Web endpoints
Theory
In their research papers, Will Schroeder and Lee Christensen found a domain escalation vector based on web endpoints vulnerable to NTLM relay attacks. The escalation vector was dubbed ESC8.
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)).]
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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 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 and Machine/Computer templates match those criteria and are very often enabled.
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.).
Practice
1 - Setting up the relay servers 🛠️
From UNIX-like systems, Impacket's ntlmrelayx (Python) can be used to conduct the ESC8 escalation scenario.
The certificate template flag (i.e. --template) can either be left blank (default to Machine at the time of writing, October 20th 2012) or chosen among the certificate templates that fill the requirements.
For instance, if the relayed principal is a domain controller, the DomainController template must be specified.
Certipy (Python) can be used to enumerate information regarding the certificate templates (EKUs allowing for authentication, allowing low-priv users to enroll, etc.) (how to enumerate).
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.
2 - Authentication coercion ⛓️
Just like any other NTLM relay attack, once the relay servers are running and waiting for incoming NTLM authentications, authentication coercion techniques can be used (e.g. PrinterBug, PetitPotam, PrivExchange) to force accounts/machines to authenticate to the relay servers.
pageMITM and coerced auths3 - 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.
Resources
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