Dirk-jan Mollema

Dirk-jan Mollema

Hacker, red teamer, researcher. Likes to write infosec-focussed Python tools. This is my personal blog containing research on topics I find interesting, such as (Azure) Active Directory internals, protocols and vulnerabilities.

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Recent Posts

The worst of both worlds: Combining NTLM Relaying and Kerberos delegation

5 minute read

After my in-depth post last month about unconstrained delegation, this post will discuss a different type of Kerberos delegation: resource-based constrained delegation. The content in this post is based on Elad Shamir’s Kerberos research and combined with my own NTLM research to present an attack that can get code execution as SYSTEM on any Windows computer in Active Directory without any credentials, if you are in the same network segment. This is another example of insecure Active Directory default abuse, and not any kind of new exploit.

“Relaying” Kerberos - Having fun with unconstrained delegation

27 minute read

There have been some interesting new developments recently to abuse Kerberos in Active Directory, and after my dive into Kerberos across trusts a few months ago, this post is about a relatively unknown (from attackers perspective), but dangerous feature: unconstrained Kerberos delegation. During the writing of this blog, this became quite a bit more relevant with the discovery of some interesting RPC calls that can get Domain Controllers to authenticate to you, which even allow for compromise across forest boundaries. Then there was the discovery of PrivExchange which can make Exchange authenticate in a similar way. Because tooling for unconstrained delegation abuse is quite scarce, I wrote a new toolkit, krbrelayx, which can abuse unconstrained delegation and get Ticket Granting Tickets (TGTs) from users connecting to your host. In this blog we will dive deeper into unconstrained delegation abuse and into some more advanced attacks that are possible with the...

Abusing Exchange: One API call away from Domain Admin

11 minute read

In most organisations using Active Directory and Exchange, Exchange servers have such high privileges that being an Administrator on an Exchange server is enough to escalate to Domain Admin. Recently I came across a blog from the ZDI, in which they detail a way to let Exchange authenticate to attackers using NTLM over HTTP. This can be combined with an NTLM relay attack to escalate from any user with a mailbox to Domain Admin in probably 90% of the organisations I’ve seen that use Exchange. This attack is possible by default and while no patches are available at the point of writing, there are mitigations that can be applied to prevent this privilege escalation. This blog details the attack, some of the more technical details and mitigations, as well as releasing a proof-of-concept tool for this attack which I’ve dubbed “PrivExchange”. Update: Patches for PrivExchange are available, see the section...

Active Directory forest trusts part 1 - How does SID filtering work?

16 minute read

This is the first post in a series on cross-forest Active Directory trusts. It will explain what exactly Forest trusts are and how they are protected with SID filtering. If you’re new to Active Directory trusts, I recommend you start by reading harmj0y’s in-depth guide about them. After reading his (excellent) post I had lots of questions about how this actually works under the hood and how trusts within the same AD forest compare with trusts between different forest. This series of blogs is both my journey and my documentation on how I researched this topic and how I understand it now. Get ready for a deep dive into trusts, Kerberos, golden tickets, mimikatz and impacket!