AD CS supports several HTTP-based enrollment methods via additional server roles that administrators can optionally install [(The certificate enrollment web interface, Certificate Enrollment Service (CES), Network Device Enrollment Service (NDES)).][...]These HTTP-based certificate enrollment interfaces are all vulnerable to NTLM relay attacks. Using NTLM relay, an attacker can impersonate an inbound-NTLM-authenticating victim user. While impersonating the victim user, an attacker could access these web interfaces and request a client authentication certificate based on the "User" or "Machine" certificate templates.
This attack, like all NTLM relay attacks, requires a victim account to authenticate to an attacker-controlled machine. An attacker can coerce authentication by many means, see MITM and coerced authentication coercion techniques. Once the incoming authentication is received by the attacker, it can be relayed to an AD CS web endpoint.
Once the relayed session is obtained, the attacker poses as the relayed account and can request a client authentication certificate. The certificate template used needs to be configured for authentication (i.e. EKUs like Client Authentication, PKINIT Client Authentication, Smart Card Logon, Any Purpose (
OID 220.127.116.11.0), or no EKU (
SubCA)) and allowing low-priv users to enroll can be abused to authenticate as any other user/machine/admin.
This allows for lateral movement, account persistence, and in some cases privilege escalation if the relayed user had powerful privileges (e.g., domain controllers or Exchange servers, domain admins etc.).
1 - Setting up the relay servers
ntlmrelayx -t "http://CA/certsrv/certfnsh.asp" --adcs --template "Template name"
2 - Authentication coercion
3 - Loot
Once incoming NTLM authentications are relayed and authenticated sessions abused, base64-encoded PFX certificates will be obtained and usable with Pass-the-Certificate to obtain a TGT and authenticate.