Certificati AD

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Introduzione

Components of a Certificate

  • The Subject of the certificate denotes its owner.
  • A Public Key is paired with a privately held key to link the certificate to its rightful owner.
  • The Validity Period, defined by NotBefore and NotAfter dates, marks the certificate's effective duration.
  • A unique Serial Number, provided by the Certificate Authority (CA), identifies each certificate.
  • The Issuer refers to the CA that has issued the certificate.
  • SubjectAlternativeName allows for additional names for the subject, enhancing identification flexibility.
  • Basic Constraints identify if the certificate is for a CA or an end entity and define usage restrictions.
  • Extended Key Usages (EKUs) delineate the certificate's specific purposes, like code signing or email encryption, through Object Identifiers (OIDs).
  • The Signature Algorithm specifies the method for signing the certificate.
  • The Signature, created with the issuer's private key, guarantees the certificate's authenticity.

Special Considerations

  • Subject Alternative Names (SANs) expand a certificate's applicability to multiple identities, crucial for servers with multiple domains. Secure issuance processes are vital to avoid impersonation risks by attackers manipulating the SAN specification.

Certificate Authorities (CAs) in Active Directory (AD)

AD CS acknowledges CA certificates in an AD forest through designated containers, each serving unique roles:

  • Certification Authorities container holds trusted root CA certificates.
  • Enrolment Services container details Enterprise CAs and their certificate templates.
  • NTAuthCertificates object includes CA certificates authorized for AD authentication.
  • AIA (Authority Information Access) container facilitates certificate chain validation with intermediate and cross CA certificates.

Certificate Acquisition: Client Certificate Request Flow

  1. The request process begins with clients finding an Enterprise CA.
  2. A CSR is created, containing a public key and other details, after generating a public-private key pair.
  3. The CA assesses the CSR against available certificate templates, issuing the certificate based on the template's permissions.
  4. Upon approval, the CA signs the certificate with its private key and returns it to the client.

Certificate Templates

Defined within AD, these templates outline the settings and permissions for issuing certificates, including permitted EKUs and enrollment or modification rights, critical for managing access to certificate services.

Certificate Enrollment

The enrollment process for certificates is initiated by an administrator who creates a certificate template, which is then published by an Enterprise Certificate Authority (CA). This makes the template available for client enrollment, a step achieved by adding the template's name to the certificatetemplates field of an Active Directory object.

For a client to request a certificate, enrollment rights must be granted. These rights are defined by security descriptors on the certificate template and the Enterprise CA itself. Permissions must be granted in both locations for a request to be successful.

Template Enrollment Rights

These rights are specified through Access Control Entries (ACEs), detailing permissions like:

  • Certificate-Enrollment and Certificate-AutoEnrollment rights, each associated with specific GUIDs.
  • ExtendedRights, allowing all extended permissions.
  • FullControl/GenericAll, providing complete control over the template.

Enterprise CA Enrollment Rights

The CA's rights are outlined in its security descriptor, accessible via the Certificate Authority management console. Some settings even allow low-privileged users remote access, which could be a security concern.

Additional Issuance Controls

Certain controls may apply, such as:

  • Manager Approval: Places requests in a pending state until approved by a certificate manager.
  • Enrolment Agents and Authorized Signatures: Specify the number of required signatures on a CSR and the necessary Application Policy OIDs.

Methods to Request Certificates

Certificates can be requested through:

  1. Windows Client Certificate Enrollment Protocol (MS-WCCE), using DCOM interfaces.
  2. ICertPassage Remote Protocol (MS-ICPR), through named pipes or TCP/IP.
  3. The certificate enrollment web interface, with the Certificate Authority Web Enrollment role installed.
  4. The Certificate Enrollment Service (CES), in conjunction with the Certificate Enrollment Policy (CEP) service.
  5. The Network Device Enrollment Service (NDES) for network devices, using the Simple Certificate Enrollment Protocol (SCEP).

Windows users can also request certificates via the GUI (certmgr.msc or certlm.msc) or command-line tools (certreq.exe or PowerShell's Get-Certificate command).

bash
# Example of requesting a certificate using PowerShell
Get-Certificate -Template "User" -CertStoreLocation "cert:\\CurrentUser\\My"

Autenticazione tramite certificato

Active Directory (AD) supporta l'autenticazione tramite certificato, utilizzando principalmente i protocolli Kerberos e Secure Channel (Schannel).

Processo di autenticazione Kerberos

Nel processo di autenticazione Kerberos, la richiesta di un utente per un Ticket Granting Ticket (TGT) viene firmata usando la private key del certificato dell'utente. Questa richiesta viene sottoposta a diverse validazioni da parte del domain controller, incluse la validity, il path e lo revocation status del certificato. Le validazioni includono anche la verifica che il certificato provenga da una fonte attendibile e la conferma della presenza dell'emittente nello NTAUTH certificate store. Validazioni riuscite portano al rilascio di un TGT. L'oggetto NTAuthCertificates in AD, trovato a:

bash
CN=NTAuthCertificates,CN=Public Key Services,CN=Services,CN=Configuration,DC=<domain>,DC=<com>

è centrale per stabilire la fiducia nell'autenticazione mediante certificati.

Autenticazione Secure Channel (Schannel)

Schannel facilita connessioni TLS/SSL sicure, nelle quali, durante l'handshake, il client presenta un certificato che, se convalidato con successo, autorizza l'accesso. L'associazione di un certificato a un account AD può coinvolgere la funzione S4U2Self di Kerberos o il Subject Alternative Name (SAN) del certificato, tra gli altri metodi.

Enumerazione dei servizi di certificazione AD

I servizi di certificazione di AD possono essere enumerati tramite query LDAP, rivelando informazioni sulle Enterprise Certificate Authorities (CAs) e le loro configurazioni. Questo è accessibile a qualsiasi utente autenticato nel dominio senza privilegi speciali. Strumenti come Certify e Certipy sono usati per l'enumerazione e la valutazione delle vulnerabilità negli ambienti AD CS.

I comandi per utilizzare questi strumenti includono:

bash
# Enumerate trusted root CA certificates, Enterprise CAs and HTTP enrollment endpoints
# Useful flags: /domain, /path, /hideAdmins, /showAllPermissions, /skipWebServiceChecks
Certify.exe cas [/ca:SERVER\ca-name | /domain:domain.local | /path:CN=Configuration,DC=domain,DC=local] [/hideAdmins] [/showAllPermissions] [/skipWebServiceChecks]

# Identify vulnerable certificate templates and filter for common abuse cases
Certify.exe find
Certify.exe find /vulnerable [/currentuser]
Certify.exe find /enrolleeSuppliesSubject   # ESC1 candidates (CT_FLAG_ENROLLEE_SUPPLIES_SUBJECT)
Certify.exe find /clientauth                # templates with client-auth EKU
Certify.exe find /showAllPermissions        # include template ACLs in output
Certify.exe find /json /outfile:C:\Temp\adcs.json

# Enumerate PKI object ACLs (Enterprise PKI container, templates, OIDs) – useful for ESC4/ESC7 discovery
Certify.exe pkiobjects [/domain:domain.local] [/showAdmins]

# Use Certipy for enumeration and identifying vulnerable templates
certipy find -vulnerable -u john@corp.local -p Passw0rd -dc-ip 172.16.126.128

# Enumerate Enterprise CAs and certificate templates with certutil
certutil.exe -TCAInfo
certutil -v -dstemplate

Riferimenti

tip

Impara e pratica il hacking AWS:HackTricks Training AWS Red Team Expert (ARTE)
Impara e pratica il hacking GCP: HackTricks Training GCP Red Team Expert (GRTE) Impara e pratica il hacking Azure: HackTricks Training Azure Red Team Expert (AzRTE)

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