July 29, 2021

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Abusing Replication: Stealing AD FS Secrets Over the Network

Abusing Replication: Stealing AD FS Secrets Over the Network

Organizations are increasingly adopting cloud-based services such as
Microsoft 365 to host applications and data. Sophisticated threat
actors are catching on and Mandiant has observed an increased focus on
long-term persistent access to Microsoft 365 as one of their primary
objectives. The focus on developing novel and hard to detect methods
to achieve this goal was highlighted with the recent detection of UNC2452
and their access to Microsoft 365
. One of this group’s key TTPs
was to steal the Token Signing Certificate from an organization’s AD
FS server to enable them to bypass MFA and access cloud services as
any user, at any time. While defenders previously associated the
defense of this certificate, and thus the entire ecosystem, with
careful access control and detection efforts around the AD FS server
and service account, this is no longer sufficient. In this blog post
we will show how a threat actor, with the right privilege, can extract
the encrypted Token Signing Certificate from anywhere on the internal
network. Once extracted, a threat actor can easily decrypt it and
begin accessing cloud services.

Active Directory Federation Services

Active Directory Federation Services (AD FS) is a feature for
Windows Servers that enables
federated identity and access management
. It is often used by
organizations to provide single sign-on functionality to access
enterprise applications such as Microsoft 365. In technical terms, AD
FS functions as an Identity Provider (IdP) and Microsoft 365 is
a Service Provider (SP). We’ll use Microsoft 365 as an example
going forward, but this technique could apply to any service that is
set up to trust AD FS. AD FS verifies a user’s identity and issues
assertions that describe the user. Microsoft 365  trusts AD FS to
verify user identities and provide it with assertions. To Microsoft
365, it doesn’t matter how AD FS performed the verification, it just
needs the assertions.

In the typical deployment (Figure 1), AD FS will verify a user’s
identity using Active Directory. At a minimum, an AD FS deployment
consists of two servers in an enterprise’s on-premises network: the
primary AD FS server, and an AD FS Web Application Proxy (WAP). The
proxy is placed in the DMZ and has no functionality besides proxying
sign-on attempts from the Internet to the AD FS server. The primary AD
FS server receives proxied requests, verifies a user’s identity, and
issues assertions that are packaged into SAML security tokens for the user.

Abusing Replication: Stealing AD FS Secrets Over the Network

Figure 1: Typical AD FS deployment
(source: Microsoft)

The SAML token
issued by AD FS proves a user’s identity to Microsoft 365 and can also
be used to make authorization decisions. The SAML token is an XML
document with two main components:

  1. Assertions: Assertions are XML elements that describe the
    user’s identity. An assertion could be a user SID, group membership
    SIDs, or other elements like the user’s department name. A single
    SAML token can have multiple assertions attached to it.
  2. Digital Signature: The assertions in the SAML token are
    digitally signed using a public/private keypair that resides on the
    AD FS server. This is called the Token Signing Certificate.

The Token Signing Certificate is the bedrock of security in AD
FS.
Microsoft 365 uses the digital signature to validate that the
SAML token is authentic, valid, and comes from an AD FS server that it
trusts. To enable this verification, an administrator shares the
public component of the Token Signing Certificate with Microsoft 365.
This is then used to cryptographically verify the digital signature in
the SAML token and prove authenticity as well as integrity of the
token. In other words, if a threat actor got hold of a Token Signing
Certificate, they could generate arbitrary SAML tokens to access any
federated application, as any user, and even bypass MFA.

Golden SAML

Golden SAML was coined in 2017 by CyberArk to describe the technique
of forging
SAML tokens to access SPs
given a valid Token Signing
Certificate. At TROOPERS 19,
I detailed how a threat actor could extract the Token Signing
Certificate from an AD FS server, as well as some mitigation
strategies for defenders.

In a default AD FS configuration, the Token Signing Certificate is
stored within a Windows Internal Database (WID) instance that is
running on the AD FS server. WID is more or less MS SQL Express,
except the database can only be accessed locally over a special named
pipe connection. In AD FS, the database is further locked down to only
the AD FS service account. The Token Signing Certificate is stored in
an encrypted state in the IdentityServerPolicy.ServiceStateSummary table.
Figure 2 contains a single row with a column that stores all the
settings that AD FS will need on service start as an XML document.

<SigningToken>
           
<IsChainIncluded>false</IsChainIncluded>
 
         
<IsChainIncludedSpecified>false</IsChainIncludedSpecified>
           
<FindValue>99FABAEE46A09CD9B34B9510AB10E2B0C0ACB99B</FindValue>
           
<RawCertificate></RawCertificate>
         
  <EncryptedPfx></EncryptedPfx>
           
<StoreNameValue>My</StoreNameValue>
       
   
<StoreLocationValue>CurrentUser</StoreLocationValue>
           
<X509FindTypeValue>FindByThumbprint</X509FindTypeValue>
        </SigningToken>

Figure 2: Example Token Signing Certificate
stored in the AD FS database

The Token Signing Certificate as it is stored in the AD FS database
is encrypted using symmetric key encryption. Windows uses a technology
called Distributed Key Management (DKM) to store the secret value used
to derive the symmetric key in an Active Directory container. The AD
FS service account can read the attributes of this container, derive
the symmetric key, and then decrypt the Token Signing Certificate.

AD FS Replication

AD FS also supports a farm configuration for high availability and
load balancing in larger enterprise networks. The individual AD FS
servers in a farm can be configured to use unique Token Signing
Certificates; however, the default is to have the servers share the
same Token Signing Certificate.  In order to stay in sync with each
other, the farm will have a primary node and secondary nodes. The
secondary nodes make use of a replication service to acquire
configuration settings and certificates from the primary AD FS server.
To facilitate this, AD FS makes use of Windows Communication
Foundation (WCF).

WCF is a framework that allows developers to build
service-oriented applications
. A WCF application has two
components: the service that will receive and process messages, and
the client that sends messages to a service and receives back
responses. The AD FS servers run a WCF service that is called the
Policy Store Transfer Service internally.

To send a message to this service, the client will connect to the
URL http://<adfs server
name>:80/adfs/services/policystoretransfer
. Note that even
though the channel is over HTTP, the actual data being exchanged is
encrypted during transit. It is also key to understand that although
there is a single primary AD FS server, all nodes in an AD FS farm run
this WCF service and can be used for replication.

Upon receipt of a message, the WCF service enforces an authorization
check to ensure the calling identity is permitted to receive the
requested information. The permission check is done by evaluating an
authorization policy that is also stored in the IdentityServerPolicy.ServiceStateSummary table of
the AD FS database. The policy permits identities whose primary SID
matches the AD FS Service account or to any identity that is
member of the AD FS server’s local administrators group. If the
identity of the client passes the authorization check, then the WCF
service will send back a message containing the requested information.

   <AuthorizationPolicy>
 
 @RuleName = “Permit Service Account”exists([Type ==
 
 
“http://schemas.microsoft.com/ws/2008/06/identity/claims/
    primarysid”, Value ==
“S-1-5-21-3508695881-2242692613
    -376241919-1107”])
=> issue(Type = “http://schemas
   
.microsoft.com/authorization/claims/permit”, Value = “
    true”);
   @RuleName = “Permit Local
Administrators”exists([Type ==
 
 “http://schemas.microsoft.com/ws/2008/06/identity/claims/group
   sid”, Value == “S-1-5-32-544”])=> issue(Type =
&quot
 
 ;http://schemas.microsoft.com/authorization/claims/permit”,
Value
    = “true”);
 
 </AuthorizationPolicy>

Figure 3: Default Authorization Policy for AD FS server

Room for Abuse

A threat actor can abuse the Policy Store Transfer Service to
acquire the encrypted Token Signing Certificate over the network,
similar to the DCSync technique for Active Directory. It is important
to note that the data is still encrypted and requires the DKM key
stored in Active Directory to decrypt. This technique, however,
requires a significant change to how defenders have secured AD FS
servers and monitored them for theft of the Token Signing Certificate.

First, previous techniques required code execution on an AD FS
server to extract the data or at least an SMB connection to transfer
the backing database files. With a strong defense in depth program
using secure credential management, EDR, and network segmentation, an
enterprise can make it very difficult for a threat actor to access an
AD FS server and the Token Signing Certificate. Abusing the AD FS
Replication service, however, requires only access to the AD FS server
over the standard HTTP port. The default installation of AD FS will
even create a Windows Firewall rule to allow HTTP traffic from any
system. Additionally, a threat actor does not need the credentials for
the AD FS service account and can instead use any account that is a
local administrator on an AD FS server. Lastly, there is no Event Log
message that is recorded when a replication event occurs on an AD FS
server. Altogether, this makes the technique both much easier to
execute and much harder to detect.

The authorization policy itself also presents an opportunity for
abuse. Because the authorization policy is stored as XML text in the
configuration database, a threat actor with enough access could modify
it to be more permissive. A threat actor could modify the
Authorization Policy to include a group SID such as domain users,
S-1-5-21-X-513. Similarly, they could add
an ACE to the DKM key container in Active Directory. This would allow
the threat actor to easily obtain the Token Signing Certificate and
decrypt it using any domain user credentials. This would give them
persistent ability to perform a Golden SAML attack with only access to
the network as a requirement.

Mandiant has not yet observed this technique used in the wild;
however, it is trivial to write a POC for and we are aware of one
public tool that will soon support it. Figure 4 shows the output of
POC code written in .NET to extract the Token Signing Certificate from
a remote AD FS server.



Figure 4: POC code output

Mitigations

The best mitigation against this technique is to use the Windows
Firewall to restrict access to port 80 TCP to only the AD FS servers
in the farm. If an organization has only a single AD FS server, then
port 80 TCP can be blocked completely. This block can be put in place
because all traffic to and from AD FS servers and proxies for user
authentication is over port 443 TCP.  

To limit inbound communications, modify the existing firewall rule
that AD FS inserts on installation.

Set-NetFirewallRule -DisplayName “AD FS HTTP
Services (TCP-In)” -RemoteAddress <ADFS1 IP
address>,<ADFS2 IP Address>

If no rule exists, the scriptlet in Figure 5 should be applied to
all ADFS servers to create one.

New-NetFirewallRule -DisplayName
“Allow ADFS Servers TCP 80” -Direction Inbound
-Action Allow  -Protocol TCP -LocalPort 80 -RemoteAddress
<ADFS1 IPAddress >,<ADFS2 IPAddress>

Figure 5: Windows Firewall – Allow ADFS Server –
TCP 80

Organizations that are monitoring the internal network can alert on
HTTP POST requests to the address that hosts the Policy Store Transfer
service. If there is an AD FS farm, then the IP addresses of the AD FS
servers will need to be whitelisted against the rule. Figure 6 shows a
sample Snort rule to detect this activity.

alert tcp any any -> any 80
(msg:”AD FS Replication”; flow:established,
to_server; content:”POST”; http_method;
content:”adfs/services/policystoretransfer”;
http_uri; threshold:type limit,track by_src,count 1,seconds
3600; priority:3; sid:7000000; rev:1;)

Figure 6: Sample snort rule

Acknowledgements

Mandiant would like to acknowledge the great work of Dr. Nestori
Syynimaa (@DrAzureAD). Dr. Syynimaa independently thought to research
the replication of configuration information between AD FS servers and
has published his findings on his blog. Mandiant would also like to
thank Microsoft for their collaboration on mitigations and detections
for this technique. Lastly, special thanks to Mike Burns of the
Mandiant Security Transformation services team for his feedback on
mitigations and detections.