This presentation was given at Sp4rkCon 2018. It covers the combination of Active Directory and host-based security descriptor backdooring and the associated security implications.
2. @harmj0y
Red teamer and offensive engineer
at SpecterOps
Adaptive Threat Division alumni
Avid blogger (http://harmj0y.net)
Co-founder of Empire,
BloodHound, Veil-Framework
2
3. @tifkin_
Red teamer, hunter, and
researcher at SpecterOps
Adaptive Threat Division alumni
Forever going after shiny things
Contributor to various
projects/blog posts
3
4. @enigma0x3
Red teamer and security
researcher at SpecterOps
Adaptive Threat Division alumni
Avid blogger
(https://enigma0x3.net/), COM
lover, CVE holder
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5. “As an offensive researcher, if you
can dream it, someone has likely
already done it...and that someone
isn’t the kind of person who speaks
at security cons.”
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Matt Graeber
“Abusing Windows Management
Instrumentation (WMI) to Build a Persistent,
Asynchronous, and Fileless Backdoor”
BlackHat 2015
7. The “True” Nature of Administrative Access
▪ Controversial statement: membership in a system’s
local administrators group isn’t what ultimately
matters!
▪ What actually matters is what local/domain groups
have access to specific remote resources (RPC,
remote reg, WMI, SQL, etc.) based on the host
service’s security descriptors
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13. From ACLs to DACLs to SACLs
▪ An Access Control List (ACL) is basically shorthand
for the DACL/SACL superset
▪ An object’s Discretionary Access Control List
(DACL) and System Access Control List (SACL) are
ordered collections of Access Control Entries (ACEs)
▫ DACL - What principals/trustees have what rights over the
object
▫ The SACL - Specifies how to audit access to the object
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15. tl;dr
▪ Security descriptors are just the mechanism that
Windows uses to define what users (principals)
can perform what actions on a specific object,
either in Active Directory or on the host
▫ When access is requested, some process enumerates
the effective security identifiers (SIDs) of the requestor,
compares them to the information in the DACL, and
decides whether to grant access
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17. Why Care?
▪ It’s often difficult to determine whether a specific security
descriptor misconfiguration was set maliciously or configured
by accident
▫ Existing misconfigurations: privesc opportunities
▫ Malicious misconfiguration changes: persistence!
▪ These changes often have a minimal forensic footprint
▪ Most defenders are not aware of this general persistence
approach, much less how to find and remediate it
▫ Nor are they aware of existing misconfigurations that affect privesc...
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21. Online vs Offline Security Descriptors
▪ Where do objects get their security descriptor?
▫ Offline - Security descriptor derived from registry, file, etc.
▫ Online - Security descriptor is in memory
▪ Our approach for enumeration:
▫ Locally as an unprivileged user
▫ Locally as a privileged user
▫ Remotely as an unprivileged user
▫ Remotely as a privileged user
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22. Example: Remote Registry
▪ Imagine this scenario: remotely dumping an
endpoint’s machine account hash as an “unprivileged”
user (i.e. not in local admins)!
▪ Backdoor Process
▫ Remotely backdoor the winreg key with an attacker-
controlled user/group (this key == remote registry access)
▫ Add malicious ACEs to the SECURITY and SYSTEM hives
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23. Example: Remote Registry
▪ (Remote) Backdoor Execution
▫ As the backdoor (domain or local) user, connect to the
remote registry service on the backdoored system
▫ Open up specific reg keys linked to LSA and extract their
classes
▫ Combine these class values and compute the BootKey
▫ Use the BootKey to decrypt the LSA key
▫ Use the LSA key to decrypt the machine account hash!
▫ EVERYONE GETS A SILVER TICKET!!
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25. Active Directory ACL Advantages
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▪ A big advantage: by default the DACLs for nearly
every AD object can be enumerated by any
authenticated user in the domain through LDAP!
▪ Other advantages of AD ACLs:
▫ Changes also have a minimal forensic footprint
▫ Changes often survive OS and domain functional level
upgrades, i.e. “misconfiguration debt”
▫ Anti-audit measures can be taken!
27. Generic Rights We Care About
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GenericAll Allows ALL generic rights to the specified object
GenericWrite Allows for the modification of (almost) all
properties on a specified object
WriteDacl Grants the ability to modify the DACL in the
object security descriptor
WriteOwner Grants the ability to take ownership of the object
28. Object-specific Rights We Care About
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Users User-Force-Change-Password or write to
the servicePrincipalName
Groups Write to the member property
Computers None outside of LAPS :(
GPOs Modification of GPC-File-Sys-Path
Domains WriteDacl to add DCSync rights
29. Example: Abusing Exchange
▪ Exchange Server introduces several schema changes,
new nested security groups, and MANY control
relationships to Active Directory, making it a perfect
spot to blend in amongst the noise!
▪ Pre Exchange Server 2007 SP1, this included the
WriteDACL privilege against the domain object itself
with Exchange Trusted Subsystem as the principal
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30. Example: Abusing Exchange
▪ Backdoor Process
▫ Identify a non-protected security group with local admin
rights on one or more Exchange servers
▫ Grant Authenticated Users full control over this security
group
▫ Change the owner of the group to an Exchange server
▫ Deny Read Permissions on this group to the Everyone
principal
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31. Example: Abusing Exchange
▪ Backdoor Execution
▫ Regain access to the Active Directory domain as any user
▫ Add your current user to the backdoored security group
▫ Use your new local admin rights on an Exchange server to
execute commands as the SYSTEM user on that computer
▫ Abuse the rights Exchange Trusted Subsystem has over
the domain object (i.e. WriteDacl!)
▫ More information: http://bit.ly/2IIK3K3
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33. ▪ Prior to joining active directory, the host is in ultimate
control of who can access its resources
▪ After a machine is joined to AD, a few things happen:
▫ The machine is no longer solely in charge of authentication
▫ A portion of key material for the host is stored in another
location (machine account hash in ntds.dit)
▫ Default domain group SIDs are added to local groups
▫ Management is no longer solely left to the host (i.e. GPOs :)
“Risks” Of Joining Active Directory
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34. Active Directory: Before and After
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Workgroup Active Directory
Security Principals Local users/groups
+ Domain
users/groups
Access/Permission
Management
Host-based Security
Descriptors
+ Default domain
groups added to
local groups
Authentication NTLM (SAM)
+ Kerberos/NTLM
(NTDS)
Resource
Administration
Manual + GPOs
35. Active Directory: Before and After
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DCOM
Service
Administrators
admin
DOMAINDomain Admins
Distributed
COM Users
DOMAINsrvcacct
DOMAINjohnDOMAINsrvadms
DOMAINlee
36. The “Actual” Attack Graph
▪ BloodHound doesn’t (currently) take host based
security descriptors into account
▪ The actual access graph that exists in a domain
includes the security descriptors for every remotely
accessible service on every host + AD descriptors
▫ Includes “unrolling” groups… this may not be (currently)
realistically possible to model in large environments ¯_(ツ)_/¯
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37. Security Implications
▪ Host-based security descriptors are the missing
link when thinking about domain attack graphs!
▪ There ARE existing misconfigurations in the security
descriptors in some host-based services!
▫ More to come this summer, stay tuned :)
▪ Host-based security descriptor modifications can be
chained with AD misconfigurations/modifications
▪ “Fills the gap” left by the lack of an AD ACL computer primitive
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38. tl;dr Security Implications of Joining Active Directory
▪ When you join a system to Active Directory, you’re
introducing additional nodes into the access graph
that may affect the security of other systems
▪ You’re also implicitly trusting the security of a
large number of other nodes in the graph as well
▫ You’re almost certainly exposing your system’s services
to more access than you realize!
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40. Case Study: Exchanging Rights
▪ We saw before that the Exchange Trusted
Subsystem group (which contains Exchange servers)
often has a huge number of rights over the domain
▪ So let’s integrate the remote registry host-based
backdoor on an Exchange box!
▫ No changes to the DC or any AD data
▫ Takes advantage of existing misconfigurations!
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43. Case Study: Abusing Existing Misconfigurations
▪ GPOs set lots of interesting settings!
▫ They can even set host-based security descriptors: )
▫ Imagine one that modifies the security descriptor for SCM
▪ We can also easily correlate GPOs to find what
systems they apply to
▪ What happens if the group SID set for the
descriptor via GPO, after unrolling, contains a
service account...
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Summary
▪ Access is more than just “local administrators” !
▪ You should really care about security descriptors!
▪ Host based security descriptors (accidentally
misconfigured or maliciously backdoored) can have far-
reaching implications for the security of other
systems in the domain!
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Questions?
You can find us at @SpecterOps:
▪ @harmj0y , @tifkin_ ,
@enigma0x3
▪ [will,lee,matt]@specterops.io