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Kerberos Unconstrained Delegation

This lab explores a security impact of unrestricted kerberos delegation enabled on a domain computer.

Overview

  • Unrestricted kerberos delegation is a privilege that can be assigned to a domain computer or a user;
  • Usually, this privilege is given to computers (in this lab, it is assigned to a computer IIS01) running services like IIS, MSSQL, etc.;
  • Those services usually require access to some back-end database (or some other server), so it can read/modify the database on the authenticated user's behalf;
  • When a user authenticates to a computer that has unresitricted kerberos delegation privilege turned on, authenticated user's TGT ticket gets saved to that computer's memory;
  • The reason TGTs get cached in memory is so the computer (with delegation rights) can impersonate the authenticated user as and when required for accessing any other services on that user's behalf.

Essentially this looks like so:
User --- authenticates to ---> IIS server ---> authenticates on behalf of the user ---> DB server

{% hint style="warning" %} Any user authentication (i.e CIFS) to the computer with unconstrained delegation enabled on it, will cache that user's TGT in memory, which can later be dumped and reused by an adversary. {% endhint %}

Setup

Let's give one of our domain computers/our victim computer IIS01 unrestricted kerberos delegation privilege:

To confirm/find computers on a domain that have unrestricted kerberos delegation property set:

Get-ADComputer -Filter {TrustedForDelegation -eq $true -and primarygroupid -eq 515} -Properties trustedfordelegation,serviceprincipalname,description

We can see our victim computer IIS01 with TrustedForDelegation field set to $true - we are good to attack:

Execution

On the computer IIS01 with kerberos delegation rights, let's do a base run of mimikatz to see what we can find in memory:

sekurlsa::tickets

Note that we do not have a TGT for offense\administrator (Domain Admin) just yet.

Let's now send an HTTP request to IIS01 from a DC01 host from the context of offense\administrator:

Invoke-WebRequest http://iis01.offense.local -UseDefaultCredentials -UseBasicParsing

We see the request got a HTTP 200 OK response:

Let's check the victim host IIS01 for new kerberos tickets in memory:

mimikatz # sekurlsa::tickets

We can see that the IIS01 has now got a TGT for offense\administrator - this means that we have effectively compromised the entire offense.local domain. We will get back to this in a moment.

First, let's export all kerberos tickets from IIS01 memory, so we can load offense\administrator ticket TGT into the current session and assume its privileges:

mimikatz::tickets /export

..but before we proceed with pass-the-ticket attack and become a DA, let's try PSRemoting to the DC01 from IIS01 and check currently available kerberos tickets in a current logon session - just to make sure we currently do not have DA rights:

Above screenshow shows that there are no tickets and PSSession could not be established - as expected.

Let's now proceed and import the previously dumped offense\administrator TGT into our current logon session on the IIS01 host:

mimikatz # kerberos::ptt C:\Users\Administrator\Desktop\mimikatz\[0;3c785]-2-0-40e10000-Administrator@krbtgt-OFFENSE.LOCAL.kirbi

Once the TGT is imported on IIS01, let's check available tickets and try connecting to the DC01 again:

As you can see from the above screengrab, the IIS01 system now contains a krbtgt for offense\administrator, which enables this session to access DC01 C$ share and establish a PSSession with an interactive shell with Domain admin privileges.

Reminder

Note that successful authentication to ANY service on the IIS01 will cache the authenticated user's TGT. Below is an example of a user offense\delegate accessing a share on IIS01 - the TGT gets cached:

Mitigation

Some of the available mitigations:

  • Disable kerberos delegation where possible
  • Be cautious of whom you give privilege Enable computer and user accounts to be trusted for delegation - these are users who can enable unrestricted kerberos delagation
  • Enable Account is sensitive and cannot be delegated for high privileged accounts

References

{% embed url="https://adsecurity.org/?p=1667" %}

{% embed url="https://blog.xpnsec.com/kerberos-attacks-part-1/" %}

{% embed url="https://blogs.technet.microsoft.com/askds/2008/03/06/kerberos-for-the-busy-admin/" %}