Feb 12 2019

Mitigating Exchange Permission Paths to Domain Admins in Active Directory

This article is a cross-post from TrimarcSecurity.com
Original article: https://www.trimarcsecurity.com/single-post/2019/02/12/Mitigating-Exchange-Permission-Paths-to-Domain-Admins-in-Active-Directory

The Issue 
Recently a blog post was published by Dirk-jan Mollema titled “Abusing Exchange: One API call away from Domain Admin ” (https://dirkjanm.io/abusing-exchange-one-api-call-away-from-domain-admin/)which highlighted several issues with Exchange permissions and a chained attack which would likely result in a regular user with a mailbox being able to become a Domain Admin in the AD forest. Tools have been released to take advantage of this issue.

He highlights the key components of the issue in the blog post introduction:

This blog combines a few known vulnerabilities and known protocol weaknesses into a new attack. There are 3 components which are combined to escalate from any user with a mailbox to Domain Admin access:

  * Exchange Servers have (too) high privileges by default
  * NTLM authentication is vulnerable to relay attacks
  * Exchange has a feature which makes it authenticate to an attacker with the computer account of the Exchange server
…
The main vulnerability here is that Exchange has high privileges in the Active Directory domain. The Exchange Windows Permissions group has WriteDacl access on the Domain object in Active Directory, which enables any member of this group to modify the domain privileges, among which is the privilege to perform DCSync operations. Users or computers with this privilege can perform synchronization operations that are normally used by Domain Controllers to replicate, which allows attackers to synchronize all the hashed passwords of users in the Active Directory.

Exchange Permissions in Active Directory 

At the Microsoft Blue Hat in 2017, Sean Metcalf, Trimarc founder and Active Directory Subject Matter Expert (SME) highlighted issues with Exchange permissions. Some slides from this presentation are shown here as representative samples (the full presentation slide deck is in the Presentations section.

More information about this issue has been highlighted in presentations by Andy Robbins (@_wald0) and Will (@Harmj0y), including at Black Hat USA 2017.

The Bloodhound tool written by Andy Robbins, Rohan Vazarkar, and Will can identify attack paths involving Exchange permissions configured in Active Directory.

Microsoft recently published an article (https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/ADV190007) on how to configure EWS throttling which will mitigate the issue(s) that Dirk-jan Mollema raised. The issue with this is that EWS throttling could negatively impact applications like Outlook for Mac and doesn’t resolve the escalation path with Exchange permissions configured in AD.

Trimarc has discovered Exchange having elevated permissions in AD in many customer environments while performing Active Directory Security Assessments (ADSAs).

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Oct 11 2018

From DNSAdmins to Domain Admin, When DNSAdmins is More than Just DNS Administration

It’s been almost 1.5 years since the Medium post by Shay Ber was published that explained how to execute a DLL as SYSTEM on a Domain Controller provided the account is a member of DNSAdmins.
I finally got around to posting here since many I speak with aren’t aware of this issue.

Shay describes this issue as follows (bolded text addedby me):

In addition to implementing their own DNS server, Microsoft has also implemented their own management protocol for that server, to allow for easy management and integration with Active Directory domains. By default, domain controllers are also DNS servers; DNS servers need to be reachable and usable by mostly every domain user. This, in turn, exposes quite some attack surface on domain controllers — on one part, the DNS protocol itself and on the other, the management protocol, which is based on RPC.

We will shallowly delve into the protocol’s implementation and detail a cute feature (certainly not a bug!) which allows us, under some circumstances, to run code as SYSTEM on domain controllers, without being a domain admin. Although this is certainly not a security vulnerability (so no panic is needed), as confirmed with Microsoft, it’s still a cute trick which can be useful as an AD privilege escalation in red team engagements.

So, how is this possible?
I will summarize Shay’s excellent technical review of this issue (this assumes DNS runs on Domain Controllers, which is the most common configuration).

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Oct 10 2018

Domain Controller Print Server + Unconstrained Kerberos Delegation = Pwned Active Directory Forest

At DerbyCon 8 (2018) over the weekend Will Schroeder (@Harmj0y), Lee Christensen (@Tifkin_), & Matt Nelson (@enigma0x3), spoke about the unintended risks of trusting AD. They cover a number of interesting persistence and privilege escalation methods, though one in particular caught my eye.
Overview
Lee figured out and presents a scenario where there’s an account with unconstrained delegation configured (which is fairly common) and the Print Spooler service running on a computer, you can get that computers credentials sent to the system with unconstrained delegation as a user.

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Aug 12 2018

Black Hat & DEF CON Presentation Slides Posted

I just uploaded the slides from my Black Hat & DEF CON talks from the past week in Vegas.  They are a bit different with the BH talk more Blue (defensive) and the DC talk mostly Red (Offensive) in focus. Also note that the only real overlap in content is the MFA & password vault sections and those were updated in my DEF CON talk to focus on the attack aspect.

An important note: The methods I show are real and work well in many real-world customer deployments. The issues with MFA and password vaults I highlight are often deployment issues and not necessarily vendor best practices. With that noted, I have seen enterprise password vaults deployed with poor security so often that I don’t think customers are very familiar with the vendor security best practices.

Slides are in the Presentations section.

Black Hat USA 2018 Talk:  “From Workstation to Domain Admin: Why Secure Administration isn’t Secure and How to Fix it”


This talk walks the audience through how AD administration has evolved over time with newer, more “secure” methods and the potential ways to exploit modern AD administration. I explore some methods to exploit current implementation weaknesses in many deployments of multi-factor authentication (MFA) and enterprise password vaults. The latter third of the talk dives into the best defenses and how to employ and deploy them appropriately.
[Slides]

Black Hat Talk Agenda:

  • Current State
  • Evolution of Administration
  • Exploiting Typical Administration
  • Common Methods of Protecting Admins (& bypassing them)
  • MFA
  • Enterprise Password Vaults
  • Admin Forest
  • Building the Best Defenses

DEF CON 26 Talk: “Exploiting Active Directory Administrator Insecurities”


This talk repeats the slide concepts from my Black Hat talk specific to exploiting current implementation weaknesses in many deployments of multi-factor authentication (MFA) and enterprise password vaults.  The talk adds in some challenges in properly discovering AD admins and some additional methods of exploiting current AD environments. I also cover how in many environments it may be possible to compromise a Read-Only Domain Controller to compromise the AD forest. This talk also includes a special, new sneaky AD persistence method which only the DEF CON audience was privy to (not in the slides, at least not directly). I will post a blog article as time allows. 🙂
[Slides]

DEF CON Talk Agenda:

  • Evolution of Admin Discovery
  • Exploiting Typical Administration
  • Multi-Factor Authentication (MFA)
  • Password Vaults
  • Admin Forest
  • Attacking RODCs

Thank you all for your support and your kind words!
– Sean

May 20 2018

NolaCon (2018) Active Directory Security Talk Slides Posted

I recently presented my talk  “Active Directory Security: The Journey” at Nolacon in New Orleans, LA.
Slides are now posted here.

On Sunday, May 19th, 2018, I spoke at NolaCon at 11am.

Here’s the talk description:

Active Directory is only the beginning.
Attackers have set their sights squarely on Active Directory when targeting a company, though this typically isn’t the primary objective. The motivation and end goals range from stealing data to impacting corporate operations. In this regard, gaining control of Active Directory is a means to an end; compromising Active Directory is an easy way to gain access to all critical corporate resources. Effectively protecting Active Directory has become critical in limiting the impact of a breach.

This talk takes the audience on a journey covering the various security milestones and challenges with Active Directory. A variety of  AD security posture are highlighted along with the challenges they encounter with securing their systems. Key elements involve how enterprise “”AD aware”” applications can weaken Active Directory security and how leveraging cloud services complicate securing infrastructure. Also explored is what an attacker can do in an environment without having Domain Admin rights. 

This talk covers the critical issues affecting organizations today, as well as the biggest challenges; current attack techniques; and the most effective defensive techniques to prevent and mitigate compromise (including limitations to these approaches).

For the curious, here’s an outline of the talk:

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Jan 01 2018

Attacking Read-Only Domain Controllers (RODCs) to Own Active Directory

I have been fascinated with Read-Only Domain Controllers (RODCs) since RODC was released as a new DC promotion option with Windows Server 2008. Microsoft customers wanted a DC that wasn’t really a DC. – something that could be deployed in a location that’s not physically secure and still be able to authenticate users.

This post covers a few different scenarios on how to attack  Read-Only Domain Controllers in order to escalate privilege. Since RODCs are typically untrusted and viewed as not having the same level of access as writable DCs, it’s possible in many environments to compromise a RODC  to escalate privileges. Given certain scenarios, it’s possible to escalate from a Read-Only Domain Controller to a full writable Domain Controller.  This post covers these scenarios and enables Red and Blue teams to better understand and check RODC configurations for issues.

The information in this post is not from any one customer environment I have seen, but a combination of several. I have found that many AD domains that have RODCs are configured very similarly: many more accounts, both user and computer, have passwords cached on RODCs than is necessary and the ability to manage RODCs is not limited or secured appropriately. This post shows what is possible given common real world RODC deployment configuration. As part of our Active Directory security review services, we scrutinize RODC configuration and identify potential issues with the configuration. Furthermore, we find that when RODCs are deployed in an environment, they are frequently configured  with weak security settings (as noted in “RODCs in the Real World” and “Attacking RODCs” below).

The information here describes what is possible in many Active Directory environments with Read-Only Domain Controllers and doesn’t highlight a misconfiguration, but common configuration issues that could be exploited to escalate privileges in the domain since the RODC is often treated as “just another server” (or worse, as a workstation). Accounts are regularly cached on RODCs (since RODCs that don’t cache passwords aren’t very useful) and once an attacker gains access to it, these passwords are available and may include delegated Active Directory admin accounts which could be compromised.

If you want to simply know how best to “harden Read-Only Domain Controllers”, skip to the end to read the “Securing RODCs Against Attack” section.

Note that throughout this post, I use the Microsoft Active Directory PowerShell cmdlets and some of the attribute names are adjusted in the output from what they are actually named in AD.

 

Enter the Read-Only Domain Controller 

When Microsoft released Windows Server 2008, a new type of Domain Controller was added called the “Read-Only Domain Controller”. The Read-Only Domain Controller (RODC) performs similar services as a writable Domain Controller except they are “read-only”. But what does that really mean?

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Nov 24 2017

Securing Microsoft Active Directory Federation Server (ADFS)

Many organizations are moving to the cloud and this often requires some level of federation. Federation, put simply, extends authentication from one system (or organization) to another.

Gerald Steere (@Darkpawh) and I spoke about cloud security at DEF CON in July 2017.
Presentation slides and video are here: “Hacking the Cloud”

One of the key items we covered was protecting Federation Servers, specifically  Microsoft Active Directory Federation Servers (ADFS).

Microsoft is currently updating guidance for securing ADFS.
This post describes key ADFS concepts and a short-list of security recommendations on how to properly protect ADFS.

Federation Overview

The federation server typically lives on the internal network with a proxy server in the DMZ. There are certificates installed on the Federation server.

ADFS uses the following certificates:

  • Service communication
  • Token-decrypting
  • Token-signing

ADFS terminology also includes:

  • Relying party trusts: cloud services and applications
  • Claim rules: determine what type of access and from where access is allowed.

Key Federation Points:

  • Federation: trust between organizations leveraging PKI (certificates matter)
  • Cloud SSO often leverages temporary or persistent browser cookies (cookies provide access)
  • Several protocols may be supported, though typically SAML. (protocols and versions matter)
  • Federation server (or proxy) is on public internet via port 443 (HTTPS).

Conceptual federation authentication flow Continue reading

Aug 11 2017

Gathering AD Data with the Active Directory PowerShell Module

Microsoft provided several Active Directory PowerShell cmdlets with Windows Server 2008 R2 (and newer) which greatly simplify tasks which previously required putting together lengthy lines of code involving ADSI.

On a Windows client, install the Remote Sever Administration Tools (RSAT) and ensure the Active Directory PowerShell module is installed.

On a Windows server (2008 R2 or newer), run the following commands in a PowerShell console (as an Adminsitrator):

Import-Module ServerManager ; Add-WindowsFeature RSAT-AD-PowerShell

Here’s my (poor) ADSI example:

$UserID = “JoeUser”
$root = [ADSI]''
$searcher = new-object System.DirectoryServices.DirectorySearcher($root)
$searcher.filter = "(&(objectClass=user)(sAMAccountName= $UserID))"
$user = $searcher.findall()
$user

Here’s the same thing with the AD PowerShell cmdlet:

Import-module ActiveDirectory
$UserID = “JoeUser”
Get-ADUser $UserID –property *

Note that with PowerShell version 3 and newer, you don’t need to run the first line since Powershell will identify the necessary module and auto load it.

Once you have the Active Directory PowerShell module loaded, you can do cool stuff like browse AD like a file system

 

Finding Useful Commands (Cmdlets):

Discover available PowerShell modules: Get-Module -ListAvailable

Discover cmdlets in a PowerShell module: Get-Command -module ActiveDirectory

 PowerShell AD Module Cmdlets:

  • Windows Server 2008 R2: 76 cmdlets
  • Windows Server 2012: 135 cmdlets
  • Windows Server 2012 R2: 147 cmdlets
  • Windows Server 2016: 147 cmdlets
(Get-Command -module ActiveDirectory).count

 

Finding Active Directory Flexible Master Single Operation (FSMO) Roles:

Active Directory Module:

  • (Get-ADForest).SchemaMaster
  • (Get-ADForest).DomainNamingMaster
  • (Get-ADDomain).InfrastructureMaster
  • (Get-ADDomain).PDCEmulator
  • (Get-ADDomain).RIDMaster

.NET Calls:

  • ([System.DirectoryServices.ActiveDirectory.Forest]::GetCurrentForest()).SchemaRoleOwner
  • ([System.DirectoryServices.ActiveDirectory.Forest]::GetCurrentForest()).NamingRoleOwner
  • ([System.DirectoryServices.ActiveDirectory.Domain]::GetCurrentDomain()).InfrastructureRoleOwner
  • ([System.DirectoryServices.ActiveDirectory.Domain]::GetCurrentDomain()).PdcRoleOwner
  • ([System.DirectoryServices.ActiveDirectory.Domain]::GetCurrentDomain()).RidRoleOwner

 

Active Directory PowerShell Module Cmdlet Examples:

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Aug 10 2017

Beyond Domain Admins – Domain Controller & AD Administration

Active Directory has several levels of administration beyond the Domain Admins group. In a previous post, I explored: “Securing Domain Controllers to Improve Active Directory Security” which explores ways to better secure Domain Controllers and by extension, Active Directory. For more information on Active Directory specific rights and permission review my post “Scanning for Active Directory Privileges & Privileged Accounts.”

This post provides information on how Active Directory is typically administered and the associated roles & rights.

  • Domain Admins is the AD group that most people think of when discussing Active Directory administration. This group has full admin rights by default on all domain-joined servers and workstations, Domain Controllers, and Active Directory. It gains admin rights on domain-joined computers since when these systems are joined to AD, the Domain Admins group is added to the computer’s Administrators group.
  • Enterprise Admins is a group in the forest root domain that has full AD rights to every domain in the AD forest. It is granted this right through membership in the Administrators group in every domain in the forest.
  • Administrators in the AD domain, is the group that has default admin rights to Active Directory and Domain Controllers and provides these rights to Domain Admins and Enterprise Admins, as well as any other members.
  • Schema Admins is a group in the forest root domain that has the ability to modify the Active Directory forest schema.

Since the Administrators group is the domain group that provides full rights to AD and Domain Controllers, it’s important to monitor this group’s membership (including all nested groups). The Active Directory PowerShell cmdlet “Get-ADGroupMember” can provide group membership information.

Default groups in Active Directory often have extensive rights – many more than typically required. For this reason, we don’t recommend using these groups for delegation. Where possible, perform custom delegation to ensure the principle of least privilege is followed. The following groups should have a “DC” prefix added to them since the scope applies to Domain Controllers by default. Furthermore, they have elevated rights on Domain Controllers and should be considered effectively Domain Controller admins.

  • Backup Operators is granted the ability to logon to, shut down, and perform backup/restore operations on Domain Controllers (assigned via the Default Domain Controllers Policy GPO). This group cannot directly modify AD admin groups, though associated privileges provides a path for escalation to AD admin. Backup Operators have the ability to schedule tasks which may provide an escalation path. They also are able to clear the event logs on Domain Controllers.
  • Print Operators is granted the ability to manage printers and load/unload device drivers on Domain Controllers as well as manage printer objects in Active Directory. By default, this group can logon to Domain Controllers and shut them down. This group cannot directly modify AD admin groups.
  • Server Operators is granted the ability to logon to, shut down, and perform backup/restore operations on Domain Controllers (assigned via the Default Domain Controllers Policy GPO). This group cannot directly modify AD admin groups, though associated privileges provides a path for escalation to AD admin.

To a lesser extend, we’ll group Remote Desktop Users into this category as well.

  • Remote Desktop Users is a domain group designed to easily provide remote access to systems. In many AD domains, this group is added to the “Allow log on through Terminal Services” right in the Default Domain Controllers Policy GPO providing potential remote logon capability to DCs.

We also see that many times the following is configured via GPO linked to the Domain Controllers OU:

  • Remote Desktop Users: often granted “Allow log on through Terminal Services” right via Group Policy linked to the Domain Controllers OU.
  • Server Operators: granted “Allow log on through Terminal Services” right via Group Policy linked to the Domain Controllers OU.
  • Server Operators: granted “Log on as a batch job” right via GPO providing the ability to schedule tasks.

Review the GPOs linked to the Domain and the Domain Controllers OU and ensure the GPO settings are appropriate.
We often find that a servers GPO is also linked to the Domain Controllers OU and it adds a “Server Admins” group to the local Administrators group. Since Domain Controllers don’t have a “local” Administrators group, the DC updates the domain Administrators group by adding Server Admins. This scenario makes all members of Server Admins Active Directory admins.

Any group/account granted logon locally rights to Domain Controllers should be scrutinized.

Server Operators & Backup Operators have elevated rights on Domain Controllers and should be monitored. The Active Directory PowerShell cmdlet “Get-ADGroupMember” can provide group membership information.

 

Other default groups with elevated rights:

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