-AllowedToActOnBehalfOfOtherIdentity property of the target
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

Download

git clone https://github.com/tothi/rbcd-attack.git

Use

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 ev1lP@sS -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 property of the target computer.

./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 Impacket example. For help and an example call the script without options.

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 Impacket example script: