Certificate Services (AD-CS)

Theory

AD CS is Microsoft’s PKI implementation that provides everything from encrypting file systems, to digital signatures, to user authentication (a large focus of our research), and more. While AD CS is not installed by default for Active Directory environments, from our experience in enterprise environments it is widely deployed, and the security ramifications of misconfigured certificate service instances are enormous. (specterops.io)

In their research papers, Will Schroeder and Lee Christensen shared their research on AD CS and identified multiple theft, escalation and persistence vectors.

Credential theft (dubbed THEFT1 to THEFT5)
Account persistence (dubbed PERSIST1 to PERSIST3)
Domain escalation (dubbed ESC1 to ESC14)
- based on misconfigured certificate templates
- based on dangerous CA configuration
- related to access control vulnerabilities
- based on an NTLM relay vulnerability related to the web and RPC endpoints of AD CS
Domain persistence (dubbed DPERSIST1 to DPERSIST3)
- by forging certificates with a stolen CA certificates
- by trusting rogue CA certificates
- by maliciously creating vulnerable access controls

Practice

Escalation techniques

Template allows SAN (ESC1)

Any purpose EKU (ESC2)

Certificate Agent EKU (ESC3)

EDITF_ATTRIBUTESUBJECTALTNAME2 (ESC6)

Web endpoint (ESC8)

No security extension (ESC9)

Weak certificate mapping (ESC10)

RPC endpoint (ESC11)

Shell access to ADCS CA with YubiHSM (ESC12)

Issuance policiy with privileged group linked (ESC13)

Weak explicit mapping (ESC14)

Certifried

Terminology

PKI (Public Key Infrastructure) — a system to manage certificates/public key encryption
AD CS (Active Directory Certificate Services) — Microsoft’s PKI implementation
CA (Certificate Authority) — PKI server that issues certificates
Enterprise CA — CA integrated with AD (as opposed to a standalone CA), offers certificate templates
Certificate Template — a collection of settings and policies that defines the contents of a certificate issued by an enterprise CA
CSR (Certificate Signing Request) — a message sent to a CA to request a signed certificate
EKU (Extended/Enhanced Key Usage) — one or more object identifiers (OIDs) that define how a certificate can be used
(specterops.io)

Recon

While AD CS offers attackers a wide range of exploitation and persistence scenarios, this set of services is not always installed, and when it is, it is a requirement to identify its different parts in the domain.

Cert Publishers

An initial indicator is the "Cert Publishers" built-in group whose members usually are the servers where AD CS is installed (i.e. PKI/CA).

From UNIX-like systems: rpc net group members "Cert Publishers" -U "DOMAIN"/"User"%"Password" -S "DomainController"
From Windows systems: net group "Cert Publishers" /domain

`pKIEnrollmentService` objects

Alternatively, information like the PKI's CA and DNS names can be gathered through LDAP.

netexec's adcs module (Python) can be used to find PKI enrollment services in AD.

netexec ldap 'domaincontroller' -d 'domain' -u 'user' -p 'password' -M adcs

windapsearch (Python) can be used to manually to the LDAP query.

windapsearch -m custom --filter '(objectCategory=pKIEnrollmentService)' --base 'CN=Configuration,DC=domain,DC=local' --attrs dn,dnshostname --dc 'domaincontroller' -d 'domain.local' -u 'user' -p 'password'

With Impacket's ntlmrelayx (Python), thanks to SAERXCIT (PR#1214), it is possible to gather information regarding ADCS like the name and host of the CA, the certificate templates enrollment rights for those allowing client authentication and not requiring manager approval, etc. With ntlmrelayx, these information can be gathered through a relayed LDAP session.

ntlmrelayx -t "ldap://domaincontroller" --dump-adcs

Attack paths

Certipy (Python) and Certify (C#) can also identify the PKI enrollment services and potential attack paths.

From UNIX-like systems, the Certipy (Python) tool can be used to operate multiple attacks and enumeration operations.

# enumerate and save text, json and bloodhound (original) outputs
certipy find -u '[email protected]' -p 'password' -dc-ip 'DC_IP' -old-bloodhound

# quickly spot vulnerable elements
certipy find -u '[email protected]' -p 'password' -dc-ip 'DC_IP' -vulnerable -stdout

Certipy also supports BloodHound. With the -old-bloodhound option, the data will be exported for the original version of BloodHound. With the -bloodhound option, the data will be exported for the modified version of BloodHound, forked by Certipy's author (default output when no flag is set).

The tool also supports multiple output types (text, json, stdout).

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.