Mandiant has seen an uptick in incidents involving Microsoft 365
(M365) and Azure Active Directory (Azure AD). Most of these incidents
are the result of a phishing email coercing a user to enter their
credentials used for accessing M365 into a phishing site. Other
incidents have been a result of password spraying, password stuffing,
or simple brute force attempts against M365 tenants. In almost all of
these incidents, the user or account was not protected by multi-factor
authentication (MFA).
These opportunistic attacks are certainly the most common form of
compromise for M365 and Azure AD, and are usually the initial vector
to establish persistence. During both incident response (IR)
engagements and proactive cloud assessments we are often asked:
In some incidents, Mandiant has witnessed attackers utilizing a
PowerShell module called
allow an attacker to vertically move from on-premises to Azure AD,
establish backdoors, steal passwords, generate user security tokens,
and bypass MFA protections. This PowerShell module has allowed
attackers to maintain persistence in the tenant even after initial
eradication efforts were conducted.
To see this module in action and understand how it works, Dr.
Nestori Syynimaa’s PSCONFEU 2020 presentation,
Active Directory: Who would you like to be today?, provides an
in-depth overview of the module.
To detect the use of AADInternals, it is important to understand how
some of these attacks work. Once an understanding is established,
abnormal usage can be detected through a combination of log analysis
and host-based indicators.
Attacker Requirements
Or
The AADInternals PowerShell module contains a function called
inserting itself as a man-in-the-middle within the Pass-through
Authentication (PTA) process that occurs between Azure AD and the
server running the PTA Agent in the on-premises environment. Commonly,
the PTA Agent runs on the same on-premises server as Azure AD Connect
(AAD Connect).
When PTA is enabled, every logon that occurs against Azure AD gets
redirected to the PTA Agent on-premises. The PTA Agent asks an
on-premises Active Directory Domain Controller if a password is valid
for an authenticating account. If valid, the PTA Agent responds back
to Azure AD to grant the requestor access. Figure 1 provides the
workflow of Pass-through Authentication and where AADInternals can
intercept the request.
Figure 1: Pass-through Authentication workflow
Once the function is running, every PTA attempt against Azure AD
will be intercepted by the installed
user’s password attempt and reply back to Azure AD on behalf of the
PTA Agent. This reply advises Azure AD the password attempt was valid
and grants the user access to the cloud, even if the password is
incorrect. If an attacker has implanted
attempts to authenticate using PTA—and will be granted access.
Additionally, all password attempts that are registered by the
on the server (Default location:
file can be decoded to reveal a user’s password in cleartext.
Figure 2: PTASpy.csv decoded passwords
Not only will this function allow an attacker to login as any user
who authenticates via PTA, but it will also act as a repository for
collecting user passwords who are legitimately logging into Azure AD.
This could allow an attacker to pivot their attack to other areas of
the network—or use these credentials against other internet accessible
portals that may leverage single-factor authentication (e.g., VPN gateway).
An attacker can use this module in one of two ways:
Method 1: On-Premises Compromise
An attacker has gained access to an on-premises domain and is able
to laterally move to the AADConnect / PTA Agent Server. From this
server, an attacker can potentially leverage the AADInternals
PowerShell module and invoke the
Method 2: Cloud Compromise
If an attacker has successfully compromised an Azure AD global admin
account, an attack can be conducted from an attacker’s own
infrastructure. An attacker can install a PTA Agent on a server they
manage and register the agent using the compromised global
administrator account (Figure 3).
Figure 3: Azure AD Portal—registered
Pass-through Authentication agents
Once registered with Azure AD, the rogue server will begin to
intercept and authorize all login attempts. As with Method 1, this
server can also be used to harvest valid credentials.
Attacker Requirements
Or
Another method of authenticating to M365 is through the usage of
federation services. When a M365 domain is configured as a federated
domain, a trust is configured between M365 and an external identify
provider. In many cases, this trust is established with an Active
Directory Federation Services (ADFS) server for an on-premises Active
Directory domain.
Once a trust is established, when a user logs into M365 using a
federated domain, their request is redirected to the external identify
provider (ADFS) where their authentication is validated (Figure 4).
Once validated, the ADFS server provides the user a security token.
This token is then trusted by M365 and grants the access to the platform.
Figure 4: Microsoft 365 Federation
Sign-in workflow
AADInternals has a PowerShell function to craft security tokens,
which mimics the ADFS authentication process. When providing the
function a valid UserPrincipalName, Immutable ID and IssuerURI, an
attacker can generate a security token as any user of the tenant.
What’s even more concerning is that once this security token is
generated, this can allow an attacker to bypass MFA.
As with Backdoor 1, this attack can either be performed from a
compromised on-premises environment or from an attacker’s own infrastructure.
Method 1: On-Premises Compromise
Once an attacker has gained access to an on-premises domain with
elevated access, they can begin to collect the required information to
craft their own security tokens to backdoor into M365 as any user. An
attacker will require:
Once an attacker has collected the necessary information, using the
AADInternals Open-AADIntOffice365Portal
command, a security token for the user can be generated granting an
attacker access to M365 (Figure 5).
Figure 5: AADInternals
Open-AADIntOffice365Portal command
Method 2: Cloud Compromise
If an attacker has a compromised an M365 Global Administrator
account, using their own infrastructure, an attacker can use their
administrative access to collect user information and reconfigure the
tenant to establish their backdoor. In this method, an attacker will require:
Figure 6: Get-MsolUser—list user UPN
& ImmutableId
Figure 7: Get-msoldomain—list of
registered domains and authentication
Figure 8:
ConvertTo-AADIntBackdoor—convert domain to federated authentication
Figure 9: Changed authentication method
Figure 10: Azure AD Portal registered domains
Note: To not interrupt production and
authentication with an existing federated domain (and to remain
undetected), an attacker may opt to register a new domain with the tenant.
Figure 11: AADInternals
Open-AADIntOffice365Portal Command using new Federated domain
Once an attacker has properly configured the tenant, using the
ImmutableId of any user, a security token can be generated by
executing the Open-AADIntOffice365Portal
command (Figure 11). This will allow an attacker to login as that user
without the need for a valid certificate or a legitimate IssuerURI.
Fortunately for defenders, this method will generate a number of
events in the unified audit log, which can be leveraged for monitoring
and alerting.
Once persistence is established, it can be extremely difficult to
detect login activity that is utilizing one of the previously
described methods. In lieu of this, it is recommended to monitor and
alert on M365 unified audit logs and Azure AD sign-in activity to
detect anomalous activity.
Being that Mandiant has seen this methodology being used in the
wild, we felt it was necessary to build these detections into our
FireEye Helix security platform. Helix engineers have created sever
new detection rules that monitor for detectable activity of an
attacker making use of the AADInternals PowerShell module.
The following five rules will monitor a server’s event logs and
alert upon the installation and usage of the AADInternals PowerShell
module (Figure 12). The detection of these activities could be high
fidelity alerts that an attacker is preparing to configure backdoors
into M365 and Azure AD environments.
Figure 12: AADInternals Helix rules
If an attacker has successfully configured a backdoor using
AADInternals, Helix will alert upon the following events registered in
the Office 365 unified audit log and Azure Activity Log as indication
of a possible event (Figure 13 and Figure 14). It is important to note
that these alerts could be triggered upon legitimate administrator
activity. When responding to these alerts, first check with your M365
and Azure AD administrator to verify the activity before raising a
security event.
Figure 13: Office 365 and Azure Helix rules
Figure 14: PTA Connector Registered alert description
If you suspect a global administrator account was compromised and
you want to review Azure AD for indicators of potential abuse, the
following should be reviewed (note that these same concepts can be
used for proactive log monitoring):
Figure 15: Azure AD Sign-ins
Figure 16: Azure AD Sign-in
logs—On-Premises Directory Synchronization Services account
From Azure AD Sign-ins, monitor and
baseline Azure AD Sign-ins to the Azure AD Application Proxy
Connector. Make sure to validate username, IP and location.
These events are typically only
generated when a new PTA agent is connected to the tenant. This could
be an indicator that an attacker has connected a rogue PTA server
hosted on an attacker’s infrastructure (Figure 17).
Figure 17: Azure AD Sign-in logs—Azure AD
Application Proxy Connector
If using Azure Sentinel, this event
will also be registered in the Azure AuditLogs table as a “
Figure 18: Register Connector—Azure
Sentinel logs
Figure 19: Azure Active Directory
Pass-through Authentication agent status
Figure 20: Unified Audit Log > Create new alert policy
Figure 21: Unified Audit Log filtered for domain related events
Figure 22: OfficeActivity Operations Azure Sentinel logs
If an attacker is able to compromise on-premises infrastructure and
access a server running AD Connect or ADFS services with the intention
of leveraging a tool such as AADInternals to expand the scope of their
access to include cloud, timely on-premises detection and containment
is key. The following methods can be leveraged to ensure optimized
visibility and detection for the scope of activities described in this post:
Figure 23: EventID 410—Installed Module
Figure 24: Get-Module command to list
installed modules
Figure 25: PTASpy.csv log activity
In order for this attack to be successful, an attacker must gain
administrative privileges on a server running Azure AD Connect and/or
gain global administrator rights within M365. Simple practices such as
limiting and properly protecting global administrator accounts as well
as properly protecting Tier 0 assets can greatly reduce the risk of an
attacker successfully using the AADInternals PowerShell against your organization.
I want to give a special thanks to Daniel Taylor, Roberto Bamberger
and Jennifer Kendall at Microsoft for collaborating with Mandiant on
the creation of this blog post.