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UnPAC the hash

Theory

When using PKINIT to obtain a TGT (Ticket Granting Ticket), the KDC (Key Distribution Center) includes in the ticket a PAC_CREDENTIAL_INFO structure containing the NTLM keys (i.e. LM and NT hashes) of the authenticating user. This feature allows users to switch to NTLM authentications when remote servers don't support Kerberos, while still relying on an asymmetric Kerberos pre-authentication verification mechanism (i.e. PKINIT).

The NTLM keys will then be recoverable after a TGS-REQ through U2U, combined with S4U2self, which is a Service Ticket request made to the KDC where the user asks to authenticate to itself (i.e. S4U2self + User-to-User authentication).

The following protocol diagram details how UnPAC-the-hash works. It allows attackers that know a user's private key, or attackers able to conduct Shadow Credentials or Golden certificate attacks, to recover the user's LM and NT hashes.

Practice

{% tabs %} {% tab title="UNIX-like" %} From UNIX-like systems, this attack can be conducted with PKINITtools (Python).

The first step consists in obtaining a TGT by validating a PKINIT pre-authentication first.

gettgtpkinit.py -cert-pfx "PATH_TO_CERTIFICATE" -pfx-pass "CERTIFICATE_PASSWORD" "FQDN_DOMAIN/TARGET_SAMNAME" "TGT_CCACHE_FILE"

Once the TGT is obtained, and the session key extracted (printed by gettgtpkinit.py), the getnthash.py script can be used to recover the NT hash.

export KRB5CCNAME="TGT_CCACHE_FILE"
getnthash.py -key 'AS-REP encryption key' 'FQDN_DOMAIN'/'TARGET_SAMNAME'

The NT hash can be used for pass-the-hash, silver ticket, or Kerberos delegations abuse. {% endtab %}

{% tab title="Windows" %} From Windows systems, Rubeus (C#) can be used to requesting a ticket using a certificate and use /getcredentials to retrieve the NT hash in the PAC.

Rubeus.exe asktgt /getcredentials /user:"TARGET_SAMNAME" /certificate:"BASE64_CERTIFICATE" /password:"CERTIFICATE_PASSWORD" /domain:"FQDN_DOMAIN" /dc:"DOMAIN_CONTROLLER" /show

{% endtab %} {% endtabs %}

Resources

{% embed url="https://shenaniganslabs.io/2021/06/21/Shadow-Credentials.html" %}

{% embed url="https://www.sstic.org/2014/presentation/secrets_dauthentification_pisode_ii__kerberos_contre-attaque/" %}